









Network Working Group                                          J. Callas

Request for Comments: 4880                               PGP Corporation

Obsoletes: 1991, 2440                                     L. Donnerhacke

Category: Standards Track                                       IKS GmbH

                                                               H. Finney

                                                         PGP Corporation

                                                                 D. Shaw

                                                               R. Thayer

                                                           November 2007





                         OpenPGP Message Format



Status of This Memo



   This document specifies an Internet standards track protocol for the

   Internet community, and requests discussion and suggestions for

   improvements.  Please refer to the current edition of the "Internet

   Official Protocol Standards" (STD 1) for the standardization state

   and status of this protocol.  Distribution of this memo is unlimited.



Abstract



   This document is maintained in order to publish all necessary

   information needed to develop interoperable applications based on the

   OpenPGP format.  It is not a step-by-step cookbook for writing an

   application.  It describes only the format and methods needed to

   read, check, generate, and write conforming packets crossing any

   network.  It does not deal with storage and implementation questions.

   It does, however, discuss implementation issues necessary to avoid

   security flaws.



   OpenPGP software uses a combination of strong public-key and

   symmetric cryptography to provide security services for electronic

   communications and data storage.  These services include

   confidentiality, key management, authentication, and digital

   signatures.  This document specifies the message formats used in

   OpenPGP.



























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Table of Contents



   1. Introduction ....................................................5

      1.1. Terms ......................................................5

   2. General functions ...............................................6

      2.1. Confidentiality via Encryption .............................6

      2.2. Authentication via Digital Signature .......................7

      2.3. Compression ................................................7

      2.4. Conversion to Radix-64 .....................................8

      2.5. Signature-Only Applications ................................8

   3. Data Element Formats ............................................8

      3.1. Scalar Numbers .............................................8

      3.2. Multiprecision Integers ....................................9

      3.3. Key IDs ....................................................9

      3.4. Text .......................................................9

      3.5. Time Fields ...............................................10

      3.6. Keyrings ..................................................10

      3.7. String-to-Key (S2K) Specifiers ............................10

           3.7.1. String-to-Key (S2K) Specifier Types ................10

                  3.7.1.1. Simple S2K ................................10

                  3.7.1.2. Salted S2K ................................11

                  3.7.1.3. Iterated and Salted S2K ...................11

           3.7.2. String-to-Key Usage ................................12

                  3.7.2.1. Secret-Key Encryption .....................12

                  3.7.2.2. Symmetric-Key Message Encryption ..........13

   4. Packet Syntax ..................................................13

      4.1. Overview ..................................................13

      4.2. Packet Headers ............................................13

           4.2.1. Old Format Packet Lengths ..........................14

           4.2.2. New Format Packet Lengths ..........................15

                  4.2.2.1. One-Octet Lengths .........................15

                  4.2.2.2. Two-Octet Lengths .........................15

                  4.2.2.3. Five-Octet Lengths ........................15

                  4.2.2.4. Partial Body Lengths ......................16

           4.2.3. Packet Length Examples .............................16

      4.3. Packet Tags ...............................................17

   5. Packet Types ...................................................17

      5.1. Public-Key Encrypted Session Key Packets (Tag 1) ..........17

      5.2. Signature Packet (Tag 2) ..................................19

           5.2.1. Signature Types ....................................19

           5.2.2. Version 3 Signature Packet Format ..................21

           5.2.3. Version 4 Signature Packet Format ..................24

                  5.2.3.1. Signature Subpacket Specification .........25

                  5.2.3.2. Signature Subpacket Types .................27

                  5.2.3.3. Notes on Self-Signatures ..................27

                  5.2.3.4. Signature Creation Time ...................28

                  5.2.3.5. Issuer ....................................28

                  5.2.3.6. Key Expiration Time .......................28







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                  5.2.3.7. Preferred Symmetric Algorithms ............28

                  5.2.3.8. Preferred Hash Algorithms .................29

                  5.2.3.9. Preferred Compression Algorithms ..........29

                  5.2.3.10. Signature Expiration Time ................29

                  5.2.3.11. Exportable Certification .................29

                  5.2.3.12. Revocable ................................30

                  5.2.3.13. Trust Signature ..........................30

                  5.2.3.14. Regular Expression .......................31

                  5.2.3.15. Revocation Key ...........................31

                  5.2.3.16. Notation Data ............................31

                  5.2.3.17. Key Server Preferences ...................32

                  5.2.3.18. Preferred Key Server .....................33

                  5.2.3.19. Primary User ID ..........................33

                  5.2.3.20. Policy URI ...............................33

                  5.2.3.21. Key Flags ................................33

                  5.2.3.22. Signer's User ID .........................34

                  5.2.3.23. Reason for Revocation ....................35

                  5.2.3.24. Features .................................36

                  5.2.3.25. Signature Target .........................36

                  5.2.3.26. Embedded Signature .......................37

           5.2.4. Computing Signatures ...............................37

                  5.2.4.1. Subpacket Hints ...........................38

      5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) .......38

      5.4. One-Pass Signature Packets (Tag 4) ........................39

      5.5. Key Material Packet .......................................40

           5.5.1. Key Packet Variants ................................40

                  5.5.1.1. Public-Key Packet (Tag 6) .................40

                  5.5.1.2. Public-Subkey Packet (Tag 14) .............40

                  5.5.1.3. Secret-Key Packet (Tag 5) .................41

                  5.5.1.4. Secret-Subkey Packet (Tag 7) ..............41

           5.5.2. Public-Key Packet Formats ..........................41

           5.5.3. Secret-Key Packet Formats ..........................43

      5.6. Compressed Data Packet (Tag 8) ............................45

      5.7. Symmetrically Encrypted Data Packet (Tag 9) ...............45

      5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) ..........46

      5.9. Literal Data Packet (Tag 11) ..............................46

      5.10. Trust Packet (Tag 12) ....................................47

      5.11. User ID Packet (Tag 13) ..................................48

      5.12. User Attribute Packet (Tag 17) ...........................48

           5.12.1. The Image Attribute Subpacket .....................48

      5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) ..49

      5.14. Modification Detection Code Packet (Tag 19) ..............52

   6. Radix-64 Conversions ...........................................53

      6.1. An Implementation of the CRC-24 in "C" ....................54

      6.2. Forming ASCII Armor .......................................54

      6.3. Encoding Binary in Radix-64 ...............................57

      6.4. Decoding Radix-64 .........................................58

      6.5. Examples of Radix-64 ......................................59







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      6.6. Example of an ASCII Armored Message .......................59

   7. Cleartext Signature Framework ..................................59

      7.1. Dash-Escaped Text .........................................60

   8. Regular Expressions ............................................61

   9. Constants ......................................................61

      9.1. Public-Key Algorithms .....................................62

      9.2. Symmetric-Key Algorithms ..................................62

      9.3. Compression Algorithms ....................................63

      9.4. Hash Algorithms ...........................................63

   10. IANA Considerations ...........................................63

      10.1. New String-to-Key Specifier Types ........................64

      10.2. New Packets ..............................................64

           10.2.1. User Attribute Types ..............................64

                  10.2.1.1. Image Format Subpacket Types .............64

           10.2.2. New Signature Subpackets ..........................64

                  10.2.2.1. Signature Notation Data Subpackets .......65

                  10.2.2.2. Key Server Preference Extensions .........65

                  10.2.2.3. Key Flags Extensions .....................65

                  10.2.2.4. Reason For Revocation Extensions .........65

                  10.2.2.5. Implementation Features ..................66

           10.2.3. New Packet Versions ...............................66

      10.3. New Algorithms ...........................................66

           10.3.1. Public-Key Algorithms .............................66

           10.3.2. Symmetric-Key Algorithms ..........................67

           10.3.3. Hash Algorithms ...................................67

           10.3.4. Compression Algorithms ............................67

   11. Packet Composition ............................................67

      11.1. Transferable Public Keys .................................67

      11.2. Transferable Secret Keys .................................69

      11.3. OpenPGP Messages .........................................69

      11.4. Detached Signatures ......................................70

   12. Enhanced Key Formats ..........................................70

      12.1. Key Structures ...........................................70

      12.2. Key IDs and Fingerprints .................................71

   13. Notes on Algorithms ...........................................72

      13.1. PKCS#1 Encoding in OpenPGP ...............................72

           13.1.1. EME-PKCS1-v1_5-ENCODE .............................73

           13.1.2. EME-PKCS1-v1_5-DECODE .............................73

           13.1.3. EMSA-PKCS1-v1_5 ...................................74

      13.2. Symmetric Algorithm Preferences ..........................75

      13.3. Other Algorithm Preferences ..............................76

           13.3.1. Compression Preferences ...........................76

           13.3.2. Hash Algorithm Preferences ........................76

      13.4. Plaintext ................................................77

      13.5. RSA ......................................................77

      13.6. DSA ......................................................77

      13.7. Elgamal ..................................................78

      13.8. Reserved Algorithm Numbers ...............................78







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      13.9. OpenPGP CFB Mode .........................................78

      13.10. Private or Experimental Parameters ......................79

      13.11. Extension of the MDC System .............................80

      13.12. Meta-Considerations for Expansion .......................80

   14. Security Considerations .......................................81

   15. Implementation Nits ...........................................84

   16. References ....................................................86

      16.1. Normative References .....................................86

      16.2. Informative References ...................................88



1.  Introduction



   This document provides information on the message-exchange packet

   formats used by OpenPGP to provide encryption, decryption, signing,

   and key management functions.  It is a revision of RFC 2440, "OpenPGP

   Message Format", which itself replaces RFC 1991, "PGP Message

   Exchange Formats" [RFC1991] [RFC2440].



1.1.  Terms



     * OpenPGP - This is a term for security software that uses PGP 5.x

       as a basis, formalized in RFC 2440 and this document.



     * PGP - Pretty Good Privacy.  PGP is a family of software systems

       developed by Philip R. Zimmermann from which OpenPGP is based.



     * PGP 2.6.x - This version of PGP has many variants, hence the term

       PGP 2.6.x.  It used only RSA, MD5, and IDEA for its cryptographic

       transforms.  An informational RFC, RFC 1991, was written

       describing this version of PGP.



     * PGP 5.x - This version of PGP is formerly known as "PGP 3" in the

       community and also in the predecessor of this document, RFC 1991.

       It has new formats and corrects a number of problems in the PGP

       2.6.x design.  It is referred to here as PGP 5.x because that

       software was the first release of the "PGP 3" code base.



     * GnuPG - GNU Privacy Guard, also called GPG.  GnuPG is an OpenPGP

       implementation that avoids all encumbered algorithms.

       Consequently, early versions of GnuPG did not include RSA public

       keys.  GnuPG may or may not have (depending on version) support

       for IDEA or other encumbered algorithms.



   "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of PGP

   Corporation and are used with permission.  The term "OpenPGP" refers

   to the protocol described in this and related documents.











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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

   document are to be interpreted as described in [RFC2119].



   The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME

   FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG

   APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in

   this document when used to describe namespace allocation are to be

   interpreted as described in [RFC2434].



2.  General functions



   OpenPGP provides data integrity services for messages and data files

   by using these core technologies:



     - digital signatures



     - encryption



     - compression



     - Radix-64 conversion



   In addition, OpenPGP provides key management and certificate

   services, but many of these are beyond the scope of this document.



2.1.  Confidentiality via Encryption



   OpenPGP combines symmetric-key encryption and public-key encryption

   to provide confidentiality.  When made confidential, first the object

   is encrypted using a symmetric encryption algorithm.  Each symmetric

   key is used only once, for a single object.  A new "session key" is

   generated as a random number for each object (sometimes referred to

   as a session).  Since it is used only once, the session key is bound

   to the message and transmitted with it.  To protect the key, it is

   encrypted with the receiver's public key.  The sequence is as

   follows:



   1.  The sender creates a message.



   2.  The sending OpenPGP generates a random number to be used as a

       session key for this message only.



   3.  The session key is encrypted using each recipient's public key.

       These "encrypted session keys" start the message.













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   4.  The sending OpenPGP encrypts the message using the session key,

       which forms the remainder of the message.  Note that the message

       is also usually compressed.



   5.  The receiving OpenPGP decrypts the session key using the

       recipient's private key.



   6.  The receiving OpenPGP decrypts the message using the session key.

       If the message was compressed, it will be decompressed.



   With symmetric-key encryption, an object may be encrypted with a

   symmetric key derived from a passphrase (or other shared secret), or

   a two-stage mechanism similar to the public-key method described

   above in which a session key is itself encrypted with a symmetric

   algorithm keyed from a shared secret.



   Both digital signature and confidentiality services may be applied to

   the same message.  First, a signature is generated for the message

   and attached to the message.  Then the message plus signature is

   encrypted using a symmetric session key.  Finally, the session key is

   encrypted using public-key encryption and prefixed to the encrypted

   block.



2.2.  Authentication via Digital Signature



   The digital signature uses a hash code or message digest algorithm,

   and a public-key signature algorithm.  The sequence is as follows:



   1.  The sender creates a message.



   2.  The sending software generates a hash code of the message.



   3.  The sending software generates a signature from the hash code

       using the sender's private key.



   4.  The binary signature is attached to the message.



   5.  The receiving software keeps a copy of the message signature.



   6.  The receiving software generates a new hash code for the received

       message and verifies it using the message's signature.  If the

       verification is successful, the message is accepted as authentic.



2.3.  Compression



   OpenPGP implementations SHOULD compress the message after applying

   the signature but before encryption.









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   If an implementation does not implement compression, its authors

   should be aware that most OpenPGP messages in the world are

   compressed.  Thus, it may even be wise for a space-constrained

   implementation to implement decompression, but not compression.



   Furthermore, compression has the added side effect that some types of

   attacks can be thwarted by the fact that slightly altered, compressed

   data rarely uncompresses without severe errors.  This is hardly

   rigorous, but it is operationally useful.  These attacks can be

   rigorously prevented by implementing and using Modification Detection

   Codes as described in sections following.



2.4.  Conversion to Radix-64



   OpenPGP's underlying native representation for encrypted messages,

   signature certificates, and keys is a stream of arbitrary octets.

   Some systems only permit the use of blocks consisting of seven-bit,

   printable text.  For transporting OpenPGP's native raw binary octets

   through channels that are not safe to raw binary data, a printable

   encoding of these binary octets is needed.  OpenPGP provides the

   service of converting the raw 8-bit binary octet stream to a stream

   of printable ASCII characters, called Radix-64 encoding or ASCII

   Armor.



   Implementations SHOULD provide Radix-64 conversions.



2.5.  Signature-Only Applications



   OpenPGP is designed for applications that use both encryption and

   signatures, but there are a number of problems that are solved by a

   signature-only implementation.  Although this specification requires

   both encryption and signatures, it is reasonable for there to be

   subset implementations that are non-conformant only in that they omit

   encryption.



3.  Data Element Formats



   This section describes the data elements used by OpenPGP.



3.1.  Scalar Numbers



   Scalar numbers are unsigned and are always stored in big-endian

   format.  Using n[k] to refer to the kth octet being interpreted, the

   value of a two-octet scalar is ((n[0] << 8) + n[1]).  The value of a

   four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +

   n[3]).











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3.2.  Multiprecision Integers



   Multiprecision integers (also called MPIs) are unsigned integers used

   to hold large integers such as the ones used in cryptographic

   calculations.



   An MPI consists of two pieces: a two-octet scalar that is the length

   of the MPI in bits followed by a string of octets that contain the

   actual integer.



   These octets form a big-endian number; a big-endian number can be

   made into an MPI by prefixing it with the appropriate length.



   Examples:



   (all numbers are in hexadecimal)



   The string of octets [00 01 01] forms an MPI with the value 1.  The

   string [00 09 01 FF] forms an MPI with the value of 511.



   Additional rules:



   The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.



   The length field of an MPI describes the length starting from its

   most significant non-zero bit.  Thus, the MPI [00 02 01] is not

   formed correctly.  It should be [00 01 01].



   Unused bits of an MPI MUST be zero.



   Also note that when an MPI is encrypted, the length refers to the

   plaintext MPI.  It may be ill-formed in its ciphertext.



3.3.  Key IDs



   A Key ID is an eight-octet scalar that identifies a key.

   Implementations SHOULD NOT assume that Key IDs are unique.  The

   section "Enhanced Key Formats" below describes how Key IDs are

   formed.



3.4.  Text



   Unless otherwise specified, the character set for text is the UTF-8

   [RFC3629] encoding of Unicode [ISO10646].















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3.5.  Time Fields



   A time field is an unsigned four-octet number containing the number

   of seconds elapsed since midnight, 1 January 1970 UTC.



3.6.  Keyrings



   A keyring is a collection of one or more keys in a file or database.

   Traditionally, a keyring is simply a sequential list of keys, but may

   be any suitable database.  It is beyond the scope of this standard to

   discuss the details of keyrings or other databases.



3.7.  String-to-Key (S2K) Specifiers



   String-to-key (S2K) specifiers are used to convert passphrase strings

   into symmetric-key encryption/decryption keys.  They are used in two

   places, currently: to encrypt the secret part of private keys in the

   private keyring, and to convert passphrases to encryption keys for

   symmetrically encrypted messages.



3.7.1.  String-to-Key (S2K) Specifier Types



   There are three types of S2K specifiers currently supported, and

   some reserved values:



       ID          S2K Type

       --          --------

       0           Simple S2K

       1           Salted S2K

       2           Reserved value

       3           Iterated and Salted S2K

       100 to 110  Private/Experimental S2K



   These are described in Sections 3.7.1.1 - 3.7.1.3.



3.7.1.1.  Simple S2K



   This directly hashes the string to produce the key data.  See below

   for how this hashing is done.



       Octet 0:        0x00

       Octet 1:        hash algorithm



   Simple S2K hashes the passphrase to produce the session key.  The

   manner in which this is done depends on the size of the session key

   (which will depend on the cipher used) and the size of the hash











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   algorithm's output.  If the hash size is greater than the session key

   size, the high-order (leftmost) octets of the hash are used as the

   key.



   If the hash size is less than the key size, multiple instances of the

   hash context are created -- enough to produce the required key data.

   These instances are preloaded with 0, 1, 2, ... octets of zeros (that

   is to say, the first instance has no preloading, the second gets

   preloaded with 1 octet of zero, the third is preloaded with two

   octets of zeros, and so forth).



   As the data is hashed, it is given independently to each hash

   context.  Since the contexts have been initialized differently, they

   will each produce different hash output.  Once the passphrase is

   hashed, the output data from the multiple hashes is concatenated,

   first hash leftmost, to produce the key data, with any excess octets

   on the right discarded.



3.7.1.2.  Salted S2K



   This includes a "salt" value in the S2K specifier -- some arbitrary

   data -- that gets hashed along with the passphrase string, to help

   prevent dictionary attacks.



       Octet 0:        0x01

       Octet 1:        hash algorithm

       Octets 2-9:     8-octet salt value



   Salted S2K is exactly like Simple S2K, except that the input to the

   hash function(s) consists of the 8 octets of salt from the S2K

   specifier, followed by the passphrase.



3.7.1.3.  Iterated and Salted S2K



   This includes both a salt and an octet count.  The salt is combined

   with the passphrase and the resulting value is hashed repeatedly.

   This further increases the amount of work an attacker must do to try

   dictionary attacks.



