









Network Working Group                                         J. Callas

Request for Comments: 2440                           Network Associates

Category: Standards Track                                L. Donnerhacke

                                     IN-Root-CA Individual Network e.V.

                                                              H. Finney

                                                     Network Associates

                                                              R. Thayer

                                                        EIS Corporation

                                                          November 1998





                         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.



Copyright Notice



   Copyright (C) The Internet Society (1998).  All Rights Reserved.



IESG Note



   This document defines many tag values, yet it doesn't describe a

   mechanism for adding new tags (for new features).  Traditionally the

   Internet Assigned Numbers Authority (IANA) handles the allocation of

   new values for future expansion and RFCs usually define the procedure

   to be used by the IANA.  However, there are subtle (and not so

   subtle) interactions that may occur in this protocol between new

   features and existing features which result in a significant

   reduction in over all security.  Therefore, this document does not

   define an extension procedure.  Instead requests to define new tag

   values (say for new encryption algorithms for example) should be

   forwarded to the IESG Security Area Directors for consideration or

   forwarding to the appropriate IETF Working Group for consideration.



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,







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   however, discuss implementation issues necessary to avoid security

   flaws.



   Open-PGP 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.



Table of Contents



            Status of this Memo                                       1

            IESG Note                                                 1

            Abstract                                                  1

            Table of Contents                                         2

   1.       Introduction                                              4

   1.1.     Terms                                                     5

   2.       General functions                                         5

   2.1.     Confidentiality via Encryption                            5

   2.2.     Authentication via Digital signature                      6

   2.3.     Compression                                               7

   2.4.     Conversion to Radix-64                                    7

   2.5.     Signature-Only Applications                               7

   3.       Data Element Formats                                      7

   3.1.     Scalar numbers                                            8

   3.2.     Multi-Precision Integers                                  8

   3.3.     Key IDs                                                   8

   3.4.     Text                                                      8

   3.5.     Time fields                                               9

   3.6.     String-to-key (S2K) specifiers                            9

   3.6.1.   String-to-key (S2k) specifier types                       9

   3.6.1.1. Simple S2K                                                9

   3.6.1.2. Salted S2K                                               10

   3.6.1.3. Iterated and Salted S2K                                  10

   3.6.2.   String-to-key usage                                      11

   3.6.2.1. Secret key encryption                                    11

   3.6.2.2. Symmetric-key message encryption                         11

   4.       Packet Syntax                                            12

   4.1.     Overview                                                 12

   4.2.     Packet Headers                                           12

   4.2.1.   Old-Format Packet Lengths                                13

   4.2.2.   New-Format Packet Lengths                                13

   4.2.2.1. One-Octet Lengths                                        14

   4.2.2.2. Two-Octet Lengths                                        14

   4.2.2.3. Five-Octet Lengths                                       14

   4.2.2.4. Partial Body Lengths                                     14

   4.2.3.   Packet Length Examples                                   14







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   4.3.     Packet Tags                                              15

