











Network Working Group                                           J. Myers

Request for Comments: 2222                       Netscape Communications

Category: Standards Track                                   October 1997





            Simple Authentication and Security Layer (SASL)



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 (1997).  All Rights Reserved.



Table of Contents



   1.    Abstract ..............................................    2

   2.    Organization of this Document .........................    2

   2.1.  How to Read This Document .............................    2

   2.2.  Conventions Used in this Document .....................    2

   2.3.  Examples ..............................................    3

   3.    Introduction and Overview .............................    3

   4.    Profiling requirements ................................    4

   5.    Specific issues .......................................    5

   5.1.  Client sends data first ...............................    5

   5.2.  Server returns success with additional data ...........    5

   5.3.  Multiple authentications ..............................    5

   6.    Registration procedures ...............................    6

   6.1.  Comments on SASL mechanism registrations ..............    6

   6.2.  Location of Registered SASL Mechanism List ............    6

   6.3.  Change Control ........................................    7

   6.4.  Registration Template .................................    7

   7.    Mechanism definitions .................................    8

   7.1.  Kerberos version 4 mechanism ..........................    8

   7.2.  GSSAPI mechanism ......................................    9

   7.2.1 Client side of authentication protocol exchange .......    9

   7.2.2 Server side of authentication protocol exchange .......   10

   7.2.3 Security layer ........................................   11

   7.3.  S/Key mechanism .......................................   11

   7.4.  External mechanism ....................................   12

   8.    References ............................................   13

   9.    Security Considerations ...............................   13

   10.   Author's Address ......................................   14







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   Appendix A. Relation of SASL to Transport Security ..........   15

   Full Copyright Statement ....................................   16



1.    Abstract



   This document describes a method for adding authentication support to

   connection-based protocols.  To use this specification, a protocol

   includes a command for identifying and authenticating a user to a

   server and for optionally negotiating protection of subsequent

   protocol interactions.  If its use is negotiated, a security layer is

   inserted between the protocol and the connection.  This document

   describes how a protocol specifies such a command, defines several

   mechanisms for use by the command, and defines the protocol used for

   carrying a negotiated security layer over the connection.



2.    Organization of this Document



2.1.  How to Read This Document



   This document is written to serve two different audiences, protocol

   designers using this specification to support authentication in their

   protocol, and implementors of clients or servers for those protocols

   using this specification.



   The sections "Introduction and Overview", "Profiling requirements",

   and "Security Considerations" cover issues that protocol designers

   need to understand and address in profiling this specification for

   use in a specific protocol.



   Implementors of a protocol using this specification need the

   protocol-specific profiling information in addition to the

   information in this document.



2.2.  Conventions Used in this Document



   In examples, "C:" and "S:" indicate lines sent by the client and

   server respectively.



   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"

   in this document are to be interpreted as defined in "Key words for

   use in RFCs to Indicate Requirement Levels" [RFC 2119].





















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2.3.  Examples



   Examples in this document are for the IMAP profile [RFC 2060] of this

   specification.  The base64 encoding of challenges and responses, as

   well as the "+ " preceding the responses are part of the IMAP4

   profile, not part of the SASL specification itself.



3.    Introduction and Overview



   The Simple Authentication and Security Layer (SASL) is a method for

   adding authentication support to connection-based protocols.  To use

   this specification, a protocol includes a command for identifying and

   authenticating a user to a server and for optionally negotiating a

   security layer for subsequent protocol interactions.



   The command has a required argument identifying a SASL mechanism.

   SASL mechanisms are named by strings, from 1 to 20 characters in

   length, consisting of upper-case letters, digits, hyphens, and/or

   underscores.  SASL mechanism names must be registered with the IANA.

   Procedures for registering new SASL mechanisms are given in the

   section "Registration procedures"



   If a server supports the requested mechanism, it initiates an

   authentication protocol exchange.  This consists of a series of

   server challenges and client responses that are specific to the

   requested mechanism.  The challenges and responses are defined by the

   mechanisms as binary tokens of arbitrary length.  The protocol's

   profile then specifies how these binary tokens are then encoded for

   transfer over the connection.



   After receiving the authentication command or any client response, a

   server may issue a challenge, indicate failure, or indicate

   completion.  The protocol's profile specifies how the server

   indicates which of the above it is doing.