       Octet  0:        0x03

       Octet  1:        hash algorithm

       Octets 2-9:      8-octet salt value

       Octet  10:       count, a one-octet, coded value

















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   The count is coded into a one-octet number using the following

   formula:



       #define EXPBIAS 6

           count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);



   The above formula is in C, where "Int32" is a type for a 32-bit

   integer, and the variable "c" is the coded count, Octet 10.



   Iterated-Salted S2K hashes the passphrase and salt data multiple

   times.  The total number of octets to be hashed is specified in the

   encoded count in the S2K specifier.  Note that the resulting count

   value is an octet count of how many octets will be hashed, not an

   iteration count.



   Initially, one or more hash contexts are set up as with the other S2K

   algorithms, depending on how many octets of key data are needed.

   Then the salt, followed by the passphrase data, is repeatedly hashed

   until the number of octets specified by the octet count has been

   hashed.  The one exception is that if the octet count is less than

   the size of the salt plus passphrase, the full salt plus passphrase

   will be hashed even though that is greater than the octet count.

   After the hashing is done, the data is unloaded from the hash

   context(s) as with the other S2K algorithms.



3.7.2.  String-to-Key Usage



   Implementations SHOULD use salted or iterated-and-salted S2K

   specifiers, as simple S2K specifiers are more vulnerable to

   dictionary attacks.



3.7.2.1.  Secret-Key Encryption



   An S2K specifier can be stored in the secret keyring to specify how

   to convert the passphrase to a key that unlocks the secret data.

   Older versions of PGP just stored a cipher algorithm octet preceding

   the secret data or a zero to indicate that the secret data was

   unencrypted.  The MD5 hash function was always used to convert the

   passphrase to a key for the specified cipher algorithm.



   For compatibility, when an S2K specifier is used, the special value

   254 or 255 is stored in the position where the hash algorithm octet

   would have been in the old data structure.  This is then followed

   immediately by a one-octet algorithm identifier, and then by the S2K

   specifier as encoded above.













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   Therefore, preceding the secret data there will be one of these

   possibilities:



       0:           secret data is unencrypted (no passphrase)

       255 or 254:  followed by algorithm octet and S2K specifier

       Cipher alg:  use Simple S2K algorithm using MD5 hash



   This last possibility, the cipher algorithm number with an implicit

   use of MD5 and IDEA, is provided for backward compatibility; it MAY

   be understood, but SHOULD NOT be generated, and is deprecated.



   These are followed by an Initial Vector of the same length as the

   block size of the cipher for the decryption of the secret values, if

   they are encrypted, and then the secret-key values themselves.



3.7.2.2.  Symmetric-Key Message Encryption



   OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet

   at the front of a message.  This is used to allow S2K specifiers to

   be used for the passphrase conversion or to create messages with a

   mix of symmetric-key ESKs and public-key ESKs.  This allows a message

   to be decrypted either with a passphrase or a public-key pair.



   PGP 2.X always used IDEA with Simple string-to-key conversion when

   encrypting a message with a symmetric algorithm.  This is deprecated,

   but MAY be used for backward-compatibility.



4.  Packet Syntax



   This section describes the packets used by OpenPGP.



4.1.  Overview



   An OpenPGP message is constructed from a number of records that are

   traditionally called packets.  A packet is a chunk of data that has a

   tag specifying its meaning.  An OpenPGP message, keyring,

   certificate, and so forth consists of a number of packets.  Some of

   those packets may contain other OpenPGP packets (for example, a

   compressed data packet, when uncompressed, contains OpenPGP packets).



   Each packet consists of a packet header, followed by the packet body.

   The packet header is of variable length.



4.2.  Packet Headers



   The first octet of the packet header is called the "Packet Tag".  It

   determines the format of the header and denotes the packet contents.

   The remainder of the packet header is the length of the packet.







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   Note that the most significant bit is the leftmost bit, called bit 7.

   A mask for this bit is 0x80 in hexadecimal.



              +---------------+

         PTag |7 6 5 4 3 2 1 0|

              +---------------+

         Bit 7 -- Always one

         Bit 6 -- New packet format if set



   PGP 2.6.x only uses old format packets.  Thus, software that

   interoperates with those versions of PGP must only use old format

   packets.  If interoperability is not an issue, the new packet format

   is RECOMMENDED.  Note that old format packets have four bits of

   packet tags, and new format packets have six; some features cannot be

   used and still be backward-compatible.



   Also note that packets with a tag greater than or equal to 16 MUST

   use new format packets.  The old format packets can only express tags

   less than or equal to 15.



   Old format packets contain:



         Bits 5-2 -- packet tag

         Bits 1-0 -- length-type



   New format packets contain:



         Bits 5-0 -- packet tag



4.2.1.  Old Format Packet Lengths



   The meaning of the length-type in old format packets is:



   0 - The packet has a one-octet length.  The header is 2 octets long.



   1 - The packet has a two-octet length.  The header is 3 octets long.



   2 - The packet has a four-octet length.  The header is 5 octets long.



   3 - The packet is of indeterminate length.  The header is 1 octet

       long, and the implementation must determine how long the packet

       is.  If the packet is in a file, this means that the packet

       extends until the end of the file.  In general, an implementation

       SHOULD NOT use indeterminate-length packets except where the end

       of the data will be clear from the context, and even then it is

       better to use a definite length, or a new format header.  The new

       format headers described below have a mechanism for precisely

       encoding data of indeterminate length.







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4.2.2.  New Format Packet Lengths



   New format packets have four possible ways of encoding length:



   1. A one-octet Body Length header encodes packet lengths of up to 191

      octets.



   2. A two-octet Body Length header encodes packet lengths of 192 to

      8383 octets.



   3. A five-octet Body Length header encodes packet lengths of up to

      4,294,967,295 (0xFFFFFFFF) octets in length.  (This actually

      encodes a four-octet scalar number.)



   4. When the length of the packet body is not known in advance by the

      issuer, Partial Body Length headers encode a packet of

      indeterminate length, effectively making it a stream.



4.2.2.1.  One-Octet Lengths



   A one-octet Body Length header encodes a length of 0 to 191 octets.

   This type of length header is recognized because the one octet value

   is less than 192.  The body length is equal to:



       bodyLen = 1st_octet;



4.2.2.2.  Two-Octet Lengths



   A two-octet Body Length header encodes a length of 192 to 8383

   octets.  It is recognized because its first octet is in the range 192

   to 223.  The body length is equal to:



       bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192



4.2.2.3.  Five-Octet Lengths



   A five-octet Body Length header consists of a single octet holding

   the value 255, followed by a four-octet scalar.  The body length is

   equal to:



       bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |

                 (4th_octet << 8)  | 5th_octet



   This basic set of one, two, and five-octet lengths is also used

   internally to some packets.













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4.2.2.4.  Partial Body Lengths



   A Partial Body Length header is one octet long and encodes the length

   of only part of the data packet.  This length is a power of 2, from 1

   to 1,073,741,824 (2 to the 30th power).  It is recognized by its one

   octet value that is greater than or equal to 224, and less than 255.

   The Partial Body Length is equal to:



       partialBodyLen = 1 << (1st_octet & 0x1F);



   Each Partial Body Length header is followed by a portion of the

   packet body data.  The Partial Body Length header specifies this

   portion's length.  Another length header (one octet, two-octet,

   five-octet, or partial) follows that portion.  The last length header

   in the packet MUST NOT be a Partial Body Length header.  Partial Body

   Length headers may only be used for the non-final parts of the

   packet.



   Note also that the last Body Length header can be a zero-length

   header.



   An implementation MAY use Partial Body Lengths for data packets, be

   they literal, compressed, or encrypted.  The first partial length

   MUST be at least 512 octets long.  Partial Body Lengths MUST NOT be

   used for any other packet types.



4.2.3.  Packet Length Examples



   These examples show ways that new format packets might encode the

   packet lengths.



   A packet with length 100 may have its length encoded in one octet:

   0x64.  This is followed by 100 octets of data.



   A packet with length 1723 may have its length encoded in two octets:

   0xC5, 0xFB.  This header is followed by the 1723 octets of data.



   A packet with length 100000 may have its length encoded in five

   octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.



   It might also be encoded in the following octet stream: 0xEF, first

   32768 octets of data; 0xE1, next two octets of data; 0xE0, next one

   octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693

   octets of data.  This is just one possible encoding, and many

   variations are possible on the size of the Partial Body Length

   headers, as long as a regular Body Length header encodes the last

   portion of the data.









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   Please note that in all of these explanations, the total length of

   the packet is the length of the header(s) plus the length of the

   body.



4.3.  Packet Tags



   The packet tag denotes what type of packet the body holds.  Note that

   old format headers can only have tags less than 16, whereas new

   format headers can have tags as great as 63.  The defined tags (in

   decimal) are as follows:



       0        -- Reserved - a packet tag MUST NOT have this value

       1        -- Public-Key Encrypted Session Key Packet

       2        -- Signature Packet

       3        -- Symmetric-Key Encrypted Session Key Packet

       4        -- One-Pass Signature Packet

       5        -- Secret-Key Packet

       6        -- Public-Key Packet

       7        -- Secret-Subkey Packet

       8        -- Compressed Data Packet

       9        -- Symmetrically Encrypted Data Packet

       10       -- Marker Packet

       11       -- Literal Data Packet

       12       -- Trust Packet

       13       -- User ID Packet

       14       -- Public-Subkey Packet

       17       -- User Attribute Packet

       18       -- Sym. Encrypted and Integrity Protected Data Packet

       19       -- Modification Detection Code Packet

       60 to 63 -- Private or Experimental Values



5.  Packet Types



5.1.  Public-Key Encrypted Session Key Packets (Tag 1)



   A Public-Key Encrypted Session Key packet holds the session key used

   to encrypt a message.  Zero or more Public-Key Encrypted Session Key

   packets and/or Symmetric-Key Encrypted Session Key packets may

   precede a Symmetrically Encrypted Data Packet, which holds an

   encrypted message.  The message is encrypted with the session key,

   and the session key is itself encrypted and stored in the Encrypted

   Session Key packet(s).  The Symmetrically Encrypted Data Packet is

   preceded by one Public-Key Encrypted Session Key packet for each

   OpenPGP key to which the message is encrypted.  The recipient of the

   message finds a session key that is encrypted to their public key,

   decrypts the session key, and then uses the session key to decrypt

   the message.









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   The body of this packet consists of:



     - A one-octet number giving the version number of the packet type.

       The currently defined value for packet version is 3.



     - An eight-octet number that gives the Key ID of the public key to

       which the session key is encrypted.  If the session key is

       encrypted to a subkey, then the Key ID of this subkey is used

       here instead of the Key ID of the primary key.



     - A one-octet number giving the public-key algorithm used.



     - A string of octets that is the encrypted session key.  This

       string takes up the remainder of the packet, and its contents are

       dependent on the public-key algorithm used.



   Algorithm Specific Fields for RSA encryption



     - multiprecision integer (MPI) of RSA encrypted value m**e mod n.



   Algorithm Specific Fields for Elgamal encryption:



     - MPI of Elgamal (Diffie-Hellman) value g**k mod p.



     - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.



   The value "m" in the above formulas is derived from the session key

   as follows.  First, the session key is prefixed with a one-octet

   algorithm identifier that specifies the symmetric encryption

   algorithm used to encrypt the following Symmetrically Encrypted Data

   Packet.  Then a two-octet checksum is appended, which is equal to the

   sum of the preceding session key octets, not including the algorithm

   identifier, modulo 65536.  This value is then encoded as described in

   PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to

   form the "m" value used in the formulas above.  See Section 13.1 of

   this document for notes on OpenPGP's use of PKCS#1.



   Note that when an implementation forms several PKESKs with one

   session key, forming a message that can be decrypted by several keys,

   the implementation MUST make a new PKCS#1 encoding for each key.



   An implementation MAY accept or use a Key ID of zero as a "wild card"

   or "speculative" Key ID.  In this case, the receiving implementation

   would try all available private keys, checking for a valid decrypted

   session key.  This format helps reduce traffic analysis of messages.













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5.2.  Signature Packet (Tag 2)



   A Signature packet describes a binding between some public key and

   some data.  The most common signatures are a signature of a file or a

   block of text, and a signature that is a certification of a User ID.



   Two versions of Signature packets are defined.  Version 3 provides

   basic signature information, while version 4 provides an expandable

   format with subpackets that can specify more information about the

   signature.  PGP 2.6.x only accepts version 3 signatures.



   Implementations SHOULD accept V3 signatures.  Implementations SHOULD

   generate V4 signatures.



   Note that if an implementation is creating an encrypted and signed

   message that is encrypted to a V3 key, it is reasonable to create a

   V3 signature.



5.2.1.  Signature Types



   There are a number of possible meanings for a signature, which are

   indicated in a signature type octet in any given signature.  Please

   note that the vagueness of these meanings is not a flaw, but a

   feature of the system.  Because OpenPGP places final authority for

   validity upon the receiver of a signature, it may be that one

   signer's casual act might be more rigorous than some other

   authority's positive act.  See Section 5.2.4, "Computing Signatures",

   for detailed information on how to compute and verify signatures of

   each type.



   These meanings are as follows:



   0x00: Signature of a binary document.

       This means the signer owns it, created it, or certifies that it

       has not been modified.



   0x01: Signature of a canonical text document.

       This means the signer owns it, created it, or certifies that it

       has not been modified.  The signature is calculated over the text

       data with its line endings converted to <CR><LF>.



   0x02: Standalone signature.

       This signature is a signature of only its own subpacket contents.

       It is calculated identically to a signature over a zero-length

       binary document.  Note that it doesn't make sense to have a V3

       standalone signature.











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   0x10: Generic certification of a User ID and Public-Key packet.

       The issuer of this certification does not make any particular

       assertion as to how well the certifier has checked that the owner

       of the key is in fact the person described by the User ID.



   0x11: Persona certification of a User ID and Public-Key packet.

       The issuer of this certification has not done any verification of

       the claim that the owner of this key is the User ID specified.



   0x12: Casual certification of a User ID and Public-Key packet.

       The issuer of this certification has done some casual

       verification of the claim of identity.



   0x13: Positive certification of a User ID and Public-Key packet.

       The issuer of this certification has done substantial

       verification of the claim of identity.



       Most OpenPGP implementations make their "key signatures" as 0x10

       certifications.  Some implementations can issue 0x11-0x13

       certifications, but few differentiate between the types.



   0x18: Subkey Binding Signature

       This signature is a statement by the top-level signing key that

       indicates that it owns the subkey.  This signature is calculated

       directly on the primary key and subkey, and not on any User ID or

       other packets.  A signature that binds a signing subkey MUST have

       an Embedded Signature subpacket in this binding signature that

       contains a 0x19 signature made by the signing subkey on the

       primary key and subkey.



   0x19: Primary Key Binding Signature

       This signature is a statement by a signing subkey, indicating

       that it is owned by the primary key and subkey.  This signature

       is calculated the same way as a 0x18 signature: directly on the

       primary key and subkey, and not on any User ID or other packets.



   0x1F: Signature directly on a key

       This signature is calculated directly on a key.  It binds the

       information in the Signature subpackets to the key, and is

       appropriate to be used for subpackets that provide information

       about the key, such as the Revocation Key subpacket.  It is also

       appropriate for statements that non-self certifiers want to make

       about the key itself, rather than the binding between a key and a

       name.















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   0x20: Key revocation signature

       The signature is calculated directly on the key being revoked.  A

       revoked key is not to be used.  Only revocation signatures by the

       key being revoked, or by an authorized revocation key, should be

       considered valid revocation signatures.



   0x28: Subkey revocation signature

       The signature is calculated directly on the subkey being revoked.

       A revoked subkey is not to be used.  Only revocation signatures

       by the top-level signature key that is bound to this subkey, or

       by an authorized revocation key, should be considered valid

       revocation signatures.



   0x30: Certification revocation signature

       This signature revokes an earlier User ID certification signature

       (signature class 0x10 through 0x13) or direct-key signature

       (0x1F).  It should be issued by the same key that issued the

       revoked signature or an authorized revocation key.  The signature

       is computed over the same data as the certificate that it

       revokes, and should have a later creation date than that

       certificate.



   0x40: Timestamp signature.

       This signature is only meaningful for the timestamp contained in

       it.



   0x50: Third-Party Confirmation signature.

       This signature is a signature over some other OpenPGP Signature

       packet(s).  It is analogous to a notary seal on the signed data.

       A third-party signature SHOULD include Signature Target

       subpacket(s) to give easy identification.  Note that we really do

       mean SHOULD.  There are plausible uses for this (such as a blind

       party that only sees the signature, not the key or source

       document) that cannot include a target subpacket.



5.2.2.  Version 3 Signature Packet Format



   The body of a version 3 Signature Packet contains:



     - One-octet version number (3).



     - One-octet length of following hashed material.  MUST be 5.



         - One-octet signature type.



         - Four-octet creation time.



     - Eight-octet Key ID of signer.







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     - One-octet public-key algorithm.



     - One-octet hash algorithm.



     - Two-octet field holding left 16 bits of signed hash value.



     - One or more multiprecision integers comprising the signature.

       This portion is algorithm specific, as described below.



   The concatenation of the data to be signed, the signature type, and

   creation time from the Signature packet (5 additional octets) is

   hashed.  The resulting hash value is used in the signature algorithm.

   The high 16 bits (first two octets) of the hash are included in the

   Signature packet to provide a quick test to reject some invalid

   signatures.



   Algorithm-Specific Fields for RSA signatures:



     - multiprecision integer (MPI) of RSA signature value m**d mod n.



   Algorithm-Specific Fields for DSA signatures:



     - MPI of DSA value r.



     - MPI of DSA value s.



   The signature calculation is based on a hash of the signed data, as

   described above.  The details of the calculation are different for

   DSA signatures than for RSA signatures.



   With RSA signatures, the hash value is encoded using PKCS#1 encoding

   type EMSA-PKCS1-v1_5 as described in Section 9.2 of RFC 3447.  This

   requires inserting the hash value as an octet string into an ASN.1

   structure.  The object identifier for the type of hash being used is

   included in the structure.  The hexadecimal representations for the

   currently defined hash algorithms are as follows:



     - MD5:        0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05



     - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01



     - SHA-1:      0x2B, 0x0E, 0x03, 0x02, 0x1A



     - SHA224:     0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04



     - SHA256:     0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01



     - SHA384:     0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02







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     - SHA512:     0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03



   The ASN.1 Object Identifiers (OIDs) are as follows:



     - MD5:        1.2.840.113549.2.5



     - RIPEMD-160: 1.3.36.3.2.1



     - SHA-1:      1.3.14.3.2.26



     - SHA224:     2.16.840.1.101.3.4.2.4



     - SHA256:     2.16.840.1.101.3.4.2.1



     - SHA384:     2.16.840.1.101.3.4.2.2



     - SHA512:     2.16.840.1.101.3.4.2.3



   The full hash prefixes for these are as follows:



       MD5:        0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,

                   0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,

                   0x04, 0x10



       RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,

                   0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14



       SHA-1:      0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,

                   0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14



       SHA224:     0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,

                   0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05,

                   0x00, 0x04, 0x1C



       SHA256:     0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,

                   0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,

                   0x00, 0x04, 0x20



       SHA384:     0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,

                   0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,

                   0x00, 0x04, 0x30



       SHA512:     0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,

                   0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,

                   0x00, 0x04, 0x40



   DSA signatures MUST use hashes that are equal in size to the number

   of bits of q, the group generated by the DSA key's generator value.