   5.       Packet Types                                             16

   5.1.     Public-Key Encrypted Session Key Packets (Tag 1)         16

   5.2.     Signature Packet (Tag 2)                                 17

   5.2.1.   Signature Types                                          17

   5.2.2.   Version 3 Signature Packet Format                        19

   5.2.3.   Version 4 Signature Packet Format                        21

   5.2.3.1. Signature Subpacket Specification                        22

   5.2.3.2. Signature Subpacket Types                                24

   5.2.3.3. Signature creation time                                  25

   5.2.3.4. Issuer                                                   25

   5.2.3.5. Key expiration time                                      25

   5.2.3.6. Preferred symmetric algorithms                           25

   5.2.3.7. Preferred hash algorithms                                25

   5.2.3.8. Preferred compression algorithms                         26

   5.2.3.9. Signature expiration time                                26

   5.2.3.10.Exportable Certification                                 26

   5.2.3.11.Revocable                                                27

   5.2.3.12.Trust signature                                          27

   5.2.3.13.Regular expression                                       27

   5.2.3.14.Revocation key                                           27

   5.2.3.15.Notation Data                                            28

   5.2.3.16.Key server preferences                                   28

   5.2.3.17.Preferred key server                                     29

   5.2.3.18.Primary user id                                          29

   5.2.3.19.Policy URL                                               29

   5.2.3.20.Key Flags                                                29

   5.2.3.21.Signer's User ID                                         30

   5.2.3.22.Reason for Revocation                                    30

   5.2.4.   Computing Signatures                                     31

   5.2.4.1. Subpacket Hints                                          32

   5.3.     Symmetric-Key Encrypted Session-Key Packets (Tag 3)      32

   5.4.     One-Pass Signature Packets (Tag 4)                       33

   5.5.     Key Material Packet                                      34

   5.5.1.   Key Packet Variants                                      34

   5.5.1.1. Public Key Packet (Tag 6)                                34

   5.5.1.2. Public Subkey Packet (Tag 14)                            34

   5.5.1.3. Secret Key Packet (Tag 5)                                35

   5.5.1.4. Secret Subkey Packet (Tag 7)                             35

   5.5.2.   Public Key Packet Formats                                35

   5.5.3.   Secret Key Packet Formats                                37

   5.6.     Compressed Data Packet (Tag 8)                           38

   5.7.     Symmetrically Encrypted Data Packet (Tag 9)              39

   5.8.     Marker Packet (Obsolete Literal Packet) (Tag 10)         39

   5.9.     Literal Data Packet (Tag 11)                             40

   5.10.    Trust Packet (Tag 12)                                    40

   5.11.    User ID Packet (Tag 13)                                  41

   6.       Radix-64 Conversions                                     41







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   6.1.     An Implementation of the CRC-24 in "C"                   42

   6.2.     Forming ASCII Armor                                      42

   6.3.     Encoding Binary in Radix-64                              44

   6.4.     Decoding Radix-64                                        46

   6.5.     Examples of Radix-64                                     46

   6.6.     Example of an ASCII Armored Message                      47

   7.       Cleartext signature framework                            47

   7.1.     Dash-Escaped Text                                        47

   8.       Regular Expressions                                      48

   9.       Constants                                                49

   9.1.     Public Key Algorithms                                    49

   9.2.     Symmetric Key Algorithms                                 49

   9.3.     Compression Algorithms                                   50

   9.4.     Hash Algorithms                                          50

   10.      Packet Composition                                       50

   10.1.    Transferable Public Keys                                 50

   10.2.    OpenPGP Messages                                         52

   10.3.    Detached Signatures                                      52

   11.      Enhanced Key Formats                                     52

   11.1.    Key Structures                                           52

   11.2.    Key IDs and Fingerprints                                 53

   12.      Notes on Algorithms                                      54

   12.1.    Symmetric Algorithm Preferences                          54

   12.2.    Other Algorithm Preferences                              55

   12.2.1.  Compression Preferences                                  56

   12.2.2.  Hash Algorithm Preferences                               56

   12.3.    Plaintext                                                56

   12.4.    RSA                                                      56

   12.5.    Elgamal                                                  57

   12.6.    DSA                                                      58

   12.7.    Reserved Algorithm Numbers                               58

   12.8.    OpenPGP CFB mode                                         58

   13.      Security Considerations                                  59

   14.      Implementation Nits                                      60

   15.      Authors and Working Group Chair                          62

   16.      References                                               63

   17.      Full Copyright Statement                                 65



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 builds on the foundation provided in

   RFC 1991 "PGP Message Exchange Formats."















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1.1. Terms



     * OpenPGP - This is a definition for security software that uses

       PGP 5.x as a basis.



     * 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.



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

   Network Associates, Inc. and are used with permission.



   This document uses the terms "MUST", "SHOULD", and "MAY" as defined

   in RFC 2119, along with the negated forms of those terms.



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 uses two encryption methods to provide confidentiality:

   symmetric-key encryption and public key encryption. With public-key

   encryption, the object is encrypted using a symmetric encryption

   algorithm.  Each symmetric key is used only once. A new "session key"

   is generated as a random number for each message. Since it is used







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   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.



   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.







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   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 MAY compress the message after applying the

   signature but before encryption.



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.



   Note that many applications, particularly messaging applications,

   will want more advanced features as described in the OpenPGP-MIME

   document, RFC 2015. An application that implements OpenPGP for

   messaging SHOULD implement OpenPGP-MIME.



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-comformant only in that they omit

   encryption.



3. Data Element Formats



   This section describes the data elements used by OpenPGP.

















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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]).



3.2. Multi-Precision Integers



   Multi-Precision 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].



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



   The default character set for text is the UTF-8 [RFC2279] 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. 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.



3.6.1. String-to-key (S2k) specifier types



   There are three types of S2K specifiers currently supported, as

   follows:



3.6.1.1. 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

   algorithm's output. If the hash size is greater than or equal to 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.











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3.6.1.2. 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.



3.6.1.3. 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



   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.







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

   context(s) as with the other S2K algorithms.



3.6.2. String-to-key usage



   Implementations SHOULD use salted or iterated-and-salted S2K

   specifiers, as simple S2K specifiers are more vulnerable to

   dictionary attacks.



3.6.2.1. 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

   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.



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

   possibilities:



       0:           secret data is unencrypted (no pass phrase)

       255:         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 8-octet Initial Vector for the decryption of

   the secret values, if they are encrypted, and then the secret key

   values themselves.



3.6.2.2. 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.











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   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.



   Note that the most significant bit is the left-most 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, either format may be

   used. Note that old format packets have four bits of content tags,

   and new format packets have six; some features cannot be used and

   still be backward-compatible.



   Old format packets contain:



         Bits 5-2 -- content tag

         Bits 1-0 - length-type











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   New format packets contain:



         Bits 5-0 -- content 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.



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.





