   After receiving a challenge, a client may issue a response or abort

   the exchange.  The protocol's profile specifies how the client

   indicates which of the above it is doing.



   During the authentication protocol exchange, the mechanism performs

   authentication, transmits an authorization identity (frequently known

   as a userid) from the client to server, and negotiates the use of a

   mechanism-specific security layer.  If the use of a security layer is

   agreed upon, then the mechanism must also define or negotiate the

   maximum cipher-text buffer size that each side is able to receive.













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   The transmitted authorization identity may be different than the

   identity in the client's authentication credentials.  This permits

   agents such as proxy servers to authenticate using their own

   credentials, yet request the access privileges of the identity for

   which they are proxying.  With any mechanism, transmitting an

   authorization identity of the empty string directs the server to

   derive an authorization identity from the client's authentication

   credentials.



   If use of a security layer is negotiated, it is applied to all

   subsequent data sent over the connection.  The security layer takes

   effect immediately following the last response of the authentication

   exchange for data sent by the client and the completion indication

   for data sent by the server.  Once the security layer is in effect,

   the protocol stream is processed by the security layer into buffers

   of cipher-text.  Each buffer is transferred over the connection as a

   stream of octets prepended with a four octet field in network byte

   order that represents the length of the following buffer.  The length

   of the cipher-text buffer must be no larger than the maximum size

   that was defined or negotiated by the other side.



4.    Profiling requirements



   In order to use this specification, a protocol definition must supply

   the following information:



   1. A service name, to be selected from the IANA registry of "service"

      elements for the GSSAPI host-based service name form [RFC 2078].



   2. A definition of the command to initiate the authentication

      protocol exchange.  This command must have as a parameter the

      mechanism name being selected by the client.



      The command SHOULD have an optional parameter giving an initial

      response.  This optional parameter allows the client to avoid a

      round trip when using a mechanism which is defined to have the

      client send data first.  When this initial response is sent by the

      client and the selected mechanism is defined to have the server

      start with an initial challenge, the command fails.  See section

      5.1 of this document for further information.



   3. A definition of the method by which the authentication protocol

      exchange is carried out, including how the challenges and

      responses are encoded, how the server indicates completion or

      failure of the exchange, how the client aborts an exchange, and

      how the exchange method interacts with any line length limits in

      the protocol.









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   4. Identification of the octet where any negotiated security layer

      starts to take effect, in both directions.



   5. A specification of how the authorization identity passed from the

      client to the server is to be interpreted.



5.    Specific issues



5.1.  Client sends data first



   Some mechanisms specify that the first data sent in the

   authentication protocol exchange is from the client to the server.



   If a protocol's profile permits the command which initiates an

   authentication protocol exchange to contain an initial client

   response, this parameter SHOULD be used with such mechanisms.



   If the initial client response parameter is not given, or if a

   protocol's profile does not permit the command which initiates an

   authentication protocol exchange to contain an initial client

   response, then the server issues a challenge with no data.  The

   client's response to this challenge is then used as the initial

   client response.  (The server then proceeds to send the next

   challenge, indicates completion, or indicates failure.)



5.2.  Server returns success with additional data



   Some mechanisms may specify that server challenge data be sent to the

   client along with an indication of successful completion of the

   exchange.  This data would, for example, authenticate the server to

   the client.



   If a protocol's profile does not permit this server challenge to be

   returned with a success indication, then the server issues the server

   challenge without an indication of successful completion.  The client

   then responds with no data.  After receiving this empty response, the

   server then indicates successful completion.



5.3.  Multiple authentications



   Unless otherwise stated by the protocol's profile, only one

   successful SASL negotiation may occur in a protocol session.  In this

   case, once an authentication protocol exchange has successfully

   completed, further attempts to initiate an authentication protocol

   exchange fail.













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   In the case that a profile explicitly permits multiple successful

   SASL negotiations to occur, then in no case may multiple security

   layers be simultaneously in effect.  If a security layer is in effect

   and a subsequent SASL negotiation selects no security layer, the

   original security layer remains in effect.  If a security layer is in

   effect and a subsequent SASL negotiation selects a second security

   layer, then the second security layer replaces the first.



6.    Registration procedures



   Registration of a SASL mechanism is done by filling in the template

   in section 6.4 and sending it in to iana@isi.edu.  IANA has the right

   to reject obviously bogus registrations, but will perform no review

   of clams made in the registration form.