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   If the output size of the chosen hash is larger than the number of

   bits of q, the hash result is truncated to fit by taking the number

   of leftmost bits equal to the number of bits of q.  This (possibly

   truncated) hash function result is treated as a number and used

   directly in the DSA signature algorithm.



5.2.3.  Version 4 Signature Packet Format



   The body of a version 4 Signature packet contains:



     - One-octet version number (4).



     - One-octet signature type.



     - One-octet public-key algorithm.



     - One-octet hash algorithm.



     - Two-octet scalar octet count for following hashed subpacket data.

       Note that this is the length in octets of all of the hashed

       subpackets; a pointer incremented by this number will skip over

       the hashed subpackets.



     - Hashed subpacket data set (zero or more subpackets).



     - Two-octet scalar octet count for the following unhashed subpacket

       data.  Note that this is the length in octets of all of the

       unhashed subpackets; a pointer incremented by this number will

       skip over the unhashed subpackets.



     - Unhashed subpacket data set (zero or more subpackets).



     - Two-octet field holding the left 16 bits of the signed hash

       value.



     - One or more multiprecision integers comprising the signature.

       This portion is algorithm specific, as described above.



   The concatenation of the data being signed and the signature data

   from the version number through the hashed subpacket data (inclusive)

   is hashed.  The resulting hash value is what is signed.  The left 16

   bits of the hash are included in the Signature packet to provide a

   quick test to reject some invalid signatures.



   There are two fields consisting of Signature subpackets.  The first

   field is hashed with the rest of the signature data, while the second

   is unhashed.  The second set of subpackets is not cryptographically









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   protected by the signature and should include only advisory

   information.



   The algorithms for converting the hash function result to a signature

   are described in a section below.



5.2.3.1.  Signature Subpacket Specification



   A subpacket data set consists of zero or more Signature subpackets.

   In Signature packets, the subpacket data set is preceded by a two-

   octet scalar count of the length in octets of all the subpackets.  A

   pointer incremented by this number will skip over the subpacket data

   set.



   Each subpacket consists of a subpacket header and a body.  The header

   consists of:



     - the subpacket length (1, 2, or 5 octets),



     - the subpacket type (1 octet),



   and is followed by the subpacket-specific data.



   The length includes the type octet but not this length.  Its format

   is similar to the "new" format packet header lengths, but cannot have

   Partial Body Lengths.  That is:



       if the 1st octet <  192, then

           lengthOfLength = 1

           subpacketLen = 1st_octet



       if the 1st octet >= 192 and < 255, then

           lengthOfLength = 2

           subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192



       if the 1st octet = 255, then

           lengthOfLength = 5

           subpacket length = [four-octet scalar starting at 2nd_octet]



   The value of the subpacket type octet may be:



            0 = Reserved

            1 = Reserved

            2 = Signature Creation Time

            3 = Signature Expiration Time

            4 = Exportable Certification

            5 = Trust Signature

            6 = Regular Expression







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            7 = Revocable

            8 = Reserved

            9 = Key Expiration Time

           10 = Placeholder for backward compatibility

           11 = Preferred Symmetric Algorithms

           12 = Revocation Key

           13 = Reserved

           14 = Reserved

           15 = Reserved

           16 = Issuer

           17 = Reserved

           18 = Reserved

           19 = Reserved

           20 = Notation Data

           21 = Preferred Hash Algorithms

           22 = Preferred Compression Algorithms

           23 = Key Server Preferences

           24 = Preferred Key Server

           25 = Primary User ID

           26 = Policy URI

           27 = Key Flags

           28 = Signer's User ID

           29 = Reason for Revocation

           30 = Features

           31 = Signature Target

           32 = Embedded Signature

   100 To 110 = Private or experimental



   An implementation SHOULD ignore any subpacket of a type that it does

   not recognize.



   Bit 7 of the subpacket type is the "critical" bit.  If set, it

   denotes that the subpacket is one that is critical for the evaluator

   of the signature to recognize.  If a subpacket is encountered that is

   marked critical but is unknown to the evaluating software, the

   evaluator SHOULD consider the signature to be in error.



   An evaluator may "recognize" a subpacket, but not implement it.  The

   purpose of the critical bit is to allow the signer to tell an

   evaluator that it would prefer a new, unknown feature to generate an

   error than be ignored.



   Implementations SHOULD implement the three preferred algorithm

   subpackets (11, 21, and 22), as well as the "Reason for Revocation"

   subpacket.  Note, however, that if an implementation chooses not to

   implement some of the preferences, it is required to behave in a

   polite manner to respect the wishes of those users who do implement

   these preferences.







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5.2.3.2.  Signature Subpacket Types



   A number of subpackets are currently defined.  Some subpackets apply

   to the signature itself and some are attributes of the key.

   Subpackets that are found on a self-signature are placed on a

   certification made by the key itself.  Note that a key may have more

   than one User ID, and thus may have more than one self-signature, and

   differing subpackets.



   A subpacket may be found either in the hashed or unhashed subpacket

   sections of a signature.  If a subpacket is not hashed, then the

   information in it cannot be considered definitive because it is not

   part of the signature proper.



5.2.3.3.  Notes on Self-Signatures



   A self-signature is a binding signature made by the key to which the

   signature refers.  There are three types of self-signatures, the

   certification signatures (types 0x10-0x13), the direct-key signature

   (type 0x1F), and the subkey binding signature (type 0x18).  For

   certification self-signatures, each User ID may have a self-

   signature, and thus different subpackets in those self-signatures.

   For subkey binding signatures, each subkey in fact has a self-

   signature.  Subpackets that appear in a certification self-signature

   apply to the user name, and subpackets that appear in the subkey

   self-signature apply to the subkey.  Lastly, subpackets on the

   direct-key signature apply to the entire key.



   Implementing software should interpret a self-signature's preference

   subpackets as narrowly as possible.  For example, suppose a key has

   two user names, Alice and Bob.  Suppose that Alice prefers the

   symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES.  If the

   software locates this key via Alice's name, then the preferred

   algorithm is CAST5; if software locates the key via Bob's name, then

   the preferred algorithm is IDEA.  If the key is located by Key ID,

   the algorithm of the primary User ID of the key provides the

   preferred symmetric algorithm.



   Revoking a self-signature or allowing it to expire has a semantic

   meaning that varies with the signature type.  Revoking the self-

   signature on a User ID effectively retires that user name.  The

   self-signature is a statement, "My name X is tied to my signing key

   K" and is corroborated by other users' certifications.  If another

   user revokes their certification, they are effectively saying that

   they no longer believe that name and that key are tied together.

   Similarly, if the users themselves revoke their self-signature, then

   the users no longer go by that name, no longer have that email

   address, etc.  Revoking a binding signature effectively retires that







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   subkey.  Revoking a direct-key signature cancels that signature.

   Please see the "Reason for Revocation" subpacket (Section 5.2.3.23)

   for more relevant detail.



   Since a self-signature contains important information about the key's

   use, an implementation SHOULD allow the user to rewrite the self-

   signature, and important information in it, such as preferences and

   key expiration.



   It is good practice to verify that a self-signature imported into an

   implementation doesn't advertise features that the implementation

   doesn't support, rewriting the signature as appropriate.



   An implementation that encounters multiple self-signatures on the

   same object may resolve the ambiguity in any way it sees fit, but it

   is RECOMMENDED that priority be given to the most recent self-

   signature.



5.2.3.4.  Signature Creation Time



   (4-octet time field)



   The time the signature was made.



   MUST be present in the hashed area.



5.2.3.5.  Issuer



   (8-octet Key ID)



   The OpenPGP Key ID of the key issuing the signature.



5.2.3.6.  Key Expiration Time



   (4-octet time field)



   The validity period of the key.  This is the number of seconds after

   the key creation time that the key expires.  If this is not present

   or has a value of zero, the key never expires.  This is found only on

   a self-signature.



5.2.3.7.  Preferred Symmetric Algorithms



   (array of one-octet values)



   Symmetric algorithm numbers that indicate which algorithms the key

   holder prefers to use.  The subpacket body is an ordered list of

   octets with the most preferred listed first.  It is assumed that only







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   algorithms listed are supported by the recipient's software.

   Algorithm numbers are in Section 9.  This is only found on a self-

   signature.



5.2.3.8.  Preferred Hash Algorithms



   (array of one-octet values)



   Message digest algorithm numbers that indicate which algorithms the

   key holder prefers to receive.  Like the preferred symmetric

   algorithms, the list is ordered.  Algorithm numbers are in Section 9.

   This is only found on a self-signature.



5.2.3.9.  Preferred Compression Algorithms



   (array of one-octet values)



   Compression algorithm numbers that indicate which algorithms the key

   holder prefers to use.  Like the preferred symmetric algorithms, the

   list is ordered.  Algorithm numbers are in Section 9.  If this

   subpacket is not included, ZIP is preferred.  A zero denotes that

   uncompressed data is preferred; the key holder's software might have

   no compression software in that implementation.  This is only found

   on a self-signature.



5.2.3.10.  Signature Expiration Time



   (4-octet time field)



   The validity period of the signature.  This is the number of seconds

   after the signature creation time that the signature expires.  If

   this is not present or has a value of zero, it never expires.



5.2.3.11.  Exportable Certification



   (1 octet of exportability, 0 for not, 1 for exportable)



   This subpacket denotes whether a certification signature is

   "exportable", to be used by other users than the signature's issuer.

   The packet body contains a Boolean flag indicating whether the

   signature is exportable.  If this packet is not present, the

   certification is exportable; it is equivalent to a flag containing a

   1.



   Non-exportable, or "local", certifications are signatures made by a

   user to mark a key as valid within that user's implementation only.











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   Thus, when an implementation prepares a user's copy of a key for

   transport to another user (this is the process of "exporting" the

   key), any local certification signatures are deleted from the key.



   The receiver of a transported key "imports" it, and likewise trims

   any local certifications.  In normal operation, there won't be any,

   assuming the import is performed on an exported key.  However, there

   are instances where this can reasonably happen.  For example, if an

   implementation allows keys to be imported from a key database in

   addition to an exported key, then this situation can arise.



   Some implementations do not represent the interest of a single user

   (for example, a key server).  Such implementations always trim local

   certifications from any key they handle.



5.2.3.12.  Revocable



   (1 octet of revocability, 0 for not, 1 for revocable)



   Signature's revocability status.  The packet body contains a Boolean

   flag indicating whether the signature is revocable.  Signatures that

   are not revocable have any later revocation signatures ignored.  They

   represent a commitment by the signer that he cannot revoke his

   signature for the life of his key.  If this packet is not present,

   the signature is revocable.



5.2.3.13.  Trust Signature



   (1 octet "level" (depth), 1 octet of trust amount)



   Signer asserts that the key is not only valid but also trustworthy at

   the specified level.  Level 0 has the same meaning as an ordinary

   validity signature.  Level 1 means that the signed key is asserted to

   be a valid trusted introducer, with the 2nd octet of the body

   specifying the degree of trust.  Level 2 means that the signed key is

   asserted to be trusted to issue level 1 trust signatures, i.e., that

   it is a "meta introducer".  Generally, a level n trust signature

   asserts that a key is trusted to issue level n-1 trust signatures.

   The trust amount is in a range from 0-255, interpreted such that

   values less than 120 indicate partial trust and values of 120 or

   greater indicate complete trust.  Implementations SHOULD emit values

   of 60 for partial trust and 120 for complete trust.



















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5.2.3.14.  Regular Expression



   (null-terminated regular expression)



   Used in conjunction with trust Signature packets (of level > 0) to

   limit the scope of trust that is extended.  Only signatures by the

   target key on User IDs that match the regular expression in the body

   of this packet have trust extended by the trust Signature subpacket.

   The regular expression uses the same syntax as the Henry Spencer's

   "almost public domain" regular expression [REGEX] package.  A

   description of the syntax is found in Section 8 below.



5.2.3.15.  Revocation Key



   (1 octet of class, 1 octet of public-key algorithm ID, 20 octets of

   fingerprint)



   Authorizes the specified key to issue revocation signatures for this

   key.  Class octet must have bit 0x80 set.  If the bit 0x40 is set,

   then this means that the revocation information is sensitive.  Other

   bits are for future expansion to other kinds of authorizations.  This

   is found on a self-signature.



   If the "sensitive" flag is set, the keyholder feels this subpacket

   contains private trust information that describes a real-world

   sensitive relationship.  If this flag is set, implementations SHOULD

   NOT export this signature to other users except in cases where the

   data needs to be available: when the signature is being sent to the

   designated revoker, or when it is accompanied by a revocation

   signature from that revoker.  Note that it may be appropriate to

   isolate this subpacket within a separate signature so that it is not

   combined with other subpackets that need to be exported.



5.2.3.16.  Notation Data



       (4 octets of flags, 2 octets of name length (M),

                           2 octets of value length (N),

                           M octets of name data,

                           N octets of value data)



   This subpacket describes a "notation" on the signature that the

   issuer wishes to make.  The notation has a name and a value, each of

   which are strings of octets.  There may be more than one notation in

   a signature.  Notations can be used for any extension the issuer of

   the signature cares to make.  The "flags" field holds four octets of

   flags.











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   All undefined flags MUST be zero.  Defined flags are as follows:



       First octet: 0x80 = human-readable.  This note value is text.

       Other octets: none.



   Notation names are arbitrary strings encoded in UTF-8.  They reside

   in two namespaces: The IETF namespace and the user namespace.



   The IETF namespace is registered with IANA.  These names MUST NOT

   contain the "@" character (0x40).  This is a tag for the user

   namespace.



   Names in the user namespace consist of a UTF-8 string tag followed by

   "@" followed by a DNS domain name.  Note that the tag MUST NOT

   contain an "@" character.  For example, the "sample" tag used by

   Example Corporation could be "sample@example.com".



   Names in a user space are owned and controlled by the owners of that

   domain.  Obviously, it's bad form to create a new name in a DNS space

   that you don't own.



   Since the user namespace is in the form of an email address,

   implementers MAY wish to arrange for that address to reach a person

   who can be consulted about the use of the named tag.  Note that due

   to UTF-8 encoding, not all valid user space name tags are valid email

   addresses.



   If there is a critical notation, the criticality applies to that

   specific notation and not to notations in general.



5.2.3.17.  Key Server Preferences



   (N octets of flags)



   This is a list of one-bit flags that indicate preferences that the

   key holder has about how the key is handled on a key server.  All

   undefined flags MUST be zero.



   First octet: 0x80 = No-modify

       the key holder requests that this key only be modified or updated

       by the key holder or an administrator of the key server.



   This is found only on a self-signature.

















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5.2.3.18.  Preferred Key Server



   (String)



   This is a URI of a key server that the key holder prefers be used for

   updates.  Note that keys with multiple User IDs can have a preferred

   key server for each User ID.  Note also that since this is a URI, the

   key server can actually be a copy of the key retrieved by ftp, http,

   finger, etc.



5.2.3.19.  Primary User ID



   (1 octet, Boolean)



   This is a flag in a User ID's self-signature that states whether this

   User ID is the main User ID for this key.  It is reasonable for an

   implementation to resolve ambiguities in preferences, etc. by

   referring to the primary User ID.  If this flag is absent, its value

   is zero.  If more than one User ID in a key is marked as primary, the

   implementation may resolve the ambiguity in any way it sees fit, but

   it is RECOMMENDED that priority be given to the User ID with the most

   recent self-signature.



   When appearing on a self-signature on a User ID packet, this

   subpacket applies only to User ID packets.  When appearing on a

   self-signature on a User Attribute packet, this subpacket applies

   only to User Attribute packets.  That is to say, there are two

   different and independent "primaries" -- one for User IDs, and one

   for User Attributes.



5.2.3.20.  Policy URI



   (String)



   This subpacket contains a URI of a document that describes the policy

   under which the signature was issued.



5.2.3.21.  Key Flags



   (N octets of flags)



   This subpacket contains a list of binary flags that hold information

   about a key.  It is a string of octets, and an implementation MUST

   NOT assume a fixed size.  This is so it can grow over time.  If a

   list is shorter than an implementation expects, the unstated flags

   are considered to be zero.  The defined flags are as follows:











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       First octet:



       0x01 - This key may be used to certify other keys.



       0x02 - This key may be used to sign data.



       0x04 - This key may be used to encrypt communications.



       0x08 - This key may be used to encrypt storage.



       0x10 - The private component of this key may have been split

              by a secret-sharing mechanism.



       0x20 - This key may be used for authentication.



       0x80 - The private component of this key may be in the

              possession of more than one person.



   Usage notes:



   The flags in this packet may appear in self-signatures or in

   certification signatures.  They mean different things depending on

   who is making the statement -- for example, a certification signature

   that has the "sign data" flag is stating that the certification is

   for that use.  On the other hand, the "communications encryption"

   flag in a self-signature is stating a preference that a given key be

   used for communications.  Note however, that it is a thorny issue to

   determine what is "communications" and what is "storage".  This

   decision is left wholly up to the implementation; the authors of this

   document do not claim any special wisdom on the issue and realize

   that accepted opinion may change.



   The "split key" (0x10) and "group key" (0x80) flags are placed on a

   self-signature only; they are meaningless on a certification

   signature.  They SHOULD be placed only on a direct-key signature

   (type 0x1F) or a subkey signature (type 0x18), one that refers to the

   key the flag applies to.



5.2.3.22.  Signer's User ID



   (String)



   This subpacket allows a keyholder to state which User ID is

   responsible for the signing.  Many keyholders use a single key for

   different purposes, such as business communications as well as

   personal communications.  This subpacket allows such a keyholder to

   state which of their roles is making a signature.









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   This subpacket is not appropriate to use to refer to a User Attribute

   packet.



5.2.3.23.  Reason for Revocation



   (1 octet of revocation code, N octets of reason string)



   This subpacket is used only in key revocation and certification

   revocation signatures.  It describes the reason why the key or

   certificate was revoked.



   The first octet contains a machine-readable code that denotes the

   reason for the revocation:



        0  - No reason specified (key revocations or cert revocations)

        1  - Key is superseded (key revocations)

        2  - Key material has been compromised (key revocations)

        3  - Key is retired and no longer used (key revocations)

        32 - User ID information is no longer valid (cert revocations)

   100-110 - Private Use



   Following the revocation code is a string of octets that gives

   information about the Reason for Revocation in human-readable form

   (UTF-8).  The string may be null, that is, of zero length.  The

   length of the subpacket is the length of the reason string plus one.

   An implementation SHOULD implement this subpacket, include it in all

   revocation signatures, and interpret revocations appropriately.

   There are important semantic differences between the reasons, and

   there are thus important reasons for revoking signatures.



   If a key has been revoked because of a compromise, all signatures

   created by that key are suspect.  However, if it was merely

   superseded or retired, old signatures are still valid.  If the

   revoked signature is the self-signature for certifying a User ID, a

   revocation denotes that that user name is no longer in use.  Such a

   revocation SHOULD include a 0x20 code.



   Note that any signature may be revoked, including a certification on

   some other person's key.  There are many good reasons for revoking a

   certification signature, such as the case where the keyholder leaves

   the employ of a business with an email address.  A revoked

   certification is no longer a part of validity calculations.



