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4.2.2.1. One-Octet Lengths



   A one-octet Body Length header encodes a length of from 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 from 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



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 (of one of the three types --

   one octet, two-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.



4.2.3. Packet Length Examples



   These examples show ways that new-format packets might encode the

   packet lengths.









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   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 coded 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. 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.



   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:



       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









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       13       -- User ID Packet

       14       -- Public Subkey 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 Encrypted Session Key packets

   (either Public-Key or Symmetric-Key) 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.



   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

       implementation should accept, but not generate a version of 2,

       which is equivalent to V3 in all other respects.



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

       that the session key is encrypted to.



     - 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.













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   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 padded as described in

   PKCS-1 block type 02 [RFC2313] to form the "m" value used in the

   formulas above.



   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 new PKCS-1 padding 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.



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 MUST accept V3 signatures. Implementations SHOULD

   generate V4 signatures.  Implementations MAY generate a V3 signature

   that can be verified by PGP 2.6.x.



   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

   specified in a signature type octet in any given signature. These

   meanings are:



   0x00: Signature of a binary document.

         Typically, this means the signer owns it, created it, or

         certifies that it has not been modified.









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   0x01: Signature of a canonical text document.

         Typically, 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> and trailing blanks removed.



   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.



   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.  Note that all PGP "key signatures" are this type of

         certification.



   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.



         Please note that the vagueness of these certification claims is

         not a flaw, but a feature of the system. Because PGP places

         final authority for validity upon the receiver of a

         certification, it may be that one authority's casual

         certification might be more rigorous than some other

         authority's positive certification. These classifications allow

         a certification authority to issue fine-grained claims.



   0x18: Subkey Binding Signature

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

         indicates that it owns the subkey. This signature is calculated

         directly on the subkey itself, not on any User ID or other

         packets.













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   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.



   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). It should be

         issued by the same key that issued the revoked signature or an

         authorized revocation key The signature should have a later

         creation date than the signature it revokes.



   0x40: Timestamp signature.

         This signature is only meaningful for the timestamp contained

         in it.



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.



     - One-octet public key algorithm.







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     - One-octet hash algorithm.



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



     - One or more multi-precision integers comprising the signature.

       This portion is algorithm specific, as described below.



   The data being signed is hashed, and then the signature type and

   creation time from the signature packet are hashed (5 additional

   octets).  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.



   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 signature than for RSA signatures.



   With RSA signatures, the hash value is encoded as described in PKCS-1

   section 10.1.2, "Data encoding", producing an ASN.1 value of type

   DigestInfo, and then padded using PKCS-1 block type 01 [RFC2313].

   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:



     - MD2:        0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02



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



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



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

















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   The ASN.1 OIDs are:



     - MD2:        1.2.840.113549.2.2



     - MD5:        1.2.840.113549.2.5



     - RIPEMD-160: 1.3.36.3.2.1



     - SHA-1:      1.3.14.3.2.26



   The full hash prefixes for these are:



       MD2:        0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,

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

                   0x04, 0x10



       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



   DSA signatures MUST use hashes with a size of 160 bits, to match q,

   the size of the group generated by the DSA key's generator value.

   The hash function result is treated as a 160 bit 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.









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     - Hashed subpacket data. (zero or more subpackets)



     - Two-octet scalar octet count for 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. (zero or more subpackets)



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



     - One or more multi-precision integers comprising the signature.

       This portion is algorithm specific, as described above.



   The data being signed is hashed, and then 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

   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



   The subpacket fields consist of zero or more signature subpackets.

   Each set of subpackets is preceded by a two-octet scalar count of the

   length of the set of 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 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:









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       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:



       2 = signature creation time

       3 = signature expiration time

       4 = exportable certification

       5 = trust signature

       6 = regular expression

       7 = revocable

       9 = key expiration time

       10 = placeholder for backward compatibility

       11 = preferred symmetric algorithms

       12 = revocation key

       16 = issuer key ID

       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 URL

       27 = key flags

       28 = signer's user id

       29 = reason for revocation

       100 to 110 = internal or user-defined



   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.













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   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 "preferences".



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 user id

   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 self-signature is a binding signature made by the key the signature

   refers to. 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 username, 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 usernames, Alice and Bob. Suppose that Alice prefers the

   symmetric algorithm CAST5, and Bob prefers IDEA or Triple-DES. 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,

   then algorithm of the default user id of the key provides the default

   symmetric algorithm.



   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.



















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5.2.3.3. Signature creation time



   (4 octet time field)



   The time the signature was made.



   MUST be present in the hashed area.



5.2.3.4. Issuer



   (8 octet key ID)



   The OpenPGP key ID of the key issuing the signature.



5.2.3.5. 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.



5.2.3.6. Preferred symmetric algorithms



   (sequence 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

   algorithms listed are supported by the recipient's software.

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

   signature.



5.2.3.7. 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 6.

   This is only found on a self-signature.



















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5.2.3.8. 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 6. 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.



5.2.3.9. 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.