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5.2.3.24.  Features



   (N octets of flags)



   The Features subpacket denotes which advanced OpenPGP features a

   user's implementation supports.  This is so that as features are

   added to OpenPGP that cannot be backwards-compatible, a user can

   state that they can use that feature.  The flags are single bits that

   indicate that a given feature is supported.



   This subpacket is similar to a preferences subpacket, and only

   appears in a self-signature.



   An implementation SHOULD NOT use a feature listed when sending to a

   user who does not state that they can use it.



   Defined features are as follows:



       First octet:



       0x01 - Modification Detection (packets 18 and 19)



   If an implementation implements any of the defined features, it

   SHOULD implement the Features subpacket, too.



   An implementation may freely infer features from other suitable

   implementation-dependent mechanisms.



5.2.3.25.  Signature Target



   (1 octet public-key algorithm, 1 octet hash algorithm, N octets hash)



   This subpacket identifies a specific target signature to which a

   signature refers.  For revocation signatures, this subpacket

   provides explicit designation of which signature is being revoked.

   For a third-party or timestamp signature, this designates what

   signature is signed.  All arguments are an identifier of that target

   signature.



   The N octets of hash data MUST be the size of the hash of the

   signature.  For example, a target signature with a SHA-1 hash MUST

   have 20 octets of hash data.



















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5.2.3.26.  Embedded Signature



   (1 signature packet body)



   This subpacket contains a complete Signature packet body as

   specified in Section 5.2 above.  It is useful when one signature

   needs to refer to, or be incorporated in, another signature.



5.2.4.  Computing Signatures



   All signatures are formed by producing a hash over the signature

   data, and then using the resulting hash in the signature algorithm.



   For binary document signatures (type 0x00), the document data is

   hashed directly.  For text document signatures (type 0x01), the

   document is canonicalized by converting line endings to <CR><LF>,

   and the resulting data is hashed.



   When a signature is made over a key, the hash data starts with the

   octet 0x99, followed by a two-octet length of the key, and then body

   of the key packet.  (Note that this is an old-style packet header for

   a key packet with two-octet length.)  A subkey binding signature

   (type 0x18) or primary key binding signature (type 0x19) then hashes

   the subkey using the same format as the main key (also using 0x99 as

   the first octet).  Key revocation signatures (types 0x20 and 0x28)

   hash only the key being revoked.



   A certification signature (type 0x10 through 0x13) hashes the User

   ID being bound to the key into the hash context after the above

   data.  A V3 certification hashes the contents of the User ID or

   attribute packet packet, without any header.  A V4 certification

   hashes the constant 0xB4 for User ID certifications or the constant

   0xD1 for User Attribute certifications, followed by a four-octet

   number giving the length of the User ID or User Attribute data, and

   then the User ID or User Attribute data.



   When a signature is made over a Signature packet (type 0x50), the

   hash data starts with the octet 0x88, followed by the four-octet

   length of the signature, and then the body of the Signature packet.

   (Note that this is an old-style packet header for a Signature packet

   with the length-of-length set to zero.)  The unhashed subpacket data

   of the Signature packet being hashed is not included in the hash, and

   the unhashed subpacket data length value is set to zero.



   Once the data body is hashed, then a trailer is hashed.  A V3

   signature hashes five octets of the packet body, starting from the

   signature type field.  This data is the signature type, followed by

   the four-octet signature time.  A V4 signature hashes the packet body







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   starting from its first field, the version number, through the end

   of the hashed subpacket data.  Thus, the fields hashed are the

   signature version, the signature type, the public-key algorithm, the

   hash algorithm, the hashed subpacket length, and the hashed

   subpacket body.



   V4 signatures also hash in a final trailer of six octets: the

   version of the Signature packet, i.e., 0x04; 0xFF; and a four-octet,

   big-endian number that is the length of the hashed data from the

   Signature packet (note that this number does not include these final

   six octets).



   After all this has been hashed in a single hash context, the

   resulting hash field is used in the signature algorithm and placed

   at the end of the Signature packet.



5.2.4.1.  Subpacket Hints



   It is certainly possible for a signature to contain conflicting

   information in subpackets.  For example, a signature may contain

   multiple copies of a preference or multiple expiration times.  In

   most cases, an implementation SHOULD use the last subpacket in the

   signature, but MAY use any conflict resolution scheme that makes

   more sense.  Please note that we are intentionally leaving conflict

   resolution to the implementer; most conflicts are simply syntax

   errors, and the wishy-washy language here allows a receiver to be

   generous in what they accept, while putting pressure on a creator to

   be stingy in what they generate.



   Some apparent conflicts may actually make sense -- for example,

   suppose a keyholder has a V3 key and a V4 key that share the same

   RSA key material.  Either of these keys can verify a signature

   created by the other, and it may be reasonable for a signature to

   contain an issuer subpacket for each key, as a way of explicitly

   tying those keys to the signature.



5.3.  Symmetric-Key Encrypted Session Key Packets (Tag 3)



   The Symmetric-Key Encrypted Session Key packet holds the

   symmetric-key encryption of a session key used to encrypt a message.

   Zero or more Public-Key Encrypted Session Key packets and/or

   Symmetric-Key Encrypted Session Key packets may precede a

   Symmetrically Encrypted Data packet that holds an encrypted message.

   The message is encrypted with a session key, and the session key is

   itself encrypted and stored in the Encrypted Session Key packet or

   the Symmetric-Key Encrypted Session Key packet.











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   If the Symmetrically Encrypted Data packet is preceded by one or

   more Symmetric-Key Encrypted Session Key packets, each specifies a

   passphrase that may be used to decrypt the message.  This allows a

   message to be encrypted to a number of public keys, and also to one

   or more passphrases.  This packet type is new and is not generated

   by PGP 2.x or PGP 5.0.



   The body of this packet consists of:



     - A one-octet version number.  The only currently defined version

       is 4.



     - A one-octet number describing the symmetric algorithm used.



     - A string-to-key (S2K) specifier, length as defined above.



     - Optionally, the encrypted session key itself, which is decrypted

       with the string-to-key object.



   If the encrypted session key is not present (which can be detected

   on the basis of packet length and S2K specifier size), then the S2K

   algorithm applied to the passphrase produces the session key for

   decrypting the file, using the symmetric cipher algorithm from the

   Symmetric-Key Encrypted Session Key packet.



   If the encrypted session key is present, the result of applying the

   S2K algorithm to the passphrase is used to decrypt just that

   encrypted session key field, using CFB mode with an IV of all zeros.

   The decryption result consists of a one-octet algorithm identifier

   that specifies the symmetric-key encryption algorithm used to

   encrypt the following Symmetrically Encrypted Data packet, followed

   by the session key octets themselves.



   Note: because an all-zero IV is used for this decryption, the S2K

   specifier MUST use a salt value, either a Salted S2K or an

   Iterated-Salted S2K.  The salt value will ensure that the decryption

   key is not repeated even if the passphrase is reused.



5.4.  One-Pass Signature Packets (Tag 4)



   The One-Pass Signature packet precedes the signed data and contains

   enough information to allow the receiver to begin calculating any

   hashes needed to verify the signature.  It allows the Signature

   packet to be placed at the end of the message, so that the signer

   can compute the entire signed message in one pass.



   A One-Pass Signature does not interoperate with PGP 2.6.x or

   earlier.







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   The body of this packet consists of:



     - A one-octet version number.  The current version is 3.



     - A one-octet signature type.  Signature types are described in

       Section 5.2.1.



     - A one-octet number describing the hash algorithm used.



     - A one-octet number describing the public-key algorithm used.



     - An eight-octet number holding the Key ID of the signing key.



     - A one-octet number holding a flag showing whether the signature

       is nested.  A zero value indicates that the next packet is

       another One-Pass Signature packet that describes another

       signature to be applied to the same message data.



   Note that if a message contains more than one one-pass signature,

   then the Signature packets bracket the message; that is, the first

   Signature packet after the message corresponds to the last one-pass

   packet and the final Signature packet corresponds to the first

   one-pass packet.



5.5.  Key Material Packet



   A key material packet contains all the information about a public or

   private key.  There are four variants of this packet type, and two

   major versions.  Consequently, this section is complex.



5.5.1.  Key Packet Variants



5.5.1.1.  Public-Key Packet (Tag 6)



   A Public-Key packet starts a series of packets that forms an OpenPGP

   key (sometimes called an OpenPGP certificate).



5.5.1.2.  Public-Subkey Packet (Tag 14)



   A Public-Subkey packet (tag 14) has exactly the same format as a

   Public-Key packet, but denotes a subkey.  One or more subkeys may be

   associated with a top-level key.  By convention, the top-level key

   provides signature services, and the subkeys provide encryption

   services.



   Note: in PGP 2.6.x, tag 14 was intended to indicate a comment

   packet.  This tag was selected for reuse because no previous version

   of PGP ever emitted comment packets but they did properly ignore







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   them.  Public-Subkey packets are ignored by PGP 2.6.x and do not

   cause it to fail, providing a limited degree of backward

   compatibility.



5.5.1.3.  Secret-Key Packet (Tag 5)



   A Secret-Key packet contains all the information that is found in a

   Public-Key packet, including the public-key material, but also

   includes the secret-key material after all the public-key fields.



5.5.1.4.  Secret-Subkey Packet (Tag 7)



   A Secret-Subkey packet (tag 7) is the subkey analog of the Secret

   Key packet and has exactly the same format.



5.5.2.  Public-Key Packet Formats



   There are two versions of key-material packets.  Version 3 packets

   were first generated by PGP 2.6.  Version 4 keys first appeared in

   PGP 5.0 and are the preferred key version for OpenPGP.



   OpenPGP implementations MUST create keys with version 4 format.  V3

   keys are deprecated; an implementation MUST NOT generate a V3 key,

   but MAY accept it.



   A version 3 public key or public-subkey packet contains:



     - A one-octet version number (3).



     - A four-octet number denoting the time that the key was created.



     - A two-octet number denoting the time in days that this key is

       valid.  If this number is zero, then it does not expire.



     - A one-octet number denoting the public-key algorithm of this key.



     - A series of multiprecision integers comprising the key material:



           - a multiprecision integer (MPI) of RSA public modulus n;



           - an MPI of RSA public encryption exponent e.



   V3 keys are deprecated.  They contain three weaknesses.  First, it is

   relatively easy to construct a V3 key that has the same Key ID as any

   other key because the Key ID is simply the low 64 bits of the public

   modulus.  Secondly, because the fingerprint of a V3 key hashes the

   key material, but not its length, there is an increased opportunity

   for fingerprint collisions.  Third, there are weaknesses in the MD5







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   hash algorithm that make developers prefer other algorithms.  See

   below for a fuller discussion of Key IDs and fingerprints.



   V2 keys are identical to the deprecated V3 keys except for the

   version number.  An implementation MUST NOT generate them and MAY

   accept or reject them as it sees fit.



   The version 4 format is similar to the version 3 format except for

   the absence of a validity period.  This has been moved to the

   Signature packet.  In addition, fingerprints of version 4 keys are

   calculated differently from version 3 keys, as described in the

   section "Enhanced Key Formats".



   A version 4 packet contains:



     - A one-octet version number (4).



     - A four-octet number denoting the time that the key was created.



     - A one-octet number denoting the public-key algorithm of this key.



     - A series of multiprecision integers comprising the key material.

       This algorithm-specific portion is:



       Algorithm-Specific Fields for RSA public keys:



         - multiprecision integer (MPI) of RSA public modulus n;



         - MPI of RSA public encryption exponent e.



       Algorithm-Specific Fields for DSA public keys:



         - MPI of DSA prime p;



         - MPI of DSA group order q (q is a prime divisor of p-1);



         - MPI of DSA group generator g;



         - MPI of DSA public-key value y (= g**x mod p where x

           is secret).



       Algorithm-Specific Fields for Elgamal public keys:



         - MPI of Elgamal prime p;



         - MPI of Elgamal group generator g;











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         - MPI of Elgamal public key value y (= g**x mod p where x

           is secret).



5.5.3.  Secret-Key Packet Formats



   The Secret-Key and Secret-Subkey packets contain all the data of the

   Public-Key and Public-Subkey packets, with additional algorithm-

   specific secret-key data appended, usually in encrypted form.



   The packet contains:



     - A Public-Key or Public-Subkey packet, as described above.



     - One octet indicating string-to-key usage conventions.  Zero

       indicates that the secret-key data is not encrypted.  255 or 254

       indicates that a string-to-key specifier is being given.  Any

       other value is a symmetric-key encryption algorithm identifier.



     - [Optional] If string-to-key usage octet was 255 or 254, a one-

       octet symmetric encryption algorithm.



     - [Optional] If string-to-key usage octet was 255 or 254, a

       string-to-key specifier.  The length of the string-to-key

       specifier is implied by its type, as described above.



     - [Optional] If secret data is encrypted (string-to-key usage octet

       not zero), an Initial Vector (IV) of the same length as the

       cipher's block size.



     - Plain or encrypted multiprecision integers comprising the secret

       key data.  These algorithm-specific fields are as described

       below.



     - If the string-to-key usage octet is zero or 255, then a two-octet

       checksum of the plaintext of the algorithm-specific portion (sum

       of all octets, mod 65536).  If the string-to-key usage octet was

       254, then a 20-octet SHA-1 hash of the plaintext of the

       algorithm-specific portion.  This checksum or hash is encrypted

       together with the algorithm-specific fields (if string-to-key

       usage octet is not zero).  Note that for all other values, a

       two-octet checksum is required.



       Algorithm-Specific Fields for RSA secret keys:



       - multiprecision integer (MPI) of RSA secret exponent d.



       - MPI of RSA secret prime value p.









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       - MPI of RSA secret prime value q (p < q).



       - MPI of u, the multiplicative inverse of p, mod q.



       Algorithm-Specific Fields for DSA secret keys:



       - MPI of DSA secret exponent x.



       Algorithm-Specific Fields for Elgamal secret keys:



       - MPI of Elgamal secret exponent x.



   Secret MPI values can be encrypted using a passphrase.  If a string-

   to-key specifier is given, that describes the algorithm for

   converting the passphrase to a key, else a simple MD5 hash of the

   passphrase is used.  Implementations MUST use a string-to-key

   specifier; the simple hash is for backward compatibility and is

   deprecated, though implementations MAY continue to use existing

   private keys in the old format.  The cipher for encrypting the MPIs

   is specified in the Secret-Key packet.



   Encryption/decryption of the secret data is done in CFB mode using

   the key created from the passphrase and the Initial Vector from the

   packet.  A different mode is used with V3 keys (which are only RSA)

   than with other key formats.  With V3 keys, the MPI bit count prefix

   (i.e., the first two octets) is not encrypted.  Only the MPI non-

   prefix data is encrypted.  Furthermore, the CFB state is

   resynchronized at the beginning of each new MPI value, so that the

   CFB block boundary is aligned with the start of the MPI data.



   With V4 keys, a simpler method is used.  All secret MPI values are

   encrypted in CFB mode, including the MPI bitcount prefix.



   The two-octet checksum that follows the algorithm-specific portion is

   the algebraic sum, mod 65536, of the plaintext of all the algorithm-

   specific octets (including MPI prefix and data).  With V3 keys, the

   checksum is stored in the clear.  With V4 keys, the checksum is

   encrypted like the algorithm-specific data.  This value is used to

   check that the passphrase was correct.  However, this checksum is

   deprecated; an implementation SHOULD NOT use it, but should rather

   use the SHA-1 hash denoted with a usage octet of 254.  The reason for

   this is that there are some attacks that involve undetectably

   modifying the secret key.

















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5.6.  Compressed Data Packet (Tag 8)



   The Compressed Data packet contains compressed data.  Typically, this

   packet is found as the contents of an encrypted packet, or following

   a Signature or One-Pass Signature packet, and contains a literal data

   packet.



   The body of this packet consists of:



     - One octet that gives the algorithm used to compress the packet.



     - Compressed data, which makes up the remainder of the packet.



   A Compressed Data Packet's body contains an block that compresses

   some set of packets.  See section "Packet Composition" for details on

   how messages are formed.



   ZIP-compressed packets are compressed with raw RFC 1951 [RFC1951]

   DEFLATE blocks.  Note that PGP V2.6 uses 13 bits of compression.  If

   an implementation uses more bits of compression, PGP V2.6 cannot

   decompress it.



   ZLIB-compressed packets are compressed with RFC 1950 [RFC1950] ZLIB-

   style blocks.



   BZip2-compressed packets are compressed using the BZip2 [BZ2]

   algorithm.



5.7.  Symmetrically Encrypted Data Packet (Tag 9)



   The Symmetrically Encrypted Data packet contains data encrypted with

   a symmetric-key algorithm.  When it has been decrypted, it contains

   other packets (usually a literal data packet or compressed data

   packet, but in theory other Symmetrically Encrypted Data packets or

   sequences of packets that form whole OpenPGP messages).



   The body of this packet consists of:



     - Encrypted data, the output of the selected symmetric-key cipher

       operating in OpenPGP's variant of Cipher Feedback (CFB) mode.



   The symmetric cipher used may be specified in a Public-Key or

   Symmetric-Key Encrypted Session Key packet that precedes the

   Symmetrically Encrypted Data packet.  In that case, the cipher

   algorithm octet is prefixed to the session key before it is

   encrypted.  If no packets of these types precede the encrypted data,

   the IDEA algorithm is used with the session key calculated as the MD5

   hash of the passphrase, though this use is deprecated.







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   The data is encrypted in CFB mode, with a CFB shift size equal to the

   cipher's block size.  The Initial Vector (IV) is specified as all

   zeros.  Instead of using an IV, OpenPGP prefixes a string of length

   equal to the block size of the cipher plus two to the data before it

   is encrypted.  The first block-size octets (for example, 8 octets for

   a 64-bit block length) are random, and the following two octets are

   copies of the last two octets of the IV.  For example, in an 8-octet

   block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of

   octet 8.  In a cipher of length 16, octet 17 is a repeat of octet 15

   and octet 18 is a repeat of octet 16.  As a pedantic clarification,

   in both these examples, we consider the first octet to be numbered 1.



   After encrypting the first block-size-plus-two octets, the CFB state

   is resynchronized.  The last block-size octets of ciphertext are

   passed through the cipher and the block boundary is reset.



   The repetition of 16 bits in the random data prefixed to the message

   allows the receiver to immediately check whether the session key is

   incorrect.  See the "Security Considerations" section for hints on

   the proper use of this "quick check".



5.8.  Marker Packet (Obsolete Literal Packet) (Tag 10)



   An experimental version of PGP used this packet as the Literal

   packet, but no released version of PGP generated Literal packets with

   this tag.  With PGP 5.x, this packet has been reassigned and is

   reserved for use as the Marker packet.



   The body of this packet consists of:



     - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).



   Such a packet MUST be ignored when received.  It may be placed at the

   beginning of a message that uses features not available in PGP 2.6.x

   in order to cause that version to report that newer software is

   necessary to process the message.



5.9.  Literal Data Packet (Tag 11)



   A Literal Data packet contains the body of a message; data that is

   not to be further interpreted.



   The body of this packet consists of:



     - A one-octet field that describes how the data is formatted.













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   If it is a 'b' (0x62), then the Literal packet contains binary data.