5.2.3.10. 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.

   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.









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5.2.3.11. Revocable



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



   Signature's revocability status.  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.



5.2.3.12. 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.



5.2.3.13. 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 package. A description of

   the syntax is found in a section below.



5.2.3.14. Revocation key



   (1 octet of class, 1 octet of algid, 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







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   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.



5.2.3.15. 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.



   All undefined flags MUST be zero. Defined flags are:



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

                           from one person to another, and has no

                           meaning to software.

       Other octets: none.



5.2.3.16. Key server preferences



   (N octets of flags)



   This is a list of 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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5.2.3.17. Preferred key server



   (String)



   This is a URL 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 URL, the

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

   finger, etc.



5.2.3.18. 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.



5.2.3.19. Policy URL



   (String)



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

   that the signature was issued under.



5.2.3.20. Key Flags



   (Octet string)



   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:



       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.









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

       secret-sharing mechanism.



       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.



5.2.3.21. Signer's User ID



   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.



5.2.3.22. 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:















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       0x00 - No reason specified (key revocations or cert revocations)

       0x01 - Key is superceded (key revocations)

       0x02 - Key material has been compromised (key revocations)

       0x03 - Key is no longer used (key revocations)

       0x20 - User id information is no longer valid (cert revocations)



   Following the revocation code is a string of octets which 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.



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.



   The signature data is simple to compute for document signatures

   (types 0x00 and 0x01), for which the document itself is the data.

   For standalone signatures, this is a null string.



   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 signature (type 0x18)

   then hashes the subkey, using the same format as the main key. 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 name packet, without any

   header. A V4 certification hashes the constant 0xb4 (which is an

   old-style packet header with the length-of-length set to zero), a

   four-octet number giving the length of the username, and then the

   username data.



   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

   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.













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

   of the signature packet, i.e. 0x04; 0xFF; 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, 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



   An implementation SHOULD put the two mandatory subpackets, creation

   time and issuer, as the first subpackets in the subpacket list,

   simply to make it easier for the implementer to find them.



   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 an 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 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.



   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







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

   or more pass phrases. 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 insure 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.



   The body of this packet consists of:











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     - 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 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.







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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 2 packets are identical in

   format to Version 3 packets, but are generated by PGP 2.5 or before.

   V2 packets are deprecated and they MUST NOT be generated.  PGP 5.0

   introduced version 4 packets, with new fields and semantics.  PGP

   2.6.x will not accept key-material packets with versions greater than

   3.



   OpenPGP implementations SHOULD create keys with version 4 format. An

   implementation MAY generate a V3 key to ensure interoperability with

   old software; note, however, that V4 keys correct some security

   deficiencies in V3 keys. These deficiencies are described below. An

   implementation MUST NOT create a V3 key with a public key algorithm

   other than RSA.



   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 multi-precision integers comprising the key

       material:



         - a multiprecision integer (MPI) of RSA public modulus n;



         - an MPI of RSA public encryption exponent e.













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   V3 keys SHOULD only be used for backward compatibility because of

   three weaknesses in them. 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, which increases the opportunity for fingerprint collisions.

   Third, there are minor weaknesses in the MD5 hash algorithm that make

   developers prefer other algorithms. See below for a fuller discussion

   of key IDs and fingerprints.



   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 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 multi-precision 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 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 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, in encrypted form.



   The packet contains:



     - A Public Key or Public Subkey packet, as described above



     - One octet indicating string-to-key usage conventions.  0

       indicates that the secret key data is not encrypted.  255

       indicates that a string-to-key specifier is being given.  Any

       other value is a symmetric-key encryption algorithm specifier.



     - [Optional] If string-to-key usage octet was 255, a one-octet

       symmetric encryption algorithm.



     - [Optional] If string-to-key usage octet was 255, 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, eight-octet Initial

       Vector (IV).



     - Encrypted multi-precision integers comprising the secret key

       data. These algorithm-specific fields are as described below.



     - Two-octet checksum of the plaintext of the algorithm-specific

       portion (sum of all octets, mod 65536).



       Algorithm Specific Fields for RSA secret keys:



       - multiprecision integer (MPI) of RSA secret exponent d.



       - MPI of RSA secret prime value p.



       - 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.









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       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 SHOULD use a string-to-key

   specifier; the simple hash is for backward compatibility. 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 16-bit 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.



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 literal data

   packets.



   The body of this packet consists of:



     - One octet that gives the algorithm used to compress the packet.



     - The remainder of the packet is compressed data.



   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.











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   ZIP-compressed packets are compressed with raw RFC 1951 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 ZLIB-style

   blocks.



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 will

   typically contain other packets (often literal data packets or

   compressed data packets).



   The body of this packet consists of:



     - Encrypted data, the output of the selected symmetric-key cipher

       operating in PGP's variant of Cipher Feedback (CFB) mode.



   The symmetric cipher used may be specified in an 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.



   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 10-octet string to

   the data before it is encrypted.  The first eight octets are random,

   and the 9th and 10th octets are copies of the 7th and 8th octets,

   respectively. After encrypting the first 10 octets, the CFB state is

   resynchronized if the cipher block size is 8 octets or less.  The

   last 8 octets of ciphertext are passed through the cipher and the

   block boundary is reset.



   The repetition of 16 bits in the 80 bits of random data prefixed to

   the message allows the receiver to immediately check whether the

   session key is incorrect.



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 re-assigned and is

   reserved for use as the Marker packet.







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   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.



   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.  RFC

   1991 also defined a value of 'l' as a 'local' mode for machine-local

   conversions.  This use is now deprecated.



     - File name as a string (one-octet length, followed by file name),

       if the encrypted data should be saved as a file.



   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 the modification date of the

       file, or the creation time of the packet, or a zero that

       indicates the present 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,







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   along with other information that implementing software uses for

   trust information.



   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.



5.11. User ID Packet (Tag 13)



   A User ID packet consists of data that is intended to represent the

   name and email address of the key holder.  By convention, it includes

   an RFC 822 mail name, but there are no restrictions on its content.

   The packet length in the header specifies the length of the user id.

   If it is text, it is encoded in UTF-8.



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 [RFC2231,

   Section 6.8]. An OpenPGP implementation MAY use ASCII Armor to

   protect the raw binary data.



   The checksum is a 24-bit CRC converted to four characters of radix-64

   encoding by the same MIME base64 transformation, preceded by an

   equals 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 data, so OpenPGP can reconstruct the data later. 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

       natures following clearsigned messages. Note that PGP 2.x s BEGIN

       PGP MESSAGE for detached signatures.



   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.



   Currently defined Armor Header Keys are:



     - "Version", that states the OpenPGP Version used to encode the

       message.



     - "Comment", a user-defined comment.



     - "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













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       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.



     - "Hash", a comma-separated list of hash algorithms used in this

       message. This is used only in clear-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 by

       default. 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.



       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.



   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.

















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         +--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.



   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.









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6.4. Decoding Radix-64



   Any characters outside of the base64 alphabet are ignored in Radix-64

   data. Decoding software must ignore all line breaks or other

   characters not found in the table above.



   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.



   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.



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      =      =











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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 is indented by two spaces.



7. Cleartext signature framework



   It is desirable 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 RFC 2015 defines another way to

   clear sign messages for environments that support MIME.)



   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 is used for the signature. If there are no such headers,

   MD5 is used, an implementation MAY omit them for 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.



7.1. Dash-Escaped Text



   The cleartext content of the message must also be dash-escaped.







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   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. 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.



   Also, any trailing whitespace (spaces, and tabs, 0x09) at the end of

   any line is ignored when the cleartext signature is calculated.



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.













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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.



   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)

       2          - RSA Encrypt-Only

       3          - RSA Sign-Only

       16         - Elgamal (Encrypt-Only), see [ELGAMAL]

       17         - DSA (Digital Signature Standard)

       18         - Reserved for Elliptic Curve

       19         - Reserved for ECDSA

       20         - 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.

   Implementations MAY implement any other algorithm.



9.2. Symmetric Key Algorithms



       ID           Algorithm

       --           ---------

       0          - Plaintext or unencrypted data

       1          - IDEA [IDEA]

       2          - Triple-DES (DES-EDE, as per spec -

                    168 bit key derived from 192)

       3          - CAST5 (128 bit key, as per RFC 2144)

       4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]

       5          - SAFER-SK128 (13 rounds) [SAFER]

       6          - Reserved for DES/SK

       7          - Reserved for AES with 128-bit key







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       8          - Reserved for AES with 192-bit key

       9          - Reserved for AES with 256-bit key

       100 to 110 - Private/Experimental algorithm.



   Implementations MUST implement Triple-DES. Implementations SHOULD

   implement IDEA and CAST5.Implementations MAY implement any other

   algorithm.



9.3. Compression Algorithms



       ID           Algorithm

       --           ---------

       0          - Uncompressed

       1          - ZIP (RFC 1951)

       2          - ZLIB (RFC 1950)

       100 to 110 - Private/Experimental algorithm.



   Implementations MUST implement uncompressed data. Implementations

   SHOULD implement ZIP. Implementations MAY implement ZLIB.



9.4. Hash Algorithms



       ID           Algorithm                              Text Name

       --           ---------                              ---- ----

       1          - MD5                                    "MD5"

       2          - SHA-1                                  "SHA1"

       3          - RIPE-MD/160                            "RIPEMD160"

       4          - Reserved for double-width SHA (experimental)

       5          - MD2                                    "MD2"

       6          - Reserved for TIGER/192                 "TIGER192"

       7          - Reserved for HAVAL (5 pass, 160-bit)

       "HAVAL-5-160"

       100 to 110 - Private/Experimental algorithm.



   Implementations MUST implement SHA-1. Implementations SHOULD

   implement MD5.