   If it is a 't' (0x74), then it contains text data, and thus may need

   line ends converted to local form, or other text-mode changes.  The

   tag 'u' (0x75) means the same as 't', but also indicates that

   implementation believes that the literal data contains UTF-8 text.



   Early versions of PGP also defined a value of 'l' as a 'local' mode

   for machine-local conversions.  RFC 1991 [RFC1991] incorrectly stated

   this local mode flag as '1' (ASCII numeral one).  Both of these local

   modes are deprecated.



     - File name as a string (one-octet length, followed by a file

       name).  This may be a zero-length string.  Commonly, if the

       source of the encrypted data is a file, this will be the name of

       the encrypted file.  An implementation MAY consider the file name

       in the Literal packet to be a more authoritative name than the

       actual file name.



   If the special name "_CONSOLE" is used, the message is considered to

   be "for your eyes only".  This advises that the message data is

   unusually sensitive, and the receiving program should process it more

   carefully, perhaps avoiding storing the received data to disk, for

   example.



     - A four-octet number that indicates a date associated with the

       literal data.  Commonly, the date might be the modification date

       of a file, or the time the packet was created, or a zero that

       indicates no specific time.



     - The remainder of the packet is literal data.



       Text data is stored with <CR><LF> text endings (i.e., network-

       normal line endings).  These should be converted to native line

       endings by the receiving software.



5.10.  Trust Packet (Tag 12)



   The Trust packet is used only within keyrings and is not normally

   exported.  Trust packets contain data that record the user's

   specifications of which key holders are trustworthy introducers,

   along with other information that implementing software uses for

   trust information.  The format of Trust packets is defined by a given

   implementation.



   Trust packets SHOULD NOT be emitted to output streams that are

   transferred to other users, and they SHOULD be ignored on any input

   other than local keyring files.









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5.11.  User ID Packet (Tag 13)



   A User ID packet consists of UTF-8 text that is intended to represent

   the name and email address of the key holder.  By convention, it

   includes an RFC 2822 [RFC2822] mail name-addr, but there are no

   restrictions on its content.  The packet length in the header

   specifies the length of the User ID.



5.12.  User Attribute Packet (Tag 17)



   The User Attribute packet is a variation of the User ID packet.  It

   is capable of storing more types of data than the User ID packet,

   which is limited to text.  Like the User ID packet, a User Attribute

   packet may be certified by the key owner ("self-signed") or any other

   key owner who cares to certify it.  Except as noted, a User Attribute

   packet may be used anywhere that a User ID packet may be used.



   While User Attribute packets are not a required part of the OpenPGP

   standard, implementations SHOULD provide at least enough

   compatibility to properly handle a certification signature on the

   User Attribute packet.  A simple way to do this is by treating the

   User Attribute packet as a User ID packet with opaque contents, but

   an implementation may use any method desired.



   The User Attribute packet is made up of one or more attribute

   subpackets.  Each subpacket consists of a subpacket header and a

   body.  The header consists of:



     - the subpacket length (1, 2, or 5 octets)



     - the subpacket type (1 octet)



   and is followed by the subpacket specific data.



   The only currently defined subpacket type is 1, signifying an image.

   An implementation SHOULD ignore any subpacket of a type that it does

   not recognize.  Subpacket types 100 through 110 are reserved for

   private or experimental use.



5.12.1.  The Image Attribute Subpacket



   The Image Attribute subpacket is used to encode an image, presumably

   (but not required to be) that of the key owner.



   The Image Attribute subpacket begins with an image header.  The first

   two octets of the image header contain the length of the image

   header.  Note that unlike other multi-octet numerical values in this

   document, due to a historical accident this value is encoded as a







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   little-endian number.  The image header length is followed by a

   single octet for the image header version.  The only currently

   defined version of the image header is 1, which is a 16-octet image

   header.  The first three octets of a version 1 image header are thus

   0x10, 0x00, 0x01.



   The fourth octet of a version 1 image header designates the encoding

   format of the image.  The only currently defined encoding format is

   the value 1 to indicate JPEG.  Image format types 100 through 110 are

   reserved for private or experimental use.  The rest of the version 1

   image header is made up of 12 reserved octets, all of which MUST be

   set to 0.



   The rest of the image subpacket contains the image itself.  As the

   only currently defined image type is JPEG, the image is encoded in

   the JPEG File Interchange Format (JFIF), a standard file format for

   JPEG images [JFIF].



   An implementation MAY try to determine the type of an image by

   examination of the image data if it is unable to handle a particular

   version of the image header or if a specified encoding format value

   is not recognized.



5.13.  Sym. Encrypted Integrity Protected Data Packet (Tag 18)



   The Symmetrically Encrypted Integrity Protected Data packet is a

   variant of the Symmetrically Encrypted Data packet.  It is a new

   feature created for OpenPGP that addresses the problem of detecting a

   modification to encrypted data.  It is used in combination with a

   Modification Detection Code packet.



   There is a corresponding feature in the features Signature subpacket

   that denotes that an implementation can properly use this packet

   type.  An implementation MUST support decrypting these packets and

   SHOULD prefer generating them to the older Symmetrically Encrypted

   Data packet when possible.  Since this data packet protects against

   modification attacks, this standard encourages its proliferation.

   While blanket adoption of this data packet would create

   interoperability problems, rapid adoption is nevertheless important.

   An implementation SHOULD specifically denote support for this packet,

   but it MAY infer it from other mechanisms.



   For example, an implementation might infer from the use of a cipher

   such as Advanced Encryption Standard (AES) or Twofish that a user

   supports this feature.  It might place in the unhashed portion of

   another user's key signature a Features subpacket.  It might also

   present a user with an opportunity to regenerate their own self-

   signature with a Features subpacket.







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   This packet contains data encrypted with a symmetric-key algorithm

   and protected against modification by the SHA-1 hash algorithm.  When

   it has been decrypted, it will typically contain other packets (often

   a Literal Data packet or Compressed Data packet).  The last decrypted

   packet in this packet's payload MUST be a Modification Detection Code

   packet.



   The body of this packet consists of:



     - A one-octet version number.  The only currently defined value is

       1.



     - Encrypted data, the output of the selected symmetric-key cipher

       operating in Cipher Feedback mode with shift amount equal to the

       block size of the cipher (CFB-n where n is the block size).



   The symmetric cipher used MUST be specified in a Public-Key or

   Symmetric-Key Encrypted Session Key packet that precedes the

   Symmetrically Encrypted Data packet.  In either case, the cipher

   algorithm octet is prefixed to the session key before it is

   encrypted.



   The data is encrypted in CFB mode, with a CFB shift size equal to the

   cipher's block size.  The Initial Vector (IV) is specified as all

   zeros.  Instead of using an IV, OpenPGP prefixes an octet string to

   the data before it is encrypted.  The length of the octet string

   equals the block size of the cipher in octets, plus two.  The first

   octets in the group, of length equal to the block size of the cipher,

   are random; the last two octets are each copies of their 2nd

   preceding octet.  For example, with a cipher whose block size is 128

   bits or 16 octets, the prefix data will contain 16 random octets,

   then two more octets, which are copies of the 15th and 16th octets,

   respectively.  Unlike the Symmetrically Encrypted Data Packet, no

   special CFB resynchronization is done after encrypting this prefix

   data.  See "OpenPGP CFB Mode" below for more details.



   The repetition of 16 bits in the random data prefixed to the message

   allows the receiver to immediately check whether the session key is

   incorrect.



   The plaintext of the data to be encrypted is passed through the SHA-1

   hash function, and the result of the hash is appended to the

   plaintext in a Modification Detection Code packet.  The input to the

   hash function includes the prefix data described above; it includes

   all of the plaintext, and then also includes two octets of values

   0xD3, 0x14.  These represent the encoding of a Modification Detection

   Code packet tag and length field of 20 octets.









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   The resulting hash value is stored in a Modification Detection Code

   (MDC) packet, which MUST use the two octet encoding just given to

   represent its tag and length field.  The body of the MDC packet is

   the 20-octet output of the SHA-1 hash.



   The Modification Detection Code packet is appended to the plaintext

   and encrypted along with the plaintext using the same CFB context.



   During decryption, the plaintext data should be hashed with SHA-1,

   including the prefix data as well as the packet tag and length field

   of the Modification Detection Code packet.  The body of the MDC

   packet, upon decryption, is compared with the result of the SHA-1

   hash.



   Any failure of the MDC indicates that the message has been modified

   and MUST be treated as a security problem.  Failures include a

   difference in the hash values, but also the absence of an MDC packet,

   or an MDC packet in any position other than the end of the plaintext.

   Any failure SHOULD be reported to the user.



   Note: future designs of new versions of this packet should consider

   rollback attacks since it will be possible for an attacker to change

   the version back to 1.



      NON-NORMATIVE EXPLANATION



      The MDC system, as packets 18 and 19 are called, were created to

      provide an integrity mechanism that is less strong than a

      signature, yet stronger than bare CFB encryption.



      It is a limitation of CFB encryption that damage to the ciphertext

      will corrupt the affected cipher blocks and the block following.

      Additionally, if data is removed from the end of a CFB-encrypted

      block, that removal is undetectable.  (Note also that CBC mode has

      a similar limitation, but data removed from the front of the block

      is undetectable.)



      The obvious way to protect or authenticate an encrypted block is

      to digitally sign it.  However, many people do not wish to

      habitually sign data, for a large number of reasons beyond the

      scope of this document.  Suffice it to say that many people

      consider properties such as deniability to be as valuable as

      integrity.



      OpenPGP addresses this desire to have more security than raw

      encryption and yet preserve deniability with the MDC system.  An

      MDC is intentionally not a MAC.  Its name was not selected by

      accident.  It is analogous to a checksum.







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      Despite the fact that it is a relatively modest system, it has

      proved itself in the real world.  It is an effective defense to

      several attacks that have surfaced since it has been created.  It

      has met its modest goals admirably.



      Consequently, because it is a modest security system, it has

      modest requirements on the hash function(s) it employs.  It does

      not rely on a hash function being collision-free, it relies on a

      hash function being one-way.  If a forger, Frank, wishes to send

      Alice a (digitally) unsigned message that says, "I've always

      secretly loved you, signed Bob", it is far easier for him to

      construct a new message than it is to modify anything intercepted

      from Bob.  (Note also that if Bob wishes to communicate secretly

      with Alice, but without authentication or identification and with

      a threat model that includes forgers, he has a problem that

      transcends mere cryptography.)



      Note also that unlike nearly every other OpenPGP subsystem, there

      are no parameters in the MDC system.  It hard-defines SHA-1 as its

      hash function.  This is not an accident.  It is an intentional

      choice to avoid downgrade and cross-grade attacks while making a

      simple, fast system.  (A downgrade attack would be an attack that

      replaced SHA-256 with SHA-1, for example.  A cross-grade attack

      would replace SHA-1 with another 160-bit hash, such as RIPE-

      MD/160, for example.)



      However, given the present state of hash function cryptanalysis

      and cryptography, it may be desirable to upgrade the MDC system to

      a new hash function.  See Section 13.11 in the "IANA

      Considerations" for guidance.



5.14.  Modification Detection Code Packet (Tag 19)



   The Modification Detection Code packet contains a SHA-1 hash of

   plaintext data, which is used to detect message modification.  It is

   only used with a Symmetrically Encrypted Integrity Protected Data

   packet.  The Modification Detection Code packet MUST be the last

   packet in the plaintext data that is encrypted in the Symmetrically

   Encrypted Integrity Protected Data packet, and MUST appear in no

   other place.



   A Modification Detection Code packet MUST have a length of 20 octets.



















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   The body of this packet consists of:



     - A 20-octet SHA-1 hash of the preceding plaintext data of the

       Symmetrically Encrypted Integrity Protected Data packet,

       including prefix data, the tag octet, and length octet of the

       Modification Detection Code packet.



   Note that the Modification Detection Code packet MUST always use a

   new format encoding of the packet tag, and a one-octet encoding of

   the packet length.  The reason for this is that the hashing rules for

   modification detection include a one-octet tag and one-octet length

   in the data hash.  While this is a bit restrictive, it reduces

   complexity.



6.  Radix-64 Conversions



   As stated in the introduction, OpenPGP's underlying native

   representation for objects is a stream of arbitrary octets, and some

   systems desire these objects to be immune to damage caused by

   character set translation, data conversions, etc.



   In principle, any printable encoding scheme that met the requirements

   of the unsafe channel would suffice, since it would not change the

   underlying binary bit streams of the native OpenPGP data structures.

   The OpenPGP standard specifies one such printable encoding scheme to

   ensure interoperability.



   OpenPGP's Radix-64 encoding is composed of two parts: a base64

   encoding of the binary data and a checksum.  The base64 encoding is

   identical to the MIME base64 content-transfer-encoding [RFC2045].



   The checksum is a 24-bit Cyclic Redundancy Check (CRC) converted to

   four characters of radix-64 encoding by the same MIME base64

   transformation, preceded by an equal sign (=).  The CRC is computed

   by using the generator 0x864CFB and an initialization of 0xB704CE.

   The accumulation is done on the data before it is converted to

   radix-64, rather than on the converted data.  A sample implementation

   of this algorithm is in the next section.



   The checksum with its leading equal sign MAY appear on the first line

   after the base64 encoded data.



   Rationale for CRC-24: The size of 24 bits fits evenly into printable

   base64.  The nonzero initialization can detect more errors than a

   zero initialization.













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6.1.  An Implementation of the CRC-24 in "C"



      #define CRC24_INIT 0xB704CEL

      #define CRC24_POLY 0x1864CFBL



      typedef long crc24;

      crc24 crc_octets(unsigned char *octets, size_t len)

      {

          crc24 crc = CRC24_INIT;

          int i;

          while (len--) {

              crc ^= (*octets++) << 16;

              for (i = 0; i < 8; i++) {

                  crc <<= 1;

                  if (crc & 0x1000000)

                      crc ^= CRC24_POLY;

              }

          }

          return crc & 0xFFFFFFL;

      }



6.2.  Forming ASCII Armor



   When OpenPGP encodes data into ASCII Armor, it puts specific headers

   around the Radix-64 encoded data, so OpenPGP can reconstruct the data

   later.  An OpenPGP implementation MAY use ASCII armor to protect raw

   binary data.  OpenPGP informs the user what kind of data is encoded

   in the ASCII armor through the use of the headers.



   Concatenating the following data creates ASCII Armor:



     - An Armor Header Line, appropriate for the type of data



     - Armor Headers



     - A blank (zero-length, or containing only whitespace) line



     - The ASCII-Armored data



     - An Armor Checksum



     - The Armor Tail, which depends on the Armor Header Line



   An Armor Header Line consists of the appropriate header line text

   surrounded by five (5) dashes ('-', 0x2D) on either side of the

   header line text.  The header line text is chosen based upon the type

   of data that is being encoded in Armor, and how it is being encoded.

   Header line texts include the following strings:







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   BEGIN PGP MESSAGE

       Used for signed, encrypted, or compressed files.



   BEGIN PGP PUBLIC KEY BLOCK

       Used for armoring public keys.



   BEGIN PGP PRIVATE KEY BLOCK

       Used for armoring private keys.



   BEGIN PGP MESSAGE, PART X/Y

       Used for multi-part messages, where the armor is split amongst Y

       parts, and this is the Xth part out of Y.



   BEGIN PGP MESSAGE, PART X

       Used for multi-part messages, where this is the Xth part of an

       unspecified number of parts.  Requires the MESSAGE-ID Armor

       Header to be used.



   BEGIN PGP SIGNATURE

       Used for detached signatures, OpenPGP/MIME signatures, and

       cleartext signatures.  Note that PGP 2.x uses BEGIN PGP MESSAGE

       for detached signatures.



   Note that all these Armor Header Lines are to consist of a complete

   line.  That is to say, there is always a line ending preceding the

   starting five dashes, and following the ending five dashes.  The

   header lines, therefore, MUST start at the beginning of a line, and

   MUST NOT have text other than whitespace following them on the same

   line.  These line endings are considered a part of the Armor Header

   Line for the purposes of determining the content they delimit.  This

   is particularly important when computing a cleartext signature (see

   below).



   The Armor Headers are pairs of strings that can give the user or the

   receiving OpenPGP implementation some information about how to decode

   or use the message.  The Armor Headers are a part of the armor, not a

   part of the message, and hence are not protected by any signatures

   applied to the message.



   The format of an Armor Header is that of a key-value pair.  A colon

   (':' 0x38) and a single space (0x20) separate the key and value.

   OpenPGP should consider improperly formatted Armor Headers to be

   corruption of the ASCII Armor.  Unknown keys should be reported to

   the user, but OpenPGP should continue to process the message.



   Note that some transport methods are sensitive to line length.  While

   there is a limit of 76 characters for the Radix-64 data (Section

   6.3), there is no limit to the length of Armor Headers.  Care should







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   be taken that the Armor Headers are short enough to survive

   transport.  One way to do this is to repeat an Armor Header key

   multiple times with different values for each so that no one line is

   overly long.



   Currently defined Armor Header Keys are as follows:



     - "Version", which states the OpenPGP implementation and version

       used to encode the message.



     - "Comment", a user-defined comment.  OpenPGP defines all text to

       be in UTF-8.  A comment may be any UTF-8 string.  However, the

       whole point of armoring is to provide seven-bit-clean data.

       Consequently, if a comment has characters that are outside the

       US-ASCII range of UTF, they may very well not survive transport.



     - "MessageID", a 32-character string of printable characters.  The

       string must be the same for all parts of a multi-part message

       that uses the "PART X" Armor Header.  MessageID strings should be

       unique enough that the recipient of the mail can associate all

       the parts of a message with each other.  A good checksum or

       cryptographic hash function is sufficient.



       The MessageID SHOULD NOT appear unless it is in a multi-part

       message.  If it appears at all, it MUST be computed from the

       finished (encrypted, signed, etc.) message in a deterministic

       fashion, rather than contain a purely random value.  This is to

       allow the legitimate recipient to determine that the MessageID

       cannot serve as a covert means of leaking cryptographic key

       information.



     - "Hash", a comma-separated list of hash algorithms used in this

       message.  This is used only in cleartext signed messages.



     - "Charset", a description of the character set that the plaintext

       is in.  Please note that OpenPGP defines text to be in UTF-8.  An

       implementation will get best results by translating into and out

       of UTF-8.  However, there are many instances where this is easier

       said than done.  Also, there are communities of users who have no

       need for UTF-8 because they are all happy with a character set

       like ISO Latin-5 or a Japanese character set.  In such instances,

       an implementation MAY override the UTF-8 default by using this

       header key.  An implementation MAY implement this key and any

       translations it cares to; an implementation MAY ignore it and

       assume all text is UTF-8.













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       The Armor Tail Line is composed in the same manner as the Armor

       Header Line, except the string "BEGIN" is replaced by the string

       "END".



6.3.  Encoding Binary in Radix-64



   The encoding process represents 24-bit groups of input bits as output

   strings of 4 encoded characters.  Proceeding from left to right, a

   24-bit input group is formed by concatenating three 8-bit input

   groups.  These 24 bits are then treated as four concatenated 6-bit

   groups, each of which is translated into a single digit in the

   Radix-64 alphabet.  When encoding a bit stream with the Radix-64

   encoding, the bit stream must be presumed to be ordered with the most

   significant bit first.  That is, the first bit in the stream will be

   the high-order bit in the first 8-bit octet, and the eighth bit will

   be the low-order bit in the first 8-bit octet, and so on.