10. 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 describes the rules for how packets

   should be placed into sequences.



10.1. Transferable Public Keys



   OpenPGP users may transfer public keys. The essential elements of a

   transferable public key are:







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     - One Public Key packet



     - Zero or more revocation signatures



     - One or more User ID packets



     - After each User ID packet, zero or more signature packets

       (certifications)



     - Zero or more Subkey packets



     - After each Subkey packet, one signature packet, 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.



   After the User ID packets there may be one 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.



   Each Subkey packet must be followed by one Signature packet, which

   should be a subkey binding signature issued by 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.











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10.2. 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 Message :- Symmetrically Encrypted Data Packet |

               ESK Sequence, Symmetrically Encrypted Data Packet.



   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 and



   decompressing a Compressed Data packet must yield a valid OpenPGP

   Message.



10.3. 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 that they are a signature of.



11. Enhanced Key Formats



11.1. Key Structures



   The format of an OpenPGP V3 key is as follows.  Entries in square

   brackets are optional and ellipses indicate repetition.











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           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.



   The format of an OpenPGP V4 key that uses two public keys is similar

   except that the other keys are added to the end as 'subkeys' of the

   primary key.



           Primary-Key

              [Revocation Self Signature]

              [Direct Key Self Signature...]

               User ID [Signature ...]

              [User ID [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 V4 is preferred, of course.



   In the above diagram, if the binding signature of a subkey has been

   revoked, the revoked binding signature may be removed, leaving only

   one signature.



   In a key that has a main key and subkeys, the primary key MUST be a

   key capable of signing. 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 used for certifying subkeys that are used for encryption and

   signatures.



11.2. 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.







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   A V4 fingerprint is the 160-bit SHA-1 hash of the one-octet Packet

   Tag, 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)-(f) (1 octet)



  a.3) low order length octet of (b)-(f) (1 octet)



    b) version number = 4 (1 octet);



    c) time stamp 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 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.



12. Notes on Algorithms



12.1. 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 different preferences. For

   example, Alice may have TripleDES only specified for "alice@work.com"

   but CAST5, Blowfish, and TripleDES specified for "alice@home.org".







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   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 than 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.)



   An implementation that is striving for backward compatibility MAY

   consider a V3 key with a V3 self-signature to be an implicit

   preference for IDEA, and no ability to do TripleDES. This is

   technically non-compliant, but an implementation MAY violate the

   above rule in this case only and use IDEA to encrypt the message,

   provided that the message creator is warned. Ideally, though, the

   implementation would follow the rule by actually generating two

   messages, because it is possible that the OpenPGP user's

   implementation does not have IDEA, and thus could not read the

   message. Consequently, an implementation MAY, but SHOULD NOT use IDEA

   in an algorithm conflict with a V3 key.



12.2. 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.











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12.2.1. Compression Preferences



   Compression has been an integral part of PGP since its first days.

   OpenPGP and all previous versions of PGP have offered compression.

   And 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)].



12.2.2. Hash Algorithm Preferences



   Typically, the choice of a hash algorithm is something the signer

   does, rather than the verifier, because a signer does not typically

   know 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.



12.3. 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.



12.4. RSA



   There are algorithm types for RSA-signature-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 768

   bits.









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   It is permissible for an implementation to support RSA merely for

   backward compatibility; for example, such an implementation would

   support V3 keys with IDEA symmetric cryptography. Note that this is

   an exception to the other MUST-implement rules. An implementation

   that supports RSA in V4 keys MUST implement the MUST-implement

   features.



12.5. Elgamal



   If an Elgamal key is to be used for both signing and encryption,

   extra care must be taken in creating the key.



   An ElGamal key consists of a generator g, a prime modulus p, a secret

   exponent x, and a public value y = g^x mod p.



   The generator and prime must be chosen so that solving the discrete

   log problem is intractable.  The group g should generate the

   multiplicative group mod p-1 or a large subgroup of it, and the order

   of g should have at least one large prime factor.  A good choice is

   to use a "strong" Sophie-Germain prime in choosing p, so that both p

   and (p-1)/2 are primes. In fact, this choice is so good that

   implementors SHOULD do it, as it avoids a small subgroup attack.



   In addition, a result of Bleichenbacher [BLEICHENBACHER] shows that

   if the generator g has only small prime factors, and if g divides the

   order of the group it generates, then signatures can be forged.  In

   particular, choosing g=2 is a bad choice if the group order may be

   even. On the other hand, a generator of 2 is a fine choice for an

   encryption-only key, as this will make the encryption faster.



   While verifying Elgamal signatures, note that it is important to test

   that r and s are less than p.  If this test is not done then

   signatures can be trivially forged by using large r values of

   approximately twice the length of p.  This attack is also discussed

   in the Bleichenbacher paper.



   Details on safe use of Elgamal signatures may be found in [MENEZES],

   which discusses all the weaknesses described above.



   If an implementation allows Elgamal signatures, then it MUST use the

   algorithm identifier 20 for an Elgamal public key that can sign.