         +--first octet--+-second octet--+--third octet--+

         |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|

         +-----------+---+-------+-------+---+-----------+

         |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|

         +--1.index--+--2.index--+--3.index--+--4.index--+



   Each 6-bit group is used as an index into an array of 64 printable

   characters from the table below.  The character referenced by the

   index is placed in the output string.



     Value Encoding  Value Encoding  Value Encoding  Value Encoding

         0 A            17 R            34 i            51 z

         1 B            18 S            35 j            52 0

         2 C            19 T            36 k            53 1

         3 D            20 U            37 l            54 2

         4 E            21 V            38 m            55 3

         5 F            22 W            39 n            56 4

         6 G            23 X            40 o            57 5

         7 H            24 Y            41 p            58 6

         8 I            25 Z            42 q            59 7

         9 J            26 a            43 r            60 8

        10 K            27 b            44 s            61 9

        11 L            28 c            45 t            62 +

        12 M            29 d            46 u            63 /

        13 N            30 e            47 v

        14 O            31 f            48 w         (pad) =

        15 P            32 g            49 x

        16 Q            33 h            50 y



   The encoded output stream must be represented in lines of no more

   than 76 characters each.







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   Special processing is performed if fewer than 24 bits are available

   at the end of the data being encoded.  There are three possibilities:



   1. The last data group has 24 bits (3 octets).  No special processing

      is needed.



   2. The last data group has 16 bits (2 octets).  The first two 6-bit

      groups are processed as above.  The third (incomplete) data group

      has two zero-value bits added to it, and is processed as above.  A

      pad character (=) is added to the output.



   3. The last data group has 8 bits (1 octet).  The first 6-bit group

      is processed as above.  The second (incomplete) data group has

      four zero-value bits added to it, and is processed as above.  Two

      pad characters (=) are added to the output.



6.4.  Decoding Radix-64



   In Radix-64 data, characters other than those in the table, line

   breaks, and other white space probably indicate a transmission error,

   about which a warning message or even a message rejection might be

   appropriate under some circumstances.  Decoding software must ignore

   all white space.



   Because it is used only for padding at the end of the data, the

   occurrence of any "=" characters may be taken as evidence that the

   end of the data has been reached (without truncation in transit).  No

   such assurance is possible, however, when the number of octets

   transmitted was a multiple of three and no "=" characters are

   present.











































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6.5.  Examples of Radix-64



   Input data:  0x14FB9C03D97E

   Hex:     1   4    F   B    9   C     | 0   3    D   9    7   E

   8-bit:   00010100 11111011 10011100  | 00000011 11011001 11111110

   6-bit:   000101 001111 101110 011100 | 000000 111101 100111 111110

   Decimal: 5      15     46     28       0      61     37     62

   Output:  F      P      u      c        A      9      l      +

   Input data:  0x14FB9C03D9

   Hex:     1   4    F   B    9   C     | 0   3    D   9

   8-bit:   00010100 11111011 10011100  | 00000011 11011001

                                                   pad with 00

   6-bit:   000101 001111 101110 011100 | 000000 111101 100100

   Decimal: 5      15     46     28       0      61     36

                                                      pad with =

   Output:  F      P      u      c        A      9      k      =

   Input data:  0x14FB9C03

   Hex:     1   4    F   B    9   C     | 0   3

   8-bit:   00010100 11111011 10011100  | 00000011

                                          pad with 0000

   6-bit:   000101 001111 101110 011100 | 000000 110000

   Decimal: 5      15     46     28       0      48

                                               pad with =      =

   Output:  F      P      u      c        A      w      =      =



6.6.  Example of an ASCII Armored Message



   -----BEGIN PGP MESSAGE-----

   Version: OpenPrivacy 0.99



   yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS

   vBSFjNSiVHsuAA==

   =njUN

   -----END PGP MESSAGE-----



   Note that this example has extra indenting; an actual armored message

   would have no leading whitespace.



7.  Cleartext Signature Framework



   It is desirable to be able to sign a textual octet stream without

   ASCII armoring the stream itself, so the signed text is still

   readable without special software.  In order to bind a signature to

   such a cleartext, this framework is used.  (Note that this framework

   is not intended to be reversible.  RFC 3156 [RFC3156] defines another

   way to sign cleartext messages for environments that support MIME.)











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   The cleartext signed message consists of:



     - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a

       single line,



     - One or more "Hash" Armor Headers,



     - Exactly one empty line not included into the message digest,



     - The dash-escaped cleartext that is included into the message

       digest,



     - The ASCII armored signature(s) including the '-----BEGIN PGP

       SIGNATURE-----' Armor Header and Armor Tail Lines.



   If the "Hash" Armor Header is given, the specified message digest

   algorithm(s) are used for the signature.  If there are no such

   headers, MD5 is used.  If MD5 is the only hash used, then an

   implementation MAY omit this header for improved V2.x compatibility.

   If more than one message digest is used in the signature, the "Hash"

   armor header contains a comma-delimited list of used message digests.



   Current message digest names are described below with the algorithm

   IDs.



   An implementation SHOULD add a line break after the cleartext, but

   MAY omit it if the cleartext ends with a line break.  This is for

   visual clarity.



7.1.  Dash-Escaped Text



   The cleartext content of the message must also be dash-escaped.



   Dash-escaped cleartext is the ordinary cleartext where every line

   starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'

   (0x2D) and space ' ' (0x20).  This prevents the parser from

   recognizing armor headers of the cleartext itself.  An implementation

   MAY dash-escape any line, SHOULD dash-escape lines commencing "From"

   followed by a space, and MUST dash-escape any line commencing in a

   dash.  The message digest is computed using the cleartext itself, not

   the dash-escaped form.



   As with binary signatures on text documents, a cleartext signature is

   calculated on the text using canonical <CR><LF> line endings.  The

   line ending (i.e., the <CR><LF>) before the '-----BEGIN PGP

   SIGNATURE-----' line that terminates the signed text is not

   considered part of the signed text.









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   When reversing dash-escaping, an implementation MUST strip the string

   "- " if it occurs at the beginning of a line, and SHOULD warn on "-"

   and any character other than a space at the beginning of a line.



   Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at

   the end of any line is removed when the cleartext signature is

   generated.



8.  Regular Expressions



   A regular expression is zero or more branches, separated by '|'.  It

   matches anything that matches one of the branches.



   A branch is zero or more pieces, concatenated.  It matches a match

   for the first, followed by a match for the second, etc.



   A piece is an atom possibly followed by '*', '+', or '?'.  An atom

   followed by '*' matches a sequence of 0 or more matches of the atom.

   An atom followed by '+' matches a sequence of 1 or more matches of

   the atom.  An atom followed by '?' matches a match of the atom, or

   the null string.



   An atom is a regular expression in parentheses (matching a match for

   the regular expression), a range (see below), '.' (matching any

   single character), '^' (matching the null string at the beginning of

   the input string), '$' (matching the null string at the end of the

   input string), a '\' followed by a single character (matching that

   character), or a single character with no other significance

   (matching that character).



   A range is a sequence of characters enclosed in '[]'.  It normally

   matches any single character from the sequence.  If the sequence

   begins with '^', it matches any single character not from the rest of

   the sequence.  If two characters in the sequence are separated

   by '-', this is shorthand for the full list of ASCII characters

   between them (e.g., '[0-9]' matches any decimal digit).  To include a

   literal ']' in the sequence, make it the first character (following a

   possible '^').  To include a literal '-', make it the first or last

   character.



9.  Constants



   This section describes the constants used in OpenPGP.



   Note that these tables are not exhaustive lists; an implementation

   MAY implement an algorithm not on these lists, so long as the

   algorithm numbers are chosen from the private or experimental

   algorithm range.







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   See the section "Notes on Algorithms" below for more discussion of

   the algorithms.



9.1.  Public-Key Algorithms



      ID           Algorithm

      --           ---------

      1          - RSA (Encrypt or Sign) [HAC]

      2          - RSA Encrypt-Only [HAC]

      3          - RSA Sign-Only [HAC]

      16         - Elgamal (Encrypt-Only) [ELGAMAL] [HAC]

      17         - DSA (Digital Signature Algorithm) [FIPS186] [HAC]

      18         - Reserved for Elliptic Curve

      19         - Reserved for ECDSA

      20         - Reserved (formerly Elgamal Encrypt or Sign)

      21         - Reserved for Diffie-Hellman (X9.42,

                   as defined for IETF-S/MIME)

      100 to 110 - Private/Experimental algorithm



   Implementations MUST implement DSA for signatures, and Elgamal for

   encryption.  Implementations SHOULD implement RSA keys (1).  RSA

   Encrypt-Only (2) and RSA Sign-Only are deprecated and SHOULD NOT be

   generated, but may be interpreted.  See Section 13.5.  See Section

   13.8 for notes on Elliptic Curve (18), ECDSA (19), Elgamal Encrypt or

   Sign (20), and X9.42 (21).  Implementations MAY implement any other

   algorithm.



9.2.  Symmetric-Key Algorithms



       ID           Algorithm

       --           ---------

       0          - Plaintext or unencrypted data

       1          - IDEA [IDEA]

       2          - TripleDES (DES-EDE, [SCHNEIER] [HAC] -

                    168 bit key derived from 192)

       3          - CAST5 (128 bit key, as per [RFC2144])

       4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]

       5          - Reserved

       6          - Reserved

       7          - AES with 128-bit key [AES]

       8          - AES with 192-bit key

       9          - AES with 256-bit key

       10         - Twofish with 256-bit key [TWOFISH]

       100 to 110 - Private/Experimental algorithm



   Implementations MUST implement TripleDES.  Implementations SHOULD

   implement AES-128 and CAST5.  Implementations that interoperate with









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   PGP 2.6 or earlier need to support IDEA, as that is the only

   symmetric cipher those versions use.  Implementations MAY implement

   any other algorithm.



9.3.  Compression Algorithms



       ID           Algorithm

       --           ---------

       0          - Uncompressed

       1          - ZIP [RFC1951]

       2          - ZLIB [RFC1950]

       3          - BZip2 [BZ2]

       100 to 110 - Private/Experimental algorithm



   Implementations MUST implement uncompressed data.  Implementations

   SHOULD implement ZIP.  Implementations MAY implement any other

   algorithm.



9.4.  Hash Algorithms



      ID           Algorithm                             Text Name

      --           ---------                             ---------

      1          - MD5 [HAC]                             "MD5"

      2          - SHA-1 [FIPS180]                       "SHA1"

      3          - RIPE-MD/160 [HAC]                     "RIPEMD160"

      4          - Reserved

      5          - Reserved

      6          - Reserved

      7          - Reserved

      8          - SHA256 [FIPS180]                      "SHA256"

      9          - SHA384 [FIPS180]                      "SHA384"

      10         - SHA512 [FIPS180]                      "SHA512"

      11         - SHA224 [FIPS180]                      "SHA224"

      100 to 110 - Private/Experimental algorithm



   Implementations MUST implement SHA-1.  Implementations MAY implement

   other algorithms.  MD5 is deprecated.



10.  IANA Considerations



   OpenPGP is highly parameterized, and consequently there are a number

   of considerations for allocating parameters for extensions.  This

   section describes how IANA should look at extensions to the protocol

   as described in this document.















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10.1.  New String-to-Key Specifier Types



   OpenPGP S2K specifiers contain a mechanism for new algorithms to turn

   a string into a key.  This specification creates a registry of S2K

   specifier types.  The registry includes the S2K type, the name of the

   S2K, and a reference to the defining specification.  The initial

   values for this registry can be found in Section 3.7.1.  Adding a new

   S2K specifier MUST be done through the IETF CONSENSUS method, as

   described in [RFC2434].



10.2.  New Packets



   Major new features of OpenPGP are defined through new packet types.

   This specification creates a registry of packet types.  The registry

   includes the packet type, the name of the packet, and a reference to

   the defining specification.  The initial values for this registry can

   be found in Section 4.3.  Adding a new packet type MUST be done

   through the IETF CONSENSUS method, as described in [RFC2434].



10.2.1.  User Attribute Types



   The User Attribute packet permits an extensible mechanism for other

   types of certificate identification.  This specification creates a

   registry of User Attribute types.  The registry includes the User

   Attribute type, the name of the User Attribute, and a reference to

   the defining specification.  The initial values for this registry can

   be found in Section 5.12.  Adding a new User Attribute type MUST be

   done through the IETF CONSENSUS method, as described in [RFC2434].



10.2.1.1.  Image Format Subpacket Types



   Within User Attribute packets, there is an extensible mechanism for

   other types of image-based user attributes.  This specification

   creates a registry of Image Attribute subpacket types.  The registry

   includes the Image Attribute subpacket type, the name of the Image

   Attribute subpacket, and a reference to the defining specification.

   The initial values for this registry can be found in Section 5.12.1.

   Adding a new Image Attribute subpacket type MUST be done through the

   IETF CONSENSUS method, as described in [RFC2434].



10.2.2.  New Signature Subpackets



   OpenPGP signatures contain a mechanism for signed (or unsigned) data

   to be added to them for a variety of purposes in the Signature

   subpackets as discussed in Section 5.2.3.1.  This specification

   creates a registry of Signature subpacket types.  The registry

   includes the Signature subpacket type, the name of the subpacket, and

   a reference to the defining specification.  The initial values for







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   this registry can be found in Section 5.2.3.1.  Adding a new

   Signature subpacket MUST be done through the IETF CONSENSUS method,

   as described in [RFC2434].



10.2.2.1.  Signature Notation Data Subpackets



   OpenPGP signatures further contain a mechanism for extensions in

   signatures.  These are the Notation Data subpackets, which contain a

   key/value pair.  Notations contain a user space that is completely

   unmanaged and an IETF space.



   This specification creates a registry of Signature Notation Data

   types.  The registry includes the Signature Notation Data type, the

   name of the Signature Notation Data, its allowed values, and a

   reference to the defining specification.  The initial values for this

   registry can be found in Section 5.2.3.16.  Adding a new Signature

   Notation Data subpacket MUST be done through the EXPERT REVIEW

   method, as described in [RFC2434].



10.2.2.2.  Key Server Preference Extensions



   OpenPGP signatures contain a mechanism for preferences to be

   specified about key servers.  This specification creates a registry

   of key server preferences.  The registry includes the key server

   preference, the name of the preference, and a reference to the

   defining specification.  The initial values for this registry can be

   found in Section 5.2.3.17.  Adding a new key server preference MUST

   be done through the IETF CONSENSUS method, as described in [RFC2434].



10.2.2.3.  Key Flags Extensions



   OpenPGP signatures contain a mechanism for flags to be specified

   about key usage.  This specification creates a registry of key usage

   flags.  The registry includes the key flags value, the name of the

   flag, and a reference to the defining specification.  The initial

   values for this registry can be found in Section 5.2.3.21.  Adding a

   new key usage flag MUST be done through the IETF CONSENSUS method, as

   described in [RFC2434].



10.2.2.4.  Reason for Revocation Extensions



   OpenPGP signatures contain a mechanism for flags to be specified

   about why a key was revoked.  This specification creates a registry

   of "Reason for Revocation" flags.  The registry includes the "Reason

   for Revocation" flags value, the name of the flag, and a reference to

   the defining specification.  The initial values for this registry can

   be found in Section 5.2.3.23.  Adding a new feature flag MUST be done

   through the IETF CONSENSUS method, as described in [RFC2434].







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10.2.2.5.  Implementation Features



   OpenPGP signatures contain a mechanism for flags to be specified

   stating which optional features an implementation supports.  This

   specification creates a registry of feature-implementation flags.

   The registry includes the feature-implementation flags value, the

   name of the flag, and a reference to the defining specification.  The

   initial values for this registry can be found in Section 5.2.3.24.

   Adding a new feature-implementation flag MUST be done through the

   IETF CONSENSUS method, as described in [RFC2434].



   Also see Section 13.12 for more information about when feature flags

   are needed.



10.2.3.  New Packet Versions



   The core OpenPGP packets all have version numbers, and can be revised

   by introducing a new version of an existing packet.  This

   specification creates a registry of packet types.  The registry

   includes the packet type, the number of the version, and a reference

   to the defining specification.  The initial values for this registry

   can be found in Section 5.  Adding a new packet version MUST be done

   through the IETF CONSENSUS method, as described in [RFC2434].



10.3.  New Algorithms



   Section 9 lists the core algorithms that OpenPGP uses.  Adding in a

   new algorithm is usually simple.  For example, adding in a new

   symmetric cipher usually would not need anything more than allocating

   a constant for that cipher.  If that cipher had other than a 64-bit

   or 128-bit block size, there might need to be additional

   documentation describing how OpenPGP-CFB mode would be adjusted.

   Similarly, when DSA was expanded from a maximum of 1024-bit public

   keys to 3072-bit public keys, the revision of FIPS 186 contained

   enough information itself to allow implementation.  Changes to this

   document were made mainly for emphasis.



10.3.1.  Public-Key Algorithms



   OpenPGP specifies a number of public-key algorithms.  This

   specification creates a registry of public-key algorithm identifiers.

   The registry includes the algorithm name, its key sizes and

   parameters, and a reference to the defining specification.  The

   initial values for this registry can be found in Section 9.  Adding a

   new public-key algorithm MUST be done through the IETF CONSENSUS

   method, as described in [RFC2434].











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10.3.2.  Symmetric-Key Algorithms



   OpenPGP specifies a number of symmetric-key algorithms.  This

   specification creates a registry of symmetric-key algorithm

   identifiers.  The registry includes the algorithm name, its key sizes

   and block size, and a reference to the defining specification.  The

   initial values for this registry can be found in Section 9.  Adding a

   new symmetric-key algorithm MUST be done through the IETF CONSENSUS

   method, as described in [RFC2434].



10.3.3.  Hash Algorithms



   OpenPGP specifies a number of hash algorithms.  This specification

   creates a registry of hash algorithm identifiers.  The registry

   includes the algorithm name, a text representation of that name, its

   block size, an OID hash prefix, and a reference to the defining

   specification.  The initial values for this registry can be found in

   Section 9 for the algorithm identifiers and text names, and Section

   5.2.2 for the OIDs and expanded signature prefixes.  Adding a new

   hash algorithm MUST be done through the IETF CONSENSUS method, as

   described in [RFC2434].



10.3.4.  Compression Algorithms



   OpenPGP specifies a number of compression algorithms.  This

   specification creates a registry of compression algorithm

   identifiers.  The registry includes the algorithm name and a

   reference to the defining specification.  The initial values for this

   registry can be found in Section 9.3.  Adding a new compression key

   algorithm MUST be done through the IETF CONSENSUS method, as

   described in [RFC2434].



11.  Packet Composition



   OpenPGP packets are assembled into sequences in order to create

   messages and to transfer keys.  Not all possible packet sequences are

   meaningful and correct.  This section describes the rules for how

   packets should be placed into sequences.