   An implementation SHOULD NOT implement Elgamal keys of size less than

   768 bits. For long-term security, Elgamal keys should be 1024 bits or

   longer.













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12.6. DSA



   An implementation SHOULD NOT implement DSA keys of size less than 768

   bits. Note that present DSA is limited to a maximum of 1024 bit keys,

   which are recommended for long-term use.



12.7. 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 implementor from actually implementing the

   algorithm. These are marked in the Public Algorithms section 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.



   The reserved symmetric key algorithm, DES/SK (6), does not have

   semantics defined.



   The reserved hash algorithms, TIGER192 (6), and HAVAL-5-160 (7), do

   not have OIDs. The reserved algorithm number 4, reserved for a

   double-width variant of SHA1, is not presently defined.



   We have reserver three algorithm IDs for the US NIST's Advanced

   Encryption Standard. This algorithm will work with (at least) 128,

   192, and 256-bit keys. We expect that this algorithm will be selected

   from the candidate algorithms in the year 2000.



12.8. 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, but described in those sections above.



   OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and

   prefixes the plaintext with ten octets of random data, such that

   octets 9 and 10 match octets 7 and 8.  It does a CFB "resync" after

   encrypting those ten octets.



   Note that for an algorithm that has a larger block size than 64 bits,

   the equivalent function will be done with that entire block.  For

   example, a 16-octet block algorithm would operate on 16 octets, and

   then produce two octets of check, and then work on 16-octet blocks.









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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 8 octets of random data prefixed to

       the plaintext to produce C1-C8, the first 8 octets of ciphertext.



   4.  FR is loaded with C1-C8.



   5.  FR is encrypted to produce FRE, the encryption of the first 8

       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 C9-C10,

       the next two octets of ciphertext.



   7.  (The resync step) FR is loaded with C3-C10.



   8.  FR is encrypted to produce FRE.



   9.  FRE is xored with the first 8 octets of the given plaintext, now

       that we have finished encrypting the 10 octets of prefixed data.

       This produces C11-C18, the next 8 octets of ciphertext.



   10.  FR is loaded with C11-C18



   11.  FR is encrypted to produce FRE.



   12.  FRE is xored with the next 8 octets of plaintext, to produce the

       next 8 octets of ciphertext.  These are loaded into FR and the

       process is repeated until the plaintext is used up.



13. 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.

   Possession of the private key portion of a public-private key pair is

   assumed to be controlled 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 RFC 1750.







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   The MD5 hash algorithm has been found to have weaknesses (pseudo-

   collisions in the compress function) that make some people deprecate

   its use.  They consider the SHA-1 algorithm better.



   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

   implementor promote dual-use keys, you should at least be aware of

   this controversy.



   The DSA algorithm will work with any 160-bit hash, but it is

   sensitive to the quality of the hash algorithm, if the hash algorithm

   is broken, it can leak the secret key. The Digital Signature Standard

   (DSS) specifies that DSA be used with SHA-1.  RIPEMD-160 is

   considered by many cryptographers to be as strong. An implementation

   should take care which hash algorithms are used with DSA, as a weak

   hash can not only allow a signature to be forged, but could leak the

   secret key. These same considerations about the quality of the hash

   algorithm apply to Elgamal signatures.



   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 Triple-

   DES. Other algorithms may have other controversies surrounding them.



   Some technologies mentioned here may be subject to government control

   in some countries.



14. 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 list of potential problems

   and gotchas for a developer who is trying to be backward-compatible.



     * PGP 5.x does not accept V4 signatures for anything other than

       key material.



     * PGP 5.x does not recognize the "five-octet" lengths in new-format

       headers or in signature subpacket lengths.







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     * PGP 5.0 rejects an encrypted session key if the keylength differs

       from the S2K symmetric algorithm. This is a bug in its validation

       function.



     * PGP 5.0 does not handle multiple one-pass signature headers and

       trailers. Signing one will compress the one-pass signed literal

       and prefix a V3 signature instead of doing a nested one-pass

       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.



     * In a clear-signed signature, PGP 5.0 will figure out the correct

       hash algorithm if there is no "Hash:" header, but it will reject

       a mismatch between the header and the actual algorithm used. The

       "standard" (i.e. Zimmermann/Finney/et al.) version of PGP 2.x

       rejects the "Hash:" header and assumes MD5. There are a number of

       enhanced variants of PGP 2.6.x that have been modified for SHA-1

       signatures.



     * PGP 5.0 can read an RSA key in V4 format, but can only recognize

       it with a V3 keyid, and can properly use only a V3 format RSA

       key.



     * Neither PGP 5.x nor PGP 6.0 recognize Elgamal Encrypt and Sign

       keys. They only handle Elgamal Encrypt-only keys.



     * 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.

