11.1.  Transferable Public Keys



   OpenPGP users may transfer public keys.  The essential elements of a

   transferable public key are as follows:



     - One Public-Key packet



     - Zero or more revocation signatures









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     - One or more User ID packets



     - After each User ID packet, zero or more Signature packets

       (certifications)



     - Zero or more User Attribute packets



     - After each User Attribute packet, zero or more Signature packets

       (certifications)



     - Zero or more Subkey packets



     - After each Subkey packet, one Signature packet, plus optionally a

       revocation



   The Public-Key packet occurs first.  Each of the following User ID

   packets provides the identity of the owner of this public key.  If

   there are multiple User ID packets, this corresponds to multiple

   means of identifying the same unique individual user; for example, a

   user may have more than one email address, and construct a User ID

   for each one.



   Immediately following each User ID packet, there are zero or more

   Signature packets.  Each Signature packet is calculated on the

   immediately preceding User ID packet and the initial Public-Key

   packet.  The signature serves to certify the corresponding public key

   and User ID.  In effect, the signer is testifying to his or her

   belief that this public key belongs to the user identified by this

   User ID.



   Within the same section as the User ID packets, there are zero or

   more User Attribute packets.  Like the User ID packets, a User

   Attribute packet is followed by zero or more Signature packets

   calculated on the immediately preceding User Attribute packet and the

   initial Public-Key packet.



   User Attribute packets and User ID packets may be freely intermixed

   in this section, so long as the signatures that follow them are

   maintained on the proper User Attribute or User ID packet.



   After the User ID packet or Attribute packet, there may be zero or

   more Subkey packets.  In general, subkeys are provided in cases where

   the top-level public key is a signature-only key.  However, any V4

   key may have subkeys, and the subkeys may be encryption-only keys,

   signature-only keys, or general-purpose keys.  V3 keys MUST NOT have

   subkeys.











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   Each Subkey packet MUST be followed by one Signature packet, which

   should be a subkey binding signature issued by the top-level key.

   For subkeys that can issue signatures, the subkey binding signature

   MUST contain an Embedded Signature subpacket with a primary key

   binding signature (0x19) issued by the subkey on the top-level key.



   Subkey and Key packets may each be followed by a revocation Signature

   packet to indicate that the key is revoked.  Revocation signatures

   are only accepted if they are issued by the key itself, or by a key

   that is authorized to issue revocations via a Revocation Key

   subpacket in a self-signature by the top-level key.



   Transferable public-key packet sequences may be concatenated to allow

   transferring multiple public keys in one operation.



11.2.  Transferable Secret Keys



   OpenPGP users may transfer secret keys.  The format of a transferable

   secret key is the same as a transferable public key except that

   secret-key and secret-subkey packets are used instead of the public

   key and public-subkey packets.  Implementations SHOULD include self-

   signatures on any user IDs and subkeys, as this allows for a complete

   public key to be automatically extracted from the transferable secret

   key.  Implementations MAY choose to omit the self-signatures,

   especially if a transferable public key accompanies the transferable

   secret key.



11.3.  OpenPGP Messages



   An OpenPGP message is a packet or sequence of packets that

   corresponds to the following grammatical rules (comma represents

   sequential composition, and vertical bar separates alternatives):



   OpenPGP Message :- Encrypted Message | Signed Message |

                      Compressed Message | Literal Message.



   Compressed Message :- Compressed Data Packet.



   Literal Message :- Literal Data Packet.



   ESK :- Public-Key Encrypted Session Key Packet |

          Symmetric-Key Encrypted Session Key Packet.



   ESK Sequence :- ESK | ESK Sequence, ESK.



   Encrypted Data :- Symmetrically Encrypted Data Packet |

         Symmetrically Encrypted Integrity Protected Data Packet









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   Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.



   One-Pass Signed Message :- One-Pass Signature Packet,

               OpenPGP Message, Corresponding Signature Packet.



   Signed Message :- Signature Packet, OpenPGP Message |

               One-Pass Signed Message.



   In addition, decrypting a Symmetrically Encrypted Data packet or a

   Symmetrically Encrypted Integrity Protected Data packet as well as

   decompressing a Compressed Data packet must yield a valid OpenPGP

   Message.



11.4.  Detached Signatures



   Some OpenPGP applications use so-called "detached signatures".  For

   example, a program bundle may contain a file, and with it a second

   file that is a detached signature of the first file.  These detached

   signatures are simply a Signature packet stored separately from the

   data for which they are a signature.



12.  Enhanced Key Formats



12.1.  Key Structures



   The format of an OpenPGP V3 key is as follows.  Entries in square

   brackets are optional and ellipses indicate repetition.



           RSA Public Key

              [Revocation Self Signature]

               User ID [Signature ...]

              [User ID [Signature ...] ...]



   Each signature certifies the RSA public key and the preceding User

   ID.  The RSA public key can have many User IDs and each User ID can

   have many signatures.  V3 keys are deprecated.  Implementations MUST

   NOT generate new V3 keys, but MAY continue to use existing ones.



   The format of an OpenPGP V4 key that uses multiple public keys is

   similar except that the other keys are added to the end as "subkeys"

   of the primary key.





















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           Primary-Key

              [Revocation Self Signature]

              [Direct Key Signature...]

               User ID [Signature ...]

              [User ID [Signature ...] ...]

              [User Attribute [Signature ...] ...]

              [[Subkey [Binding-Signature-Revocation]

                      Primary-Key-Binding-Signature] ...]



   A subkey always has a single signature after it that is issued using

   the primary key to tie the two keys together.  This binding signature

   may be in either V3 or V4 format, but SHOULD be V4.  Subkeys that can

   issue signatures MUST have a V4 binding signature due to the REQUIRED

   embedded primary key binding signature.



   In the above diagram, if the binding signature of a subkey has been

   revoked, the revoked key may be removed, leaving only one key.



   In a V4 key, the primary key MUST be a key capable of certification.

   The subkeys may be keys of any other type.  There may be other

   constructions of V4 keys, too.  For example, there may be a single-

   key RSA key in V4 format, a DSA primary key with an RSA encryption

   key, or RSA primary key with an Elgamal subkey, etc.



   It is also possible to have a signature-only subkey.  This permits a

   primary key that collects certifications (key signatures), but is

   used only for certifying subkeys that are used for encryption and

   signatures.



12.2.  Key IDs and Fingerprints



   For a V3 key, the eight-octet Key ID consists of the low 64 bits of

   the public modulus of the RSA key.



   The fingerprint of a V3 key is formed by hashing the body (but not

   the two-octet length) of the MPIs that form the key material (public

   modulus n, followed by exponent e) with MD5.  Note that both V3 keys

   and MD5 are deprecated.



   A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,

   followed by the two-octet packet length, followed by the entire

   Public-Key packet starting with the version field.  The Key ID is the

   low-order 64 bits of the fingerprint.  Here are the fields of the

   hash material, with the example of a DSA key:



   a.1) 0x99 (1 octet)



   a.2) high-order length octet of (b)-(e) (1 octet)







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   a.3) low-order length octet of (b)-(e) (1 octet)



     b) version number = 4 (1 octet);



     c) timestamp of key creation (4 octets);



     d) algorithm (1 octet): 17 = DSA (example);



     e) Algorithm-specific fields.



   Algorithm-Specific Fields for DSA keys (example):



   e.1) MPI of DSA prime p;



   e.2) MPI of DSA group order q (q is a prime divisor of p-1);



   e.3) MPI of DSA group generator g;



   e.4) MPI of DSA public-key value y (= g**x mod p where x is secret).



   Note that it is possible for there to be collisions of Key IDs -- two

   different keys with the same Key ID.  Note that there is a much

   smaller, but still non-zero, probability that two different keys have

   the same fingerprint.



   Also note that if V3 and V4 format keys share the same RSA key

   material, they will have different Key IDs as well as different

   fingerprints.



   Finally, the Key ID and fingerprint of a subkey are calculated in the

   same way as for a primary key, including the 0x99 as the first octet

   (even though this is not a valid packet ID for a public subkey).



13.  Notes on Algorithms



13.1.  PKCS#1 Encoding in OpenPGP



   This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and

   EMSA-PKCS1-v1_5.  However, the calling conventions of these functions

   has changed in the past.  To avoid potential confusion and

   interoperability problems, we are including local copies in this

   document, adapted from those in PKCS#1 v2.1 [RFC3447].  RFC 3447

   should be treated as the ultimate authority on PKCS#1 for OpenPGP.

   Nonetheless, we believe that there is value in having a self-

   contained document that avoids problems in the future with needed

   changes in the conventions.











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13.1.1.  EME-PKCS1-v1_5-ENCODE



   Input:



   k  = the length in octets of the key modulus



   M  = message to be encoded, an octet string of length mLen, where

        mLen <= k - 11



   Output:



   EM = encoded message, an octet string of length k



   Error:   "message too long"



     1. Length checking: If mLen > k - 11, output "message too long" and

        stop.



     2. Generate an octet string PS of length k - mLen - 3 consisting of

        pseudo-randomly generated nonzero octets.  The length of PS will

        be at least eight octets.



     3. Concatenate PS, the message M, and other padding to form an

        encoded message EM of length k octets as



        EM = 0x00 || 0x02 || PS || 0x00 || M.



     4. Output EM.



13.1.2.  EME-PKCS1-v1_5-DECODE



   Input:



   EM = encoded message, an octet string



   Output:



   M  = message, an octet string



   Error:   "decryption error"



   To decode an EME-PKCS1_v1_5 message, separate the encoded message EM

   into an octet string PS consisting of nonzero octets and a message M

   as follows



     EM = 0x00 || 0x02 || PS || 0x00 || M.











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   If the first octet of EM does not have hexadecimal value 0x00, if the

   second octet of EM does not have hexadecimal value 0x02, if there is

   no octet with hexadecimal value 0x00 to separate PS from M, or if the

   length of PS is less than 8 octets, output "decryption error" and

   stop.  See also the security note in Section 14 regarding differences

   in reporting between a decryption error and a padding error.



13.1.3.  EMSA-PKCS1-v1_5



   This encoding method is deterministic and only has an encoding

   operation.



   Option:



   Hash - a hash function in which hLen denotes the length in octets of

         the hash function output



   Input:



   M  = message to be encoded



   mL = intended length in octets of the encoded message, at least tLen

        + 11, where tLen is the octet length of the DER encoding T of a

        certain value computed during the encoding operation



   Output:



   EM = encoded message, an octet string of length emLen



   Errors: "message too long"; "intended encoded message length too

   short"



   Steps:



     1. Apply the hash function to the message M to produce a hash value

        H:



        H = Hash(M).



        If the hash function outputs "message too long," output "message

        too long" and stop.



     2. Using the list in Section 5.2.2, produce an ASN.1 DER value for

        the hash function used.  Let T be the full hash prefix from

        Section 5.2.2, and let tLen be the length in octets of T.



     3. If emLen < tLen + 11, output "intended encoded message length

        too short" and stop.







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     4. Generate an octet string PS consisting of emLen - tLen - 3

        octets with hexadecimal value 0xFF.  The length of PS will be at

        least 8 octets.



     5. Concatenate PS, the hash prefix T, and other padding to form the

        encoded message EM as



        EM = 0x00 || 0x01 || PS || 0x00 || T.



     6. Output EM.



13.2.  Symmetric Algorithm Preferences



   The symmetric algorithm preference is an ordered list of algorithms

   that the keyholder accepts.  Since it is found on a self-signature,

   it is possible that a keyholder may have multiple, different

   preferences.  For example, Alice may have TripleDES only specified

   for "alice@work.com" but CAST5, Blowfish, and TripleDES specified for

   "alice@home.org".  Note that it is also possible for preferences to

   be in a subkey's binding signature.



   Since TripleDES is the MUST-implement algorithm, if it is not

   explicitly in the list, it is tacitly at the end.  However, it is

   good form to place it there explicitly.  Note also that if an

   implementation does not implement the preference, then it is

   implicitly a TripleDES-only implementation.



   An implementation MUST NOT use a symmetric algorithm that is not in

   the recipient's preference list.  When encrypting to more than one

   recipient, the implementation finds a suitable algorithm by taking

   the intersection of the preferences of the recipients.  Note that the

   MUST-implement algorithm, TripleDES, ensures that the intersection is

   not null.  The implementation may use any mechanism to pick an

   algorithm in the intersection.



   If an implementation can decrypt a message that a keyholder doesn't

   have in their preferences, the implementation SHOULD decrypt the

   message anyway, but MUST warn the keyholder that the protocol has

   been violated.  For example, suppose that Alice, above, has software

   that implements all algorithms in this specification.  Nonetheless,

   she prefers subsets for work or home.  If she is sent a message

   encrypted with IDEA, which is not in her preferences, the software

   warns her that someone sent her an IDEA-encrypted message, but it

   would ideally decrypt it anyway.















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13.3.  Other Algorithm Preferences



   Other algorithm preferences work similarly to the symmetric algorithm

   preference, in that they specify which algorithms the keyholder

   accepts.  There are two interesting cases that other comments need to

   be made about, though, the compression preferences and the hash

   preferences.



13.3.1.  Compression Preferences



   Compression has been an integral part of PGP since its first days.

   OpenPGP and all previous versions of PGP have offered compression.

   In this specification, the default is for messages to be compressed,

   although an implementation is not required to do so.  Consequently,

   the compression preference gives a way for a keyholder to request

   that messages not be compressed, presumably because they are using a

   minimal implementation that does not include compression.

   Additionally, this gives a keyholder a way to state that it can

   support alternate algorithms.



   Like the algorithm preferences, an implementation MUST NOT use an

   algorithm that is not in the preference vector.  If the preferences

   are not present, then they are assumed to be [ZIP(1),

   Uncompressed(0)].



   Additionally, an implementation MUST implement this preference to the

   degree of recognizing when to send an uncompressed message.  A robust

   implementation would satisfy this requirement by looking at the

   recipient's preference and acting accordingly.  A minimal

   implementation can satisfy this requirement by never generating a

   compressed message, since all implementations can handle messages

   that have not been compressed.



13.3.2.  Hash Algorithm Preferences



   Typically, the choice of a hash algorithm is something the signer

   does, rather than the verifier, because a signer rarely knows who is

   going to be verifying the signature.  This preference, though, allows

   a protocol based upon digital signatures ease in negotiation.



   Thus, if Alice is authenticating herself to Bob with a signature, it

   makes sense for her to use a hash algorithm that Bob's software uses.

   This preference allows Bob to state in his key which algorithms Alice

   may use.



   Since SHA1 is the MUST-implement hash algorithm, if it is not

   explicitly in the list, it is tacitly at the end.  However, it is

   good form to place it there explicitly.







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13.4.  Plaintext



   Algorithm 0, "plaintext", may only be used to denote secret keys that

   are stored in the clear.  Implementations MUST NOT use plaintext in

   Symmetrically Encrypted Data packets; they must use Literal Data

   packets to encode unencrypted or literal data.



13.5.  RSA



   There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only

   keys.  These types are deprecated.  The "key flags" subpacket in a

   signature is a much better way to express the same idea, and

   generalizes it to all algorithms.  An implementation SHOULD NOT

   create such a key, but MAY interpret it.



   An implementation SHOULD NOT implement RSA keys of size less than

   1024 bits.



13.6.  DSA



   An implementation SHOULD NOT implement DSA keys of size less than

   1024 bits.  It MUST NOT implement a DSA key with a q size of less

   than 160 bits.  DSA keys MUST also be a multiple of 64 bits, and the

   q size MUST be a multiple of 8 bits.  The Digital Signature Standard

   (DSS) [FIPS186] specifies that DSA be used in one of the following

   ways:



     * 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384, or

       SHA-512 hash



     * 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384, or SHA-512

       hash



     * 2048-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash



     * 3072-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash



   The above key and q size pairs were chosen to best balance the

   strength of the key with the strength of the hash.  Implementations

   SHOULD use one of the above key and q size pairs when generating DSA

   keys.  If DSS compliance is desired, one of the specified SHA hashes

   must be used as well.  [FIPS186] is the ultimate authority on DSS,

   and should be consulted for all questions of DSS compliance.



   Note that earlier versions of this standard only allowed a 160-bit q

   with no truncation allowed, so earlier implementations may not be

   able to handle signatures with a different q size or a truncated

   hash.







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13.7.  Elgamal



   An implementation SHOULD NOT implement Elgamal keys of size less than

   1024 bits.



13.8.  Reserved Algorithm Numbers



   A number of algorithm IDs have been reserved for algorithms that

   would be useful to use in an OpenPGP implementation, yet there are

   issues that prevent an implementer from actually implementing the

   algorithm.  These are marked in Section 9.1, "Public-Key Algorithms",

   as "reserved for".



   The reserved public-key algorithms, Elliptic Curve (18), ECDSA (19),

   and X9.42 (21), do not have the necessary parameters, parameter

   order, or semantics defined.



   Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures

   with a public-key identifier of 20.  These are no longer permitted.

   An implementation MUST NOT generate such keys.  An implementation

   MUST NOT generate Elgamal signatures.  See [BLEICHENBACHER].



13.9.  OpenPGP CFB Mode



   OpenPGP does symmetric encryption using a variant of Cipher Feedback

   mode (CFB mode).  This section describes the procedure it uses in

   detail.  This mode is what is used for Symmetrically Encrypted Data

   Packets; the mechanism used for encrypting secret-key material is

   similar, and is described in the sections above.



   In the description below, the value BS is the block size in octets of

   the cipher.  Most ciphers have a block size of 8 octets.  The AES and

   Twofish have a block size of 16 octets.  Also note that the

   description below assumes that the IV and CFB arrays start with an

   index of 1 (unlike the C language, which assumes arrays start with a

   zero index).



   OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and

   prefixes the plaintext with BS+2 octets of random data, such that

   octets BS+1 and BS+2 match octets BS-1 and BS.  It does a CFB

   resynchronization after encrypting those BS+2 octets.



   Thus, for an algorithm that has a block size of 8 octets (64 bits),

   the IV is 10 octets long and octets 7 and 8 of the IV are the same as

   octets 9 and 10.  For an algorithm with a block size of 16 octets

   (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate

   octets 15 and 16.  Those extra two octets are an easy check for a

   correct key.







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   Step by step, here is the procedure:



   1.  The feedback register (FR) is set to the IV, which is all zeros.



   2.  FR is encrypted to produce FRE (FR Encrypted).  This is the

       encryption of an all-zero value.



   3.  FRE is xored with the first BS octets of random data prefixed to

       the plaintext to produce C[1] through C[BS], the first BS octets

       of ciphertext.



   4.  FR is loaded with C[1] through C[BS].



   5.  FR is encrypted to produce FRE, the encryption of the first BS

       octets of ciphertext.



   6.  The left two octets of FRE get xored with the next two octets of

       data that were prefixed to the plaintext.  This produces C[BS+1]

       and C[BS+2], the next two octets of ciphertext.



   7.  (The resynchronization step) FR is loaded with C[3] through

       C[BS+2].



   8.  FR is encrypted to produce FRE.



   9.  FRE is xored with the first BS octets of the given plaintext, now

       that we have finished encrypting the BS+2 octets of prefixed

       data.  This produces C[BS+3] through C[BS+(BS+2)], the next BS

       octets of ciphertext.



   10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18 for

       an 8-octet block).



       11. FR is encrypted to produce FRE.



       12. FRE is xored with the next BS octets of plaintext, to produce

       the next BS octets of ciphertext.  These are loaded into FR, and

       the process is repeated until the plaintext is used up.



13.10.  Private or Experimental Parameters



   S2K specifiers, Signature subpacket types, user attribute types,

   image format types, and algorithms described in Section 9 all reserve

   the range 100 to 110 for private and experimental use.  Packet types

   reserve the range 60 to 63 for private and experimental use.  These

   are intentionally managed with the PRIVATE USE method, as described

   in [RFC2434].









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   However, implementations need to be careful with these and promote

   them to full IANA-managed parameters when they grow beyond the

   original, limited system.



13.11.  Extension of the MDC System



   As described in the non-normative explanation in Section 5.13, the

   MDC system is uniquely unparameterized in OpenPGP.  This was an

   intentional decision to avoid cross-grade attacks.  If the MDC system

   is extended to a stronger hash function, care must be taken to avoid

   downgrade and cross-grade attacks.



   One simple way to do this is to create new packets for a new MDC.

   For example, instead of the MDC system using packets 18 and 19, a new

   MDC could use 20 and 21.  This has obvious drawbacks (it uses two

   packet numbers for each new hash function in a space that is limited

   to a maximum of 60).



   Another simple way to extend the MDC system is to create new versions

   of packet 18, and reflect this in packet 19.  For example, suppose

   that V2 of packet 18 implicitly used SHA-256.  This would require

   packet 19 to have a length of 32 octets.  The change in the version

   in packet 18 and the size of packet 19 prevent a downgrade attack.



   There are two drawbacks to this latter approach.  The first is that

   using the version number of a packet to carry algorithm information

   is not tidy from a protocol-design standpoint.  It is possible that

   there might be several versions of the MDC system in common use, but

   this untidiness would reflect untidiness in cryptographic consensus

   about hash function security.  The second is that different versions

   of packet 19 would have to have unique sizes.  If there were two

   versions each with 256-bit hashes, they could not both have 32-octet

   packet 19s without admitting the chance of a cross-grade attack.



   Yet another, complex approach to extend the MDC system would be a

   hybrid of the two above -- create a new pair of MDC packets that are

   fully parameterized, and yet protected from downgrade and cross-

   grade.



   Any change to the MDC system MUST be done through the IETF CONSENSUS

   method, as described in [RFC2434].



13.12.  Meta-Considerations for Expansion



   If OpenPGP is extended in a way that is not backwards-compatible,

   meaning that old implementations will not gracefully handle their











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   absence of a new feature, the extension proposal can be declared in

   the key holder's self-signature as part of the Features signature

   subpacket.



   We cannot state definitively what extensions will not be upwards-

   compatible, but typically new algorithms are upwards-compatible,

   whereas new packets are not.



   If an extension proposal does not update the Features system, it

   SHOULD include an explanation of why this is unnecessary.  If the

   proposal contains neither an extension to the Features system nor an

   explanation of why such an extension is unnecessary, the proposal

   SHOULD be rejected.



14.  Security Considerations



   * As with any technology involving cryptography, you should check the

     current literature to determine if any algorithms used here have

     been found to be vulnerable to attack.



   * This specification uses Public-Key Cryptography technologies.  It

     is assumed that the private key portion of a public-private key

     pair is controlled and secured by the proper party or parties.



   * Certain operations in this specification involve the use of random

     numbers.  An appropriate entropy source should be used to generate

     these numbers (see [RFC4086]).



   * The MD5 hash algorithm has been found to have weaknesses, with

     collisions found in a number of cases.  MD5 is deprecated for use

     in OpenPGP.  Implementations MUST NOT generate new signatures using

     MD5 as a hash function.  They MAY continue to consider old

     signatures that used MD5 as valid.



   * SHA-224 and SHA-384 require the same work as SHA-256 and SHA-512,

     respectively.  In general, there are few reasons to use them

     outside of DSS compatibility.  You need a situation where one needs

     more security than smaller hashes, but does not want to have the

     full 256-bit or 512-bit data length.



   * Many security protocol designers think that it is a bad idea to use

     a single key for both privacy (encryption) and integrity

     (signatures).  In fact, this was one of the motivating forces

     behind the V4 key format with separate signature and encryption

     keys.  If you as an implementer promote dual-use keys, you should

     at least be aware of this controversy.











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   * The DSA algorithm will work with any hash, but is sensitive to the

     quality of the hash algorithm.  Verifiers should be aware that even

     if the signer used a strong hash, an attacker could have modified

     the signature to use a weak one.  Only signatures using acceptably

     strong hash algorithms should be accepted as valid.



   * As OpenPGP combines many different asymmetric, symmetric, and hash

     algorithms, each with different measures of strength, care should

     be taken that the weakest element of an OpenPGP message is still

     sufficiently strong for the purpose at hand.  While consensus about

     the strength of a given algorithm may evolve, NIST Special

     Publication 800-57 [SP800-57] recommends the following list of

     equivalent strengths:



           Asymmetric  |  Hash  |  Symmetric

            key size   |  size  |   key size

           ------------+--------+-----------

              1024        160         80

              2048        224        112

              3072        256        128

              7680        384        192

             15360        512        256



   * There is a somewhat-related potential security problem in

     signatures.  If an attacker can find a message that hashes to the

     same hash with a different algorithm, a bogus signature structure

     can be constructed that evaluates correctly.



     For example, suppose Alice DSA signs message M using hash algorithm

     H.  Suppose that Mallet finds a message M' that has the same hash

     value as M with H'.  Mallet can then construct a signature block

     that verifies as Alice's signature of M' with H'.  However, this

     would also constitute a weakness in either H or H' or both.  Should

     this ever occur, a revision will have to be made to this document

     to revise the allowed hash algorithms.



   * If you are building an authentication system, the recipient may

     specify a preferred signing algorithm.  However, the signer would

     be foolish to use a weak algorithm simply because the recipient

     requests it.



   * Some of the encryption algorithms mentioned in this document have

     been analyzed less than others.  For example, although CAST5 is

     presently considered strong, it has been analyzed less than

     TripleDES.  Other algorithms may have other controversies

     surrounding them.











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   * In late summer 2002, Jallad, Katz, and Schneier published an

     interesting attack on the OpenPGP protocol and some of its

     implementations [JKS02].  In this attack, the attacker modifies a

     message and sends it to a user who then returns the erroneously

     decrypted message to the attacker.  The attacker is thus using the

     user as a random oracle, and can often decrypt the message.



     Compressing data can ameliorate this attack.  The incorrectly

     decrypted data nearly always decompresses in ways that defeat the

     attack.  However, this is not a rigorous fix, and leaves open some

     small vulnerabilities.  For example, if an implementation does not

     compress a message before encryption (perhaps because it knows it

     was already compressed), then that message is vulnerable.  Because

     of this happenstance -- that modification attacks can be thwarted

     by decompression errors -- an implementation SHOULD treat a

     decompression error as a security problem, not merely a data

     problem.



     This attack can be defeated by the use of Modification Detection,

     provided that the implementation does not let the user naively

     return the data to the attacker.  An implementation MUST treat an

     MDC failure as a security problem, not merely a data problem.



     In either case, the implementation MAY allow the user access to the

     erroneous data, but MUST warn the user as to potential security

     problems should that data be returned to the sender.



     While this attack is somewhat obscure, requiring a special set of

     circumstances to create it, it is nonetheless quite serious as it

     permits someone to trick a user to decrypt a message.

     Consequently, it is important that:



      1. Implementers treat MDC errors and decompression failures as

         security problems.



      2. Implementers implement Modification Detection with all due

         speed and encourage its spread.



      3. Users migrate to implementations that support Modification

         Detection with all due speed.



   * PKCS#1 has been found to be vulnerable to attacks in which a system

     that reports errors in padding differently from errors in

     decryption becomes a random oracle that can leak the private key in

     mere millions of queries.  Implementations must be aware of this

     attack and prevent it from happening.  The simplest solution is to

     report a single error code for all variants of decryption errors so

     as not to leak information to an attacker.







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   * Some technologies mentioned here may be subject to government

     control in some countries.



   * In winter 2005, Serge Mister and Robert Zuccherato from Entrust

     released a paper describing a way that the "quick check" in OpenPGP

     CFB mode can be used with a random oracle to decrypt two octets of

     every cipher block [MZ05].  They recommend as prevention not using

     the quick check at all.



     Many implementers have taken this advice to heart for any data that

     is symmetrically encrypted and for which the session key is

     public-key encrypted.  In this case, the quick check is not needed

     as the public-key encryption of the session key should guarantee

     that it is the right session key.  In other cases, the

     implementation should use the quick check with care.



     On the one hand, there is a danger to using it if there is a random

     oracle that can leak information to an attacker.  In plainer

     language, there is a danger to using the quick check if timing

     information about the check can be exposed to an attacker,

     particularly via an automated service that allows rapidly repeated

     queries.



     On the other hand, it is inconvenient to the user to be informed

     that they typed in the wrong passphrase only after a petabyte of

     data is decrypted.  There are many cases in cryptographic

     engineering where the implementer must use care and wisdom, and

     this is one.



15.  Implementation Nits



   This section is a collection of comments to help an implementer,

   particularly with an eye to backward compatibility.  Previous

   implementations of PGP are not OpenPGP compliant.  Often the

   differences are small, but small differences are frequently more

   vexing than large differences.  Thus, this is a non-comprehensive

   list of potential problems and gotchas for a developer who is trying

   to be backward-compatible.



     * The IDEA algorithm is patented, and yet it is required for PGP

       2.x interoperability.  It is also the de-facto preferred

       algorithm for a V3 key with a V3 self-signature (or no self-

       signature).



     * When exporting a private key, PGP 2.x generates the header "BEGIN

       PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK".

       All previous versions ignore the implied data type, and look

       directly at the packet data type.







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     * PGP 2.0 through 2.5 generated V2 Public-Key packets.  These are

       identical to the deprecated V3 keys except for the version

       number.  An implementation MUST NOT generate them and may accept

       or reject them as it sees fit.  Some older PGP versions generated

       V2 PKESK packets (Tag 1) as well.  An implementation may accept

       or reject V2 PKESK packets as it sees fit, and MUST NOT generate

       them.



     * PGP 2.6.x will not accept key-material packets with versions

       greater than 3.



     * There are many ways possible for two keys to have the same key

       material, but different fingerprints (and thus Key IDs).  Perhaps

       the most interesting is an RSA key that has been "upgraded" to V4

       format, but since a V4 fingerprint is constructed by hashing the

       key creation time along with other things, two V4 keys created at

       different times, yet with the same key material will have

       different fingerprints.



     * If an implementation is using zlib to interoperate with PGP 2.x,

       then the "windowBits" parameter should be set to -13.



     * The 0x19 back signatures were not required for signing subkeys

       until relatively recently.  Consequently, there may be keys in

       the wild that do not have these back signatures.  Implementing

       software may handle these keys as it sees fit.



     * OpenPGP does not put limits on the size of public keys.  However,

       larger keys are not necessarily better keys.  Larger keys take

       more computation time to use, and this can quickly become

       impractical.  Different OpenPGP implementations may also use

       different upper bounds for public key sizes, and so care should

       be taken when choosing sizes to maintain interoperability.  As of

       2007 most implementations have an upper bound of 4096 bits.



     * ASCII armor is an optional feature of OpenPGP.  The OpenPGP

       working group strives for a minimal set of mandatory-to-implement

       features, and since there could be useful implementations that

       only use binary object formats, this is not a "MUST" feature for

       an implementation.  For example, an implementation that is using

       OpenPGP as a mechanism for file signatures may find ASCII armor

       unnecessary. OpenPGP permits an implementation to declare what

       features it does and does not support, but ASCII armor is not one

       of these.  Since most implementations allow binary and armored

       objects to be used indiscriminately, an implementation that does

       not implement ASCII armor may find itself with compatibility

       issues with general-purpose implementations.  Moreover,

       implementations of OpenPGP-MIME [RFC3156] already have a







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       requirement for ASCII armor so those implementations will

       necessarily have support.



16.  References



16.1.  Normative References



   [AES]            NIST, FIPS PUB 197, "Advanced Encryption Standard

                    (AES)," November 2001.

                    http://csrc.nist.gov/publications/fips/fips197/fips-

                    197.{ps,pdf}



   [BLOWFISH]       Schneier, B. "Description of a New Variable-Length

                    Key, 64-Bit Block Cipher (Blowfish)" Fast Software

                    Encryption, Cambridge Security Workshop Proceedings

                    (December 1993), Springer-Verlag, 1994, pp191-204

                    <http://www.counterpane.com/bfsverlag.html>



   [BZ2]            J. Seward, jseward@acm.org, "The Bzip2 and libbzip2

                    home page" <http://www.bzip.org/>



   [ELGAMAL]        T. Elgamal, "A Public-Key Cryptosystem and a

                    Signature Scheme Based on Discrete Logarithms," IEEE

                    Transactions on Information Theory, v. IT-31, n. 4,

                    1985, pp. 469-472.



   [FIPS180]        Secure Hash Signature Standard (SHS) (FIPS PUB 180-

                    2).

                    <http://csrc.nist.gov/publications/fips/fips180-

                    2/fips180-2withchangenotice.pdf>



   [FIPS186]        Digital Signature Standard (DSS) (FIPS PUB 186-2).

                    <http://csrc.nist.gov/publications/fips/fips186-2/

                     fips186-2-change1.pdf> FIPS 186-3 describes keys

                    greater than 1024 bits.  The latest draft is at:

                    <http://csrc.nist.gov/publications/drafts/

                    fips_186-3/Draft-FIPS-186-3%20_March2006.pdf>



   [HAC]            Alfred Menezes, Paul van Oorschot, and Scott

                    Vanstone, "Handbook of Applied Cryptography," CRC

                    Press, 1996.

                    <http://www.cacr.math.uwaterloo.ca/hac/>



   [IDEA]           Lai, X, "On the design and security of block

                    ciphers", ETH Series in Information Processing, J.L.

                    Massey (editor), Vol. 1, Hartung-Gorre Verlag

                    Knostanz, Technische Hochschule (Zurich), 1992









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   [ISO10646]       ISO/IEC 10646-1:1993. International Standard --

                    Information technology -- Universal Multiple-Octet

                    Coded Character Set (UCS) -- Part 1: Architecture

                    and Basic Multilingual Plane.



   [JFIF]           JPEG File Interchange Format (Version 1.02).  Eric

                    Hamilton, C-Cube Microsystems, Milpitas, CA,

                    September 1, 1992.



   [RFC1950]        Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data

                    Format Specification version 3.3", RFC 1950, May

                    1996.



   [RFC1951]        Deutsch, P., "DEFLATE Compressed Data Format

                    Specification version 1.3", RFC 1951, May 1996.



   [RFC2045]        Freed, N. and N. Borenstein, "Multipurpose Internet

                    Mail Extensions (MIME) Part One: Format of Internet

                    Message Bodies", RFC 2045, November 1996



   [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate

                    Requirement Levels", BCP 14, RFC 2119, March 1997.



   [RFC2144]        Adams, C., "The CAST-128 Encryption Algorithm", RFC

                    2144, May 1997.



   [RFC2434]        Narten, T. and H. Alvestrand, "Guidelines for

                    Writing an IANA Considerations Section in RFCs", BCP

                    26, RFC 2434, October 1998.



   [RFC2822]        Resnick, P., "Internet Message Format", RFC 2822,

                    April 2001.



   [RFC3156]        Elkins, M., Del Torto, D., Levien, R., and T.

                    Roessler, "MIME Security with OpenPGP", RFC 3156,

                    August 2001.



   [RFC3447]        Jonsson, J. and B. Kaliski, "Public-Key Cryptography

                    Standards (PKCS) #1: RSA Cryptography Specifications

                    Version 2.1", RFC 3447, February 2003.



   [RFC3629]        Yergeau, F., "UTF-8, a transformation format of ISO

                    10646", STD 63, RFC 3629, November 2003.



   [RFC4086]        Eastlake, D., 3rd, Schiller, J., and S. Crocker,

                    "Randomness Requirements for Security", BCP 106, RFC

                    4086, June 2005.









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   [SCHNEIER]      Schneier, B., "Applied Cryptography Second Edition:

                    protocols, algorithms, and source code in C", 1996.



   [TWOFISH]        B. Schneier, J. Kelsey, D. Whiting, D. Wagner, C.

                    Hall, and N. Ferguson, "The Twofish Encryption

                    Algorithm", John Wiley & Sons, 1999.



16.2.  Informative References



   [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating Elgamal

                    signatures without knowing the secret key,"

                    Eurocrypt 96. Note that the version in the

                    proceedings has an error. A revised version is

                    available at the time of writing from

                    <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti

                    /isc/ElGamal.ps>



   [JKS02]          Kahil Jallad, Jonathan Katz, Bruce Schneier

                    "Implementation of Chosen-Ciphertext Attacks against

                    PGP and GnuPG" http://www.counterpane.com/pgp-

                    attack.html



   [MAURER]         Ueli Maurer, "Modelling a Public-Key

                    Infrastructure", Proc. 1996 European Symposium on

                    Research in Computer Security (ESORICS' 96), Lecture

                    Notes in Computer Science, Springer-Verlag, vol.

                    1146, pp. 325-350, Sep 1996.



   [MZ05]           Serge Mister, Robert Zuccherato, "An Attack on CFB

                    Mode Encryption As Used By OpenPGP," IACR ePrint

                    Archive: Report 2005/033, 8 Feb 2005

                    http://eprint.iacr.org/2005/033



   [REGEX]          Jeffrey Friedl, "Mastering Regular Expressions,"

                    O'Reilly, ISBN 0-596-00289-0.



   [RFC1423]        Balenson, D., "Privacy Enhancement for Internet

                    Electronic Mail: Part III: Algorithms, Modes, and

                    Identifiers", RFC 1423, February 1993.



   [RFC1991]        Atkins, D., Stallings, W., and P. Zimmermann, "PGP

                    Message Exchange Formats", RFC 1991, August 1996.



   [RFC2440]        Callas, J., Donnerhacke, L., Finney, H., and R.

                    Thayer, "OpenPGP Message Format", RFC 2440, November

                    1998.











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   [SP800-57]       NIST Special Publication 800-57, Recommendation on

                    Key Management

                    <http://csrc.nist.gov/publications/nistpubs/ 800-

                    57/SP800-57-Part1.pdf>

                    <http://csrc.nist.gov/publications/nistpubs/ 800-

                    57/SP800-57-Part2.pdf>



Acknowledgements



   This memo also draws on much previous work from a number of other

   authors, including: Derek Atkins, Charles Breed, Dave Del Torto, Marc

   Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Ben Laurie,

   Raph Levien, Colin Plumb, Will Price, David Shaw, William Stallings,

   Mark Weaver, and Philip R. Zimmermann.



Authors' Addresses



   The working group can be contacted via the current chair:



      Derek Atkins

      IHTFP Consulting, Inc.

      4 Farragut Ave

      Somerville, MA  02144  USA



      EMail: derek@ihtfp.com

      Tel: +1 617 623 3745



   The principal authors of this document are as follows:



      Jon Callas

      EMail: jon@callas.org



      Lutz Donnerhacke

      IKS GmbH

      Wildenbruchstr. 15

      07745 Jena, Germany

      EMail: lutz@iks-jena.de



      Hal Finney

      EMail: hal@finney.org



      David Shaw

      EMail: dshaw@jabberwocky.com



      Rodney Thayer

      EMail: rodney@canola-jones.com











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Full Copyright Statement



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   contained in BCP 78, and except as set forth therein, the authors

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