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15. Authors and Working Group Chair



   The working group can be contacted via the current chair:



   John W. Noerenberg, II

   Qualcomm, Inc

   6455 Lusk Blvd

   San Diego, CA 92131 USA



   Phone: +1 619-658-3510

   EMail: jwn2@qualcomm.com





   The principal authors of this memo are:



   Jon Callas

   Network Associates, Inc.

   3965 Freedom Circle

   Santa Clara, CA 95054, USA



   Phone: +1 408-346-5860

   EMail: jon@pgp.com, jcallas@nai.com





   Lutz Donnerhacke

   IKS GmbH

   Wildenbruchstr. 15

   07745 Jena, Germany



   Phone: +49-3641-675642

   EMail: lutz@iks-jena.de





   Hal Finney

   Network Associates, Inc.

   3965 Freedom Circle

   Santa Clara, CA 95054, USA



   EMail: hal@pgp.com





   Rodney Thayer

   EIS Corporation

   Clearwater, FL 33767, USA



   EMail: rodney@unitran.com











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   This memo also draws on much previous work from a number of other

   authors who include: Derek Atkins, Charles Breed, Dave Del Torto,

   Marc Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Raph

   Levien, Colin Plumb, Will Price, William Stallings, Mark Weaver, and

   Philip R. Zimmermann.



16. 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>



   [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>



   [DONNERHACKE]    Donnerhacke, L., et. al, "PGP263in - an improved

                    international version of PGP", ftp://ftp.iks-

                    jena.de/mitarb/lutz/crypt/software/pgp/



   [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.



   [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



   [ISO-10646]      ISO/IEC 10646-1:1993. International Standard --

                    Information technology -- Universal Multiple-Octet

                    Coded Character Set (UCS) -- Part 1: Architecture

                    and Basic Multilingual Plane.  UTF-8 is described in

                    Annex R, adopted but not yet published.  UTF-16 is

                    described in Annex Q, adopted but not yet published.



   [MENEZES]        Alfred Menezes, Paul van Oorschot, and Scott

                    Vanstone, "Handbook of Applied Cryptography," CRC

                    Press, 1996.









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   [RFC822]         Crocker, D., "Standard for the format of ARPA

                    Internet text messages", STD 11, RFC 822, August

                    1982.



   [RFC1423]        Balenson, D., "Privacy Enhancement for Internet

                    Electronic Mail: Part III: Algorithms, Modes, and

                    Identifiers", RFC 1423, October 1993.



   [RFC1641]        Goldsmith, D. and M. Davis, "Using Unicode with

                    MIME", RFC 1641, July 1994.



   [RFC1750]        Eastlake, D., Crocker, S. and J. Schiller,

                    "Randomness Recommendations for Security", RFC 1750,

                    December 1994.



   [RFC1951]        Deutsch, P., "DEFLATE Compressed Data Format

                    Specification version 1.3.", RFC 1951, May 1996.



   [RFC1983]        Malkin, G., "Internet Users' Glossary", FYI 18, RFC

                    1983, August 1996.



   [RFC1991]        Atkins, D., Stallings, W. and P. Zimmermann, "PGP

                    Message Exchange Formats", RFC 1991, August 1996.



   [RFC2015]        Elkins, M., "MIME Security with Pretty Good Privacy

                    (PGP)", RFC 2015, October 1996.



   [RFC2231]        Borenstein, N. and N. Freed, "Multipurpose Internet

                    Mail Extensions (MIME) Part One: Format of Internet

                    Message Bodies.", RFC 2231, November 1996.



   [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate

                    Requirement Level", BCP 14, RFC 2119, March 1997.



   [RFC2144]        Adams, C., "The CAST-128 Encryption Algorithm", RFC

                    2144, May 1997.



   [RFC2279]        Yergeau., F., "UTF-8, a transformation format of

                    Unicode and ISO 10646", RFC 2279, January 1998.



   [RFC2313]        Kaliski, B., "PKCS #1: RSA Encryption Standard

                    version 1.5", RFC 2313, March 1998.



   [SAFER]          Massey, J.L. "SAFER K-64: One Year Later", B.

                    Preneel, editor, Fast Software Encryption, Second

                    International Workshop (LNCS 1008) pp212-241,

                    Springer-Verlag 1995









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17.  Full Copyright Statement



   Copyright (C) The Internet Society (1998).  All Rights Reserved.



   This document and translations of it may be copied and furnished to

   others, and derivative works that comment on or otherwise explain it

   or assist in its implementation may be prepared, copied, published

   and distributed, in whole or in part, without restriction of any

   kind, provided that the above copyright notice and this paragraph are

   included on all such copies and derivative works.  However, this

   document itself may not be modified in any way, such as by removing

   the copyright notice or references to the Internet Society or other

   Internet organizations, except as needed for the purpose of

   developing Internet standards in which case the procedures for

   copyrights defined in the Internet Standards process must be

   followed, or as required to translate it into languages other than

   English.



   The limited permissions granted above are perpetual and will not be

   revoked by the Internet Society or its successors or assigns.



   This document and the information contained herein is provided on an

   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

















































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