   There is no naming convention for SASL mechanisms; any name that

   conforms to the syntax of a SASL mechanism name can be registered.



   While the registration procedures do not require it, authors of SASL

   mechanisms are encouraged to seek community review and comment

   whenever that is feasible.  Authors may seek community review by

   posting a specification of their proposed mechanism as an internet-

   draft.  SASL mechanisms intended for widespread use should be

   standardized through the normal IETF process, when appropriate.



6.1.  Comments on SASL mechanism registrations



   Comments on registered SASL mechanisms should first be sent to the

   "owner" of the mechanism.  Submitters of comments may, after a

   reasonable attempt to contact the owner, request IANA to attach their

   comment to the SASL mechanism registration itself.  If IANA approves

   of this the comment will be made accessible in conjunction with the

   SASL mechanism registration itself.



6.2.  Location of Registered SASL Mechanism List



   SASL mechanism registrations will be posted in the anonymous FTP

   directory "ftp://ftp.isi.edu/in-notes/iana/assignments/sasl-

   mechanisms/" and all registered SASL mechanisms will be listed in the

   periodically issued "Assigned Numbers" RFC [currently STD 2, RFC

   1700].  The SASL mechanism description and other supporting material

   may also be published as an Informational RFC by sending it to "rfc-

   editor@isi.edu" (please follow the instructions to RFC authors [RFC

   2223]).















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6.3.  Change Control



   Once a SASL mechanism registration has been published by IANA, the

   author may request a change to its definition.  The change request

   follows the same procedure as the registration request.



   The owner of a SASL mechanism may pass responsibility for the SASL

   mechanism to another person or agency by informing IANA; this can be

   done without discussion or review.



   The IESG may reassign responsibility for a SASL mechanism. The most

   common case of this will be to enable changes to be made to

   mechanisms where the author of the registration has died, moved out

   of contact or is otherwise unable to make changes that are important

   to the community.



   SASL mechanism registrations may not be deleted; mechanisms which are

   no longer believed appropriate for use can be declared OBSOLETE by a

   change to their "intended use" field; such SASL mechanisms will be

   clearly marked in the lists published by IANA.



   The IESG is considered to be the owner of all SASL mechanisms which

   are on the IETF standards track.



6.4.  Registration Template



   To: iana@iana.org

   Subject: Registration of SASL mechanism X



   SASL mechanism name:



   Security considerations:



   Published specification (optional, recommended):



   Person & email address to contact for further information:



   Intended usage:



   (One of COMMON, LIMITED USE or OBSOLETE)



   Author/Change controller:



   (Any other information that the author deems interesting may be

   added below this line.)













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7.    Mechanism definitions



   The following mechanisms are hereby defined.



7.1.  Kerberos version 4 mechanism



   The mechanism name associated with Kerberos version 4 is

   "KERBEROS_V4".



   The first challenge consists of a random 32-bit number in network

   byte order.  The client responds with a Kerberos ticket and an

   authenticator for the principal "service.hostname@realm", where

   "service" is the service name specified in the protocol's profile,

   "hostname" is the first component of the host name of the server with

   all letters in lower case, and where "realm" is the Kerberos realm of

   the server.  The encrypted checksum field included within the

   Kerberos authenticator contains the server provided challenge in

   network byte order.



   Upon decrypting and verifying the ticket and authenticator, the

   server verifies that the contained checksum field equals the original

   server provided random 32-bit number.  Should the verification be

   successful, the server must add one to the checksum and construct 8

   octets of data, with the first four octets containing the incremented

   checksum in network byte order, the fifth octet containing a bit-mask

   specifying the security layers supported by the server, and the sixth

   through eighth octets containing, in network byte order, the maximum

   cipher-text buffer size the server is able to receive.  The server

   must encrypt using DES ECB mode the 8 octets of data in the session

   key and issue that encrypted data in a second challenge.  The client

   considers the server authenticated if the first four octets of the

   un-encrypted data is equal to one plus the checksum it previously

   sent.



   The client must construct data with the first four octets containing

   the original server-issued checksum in network byte order, the fifth

   octet containing the bit-mask specifying the selected security layer,

   the sixth through eighth octets containing in network byte order the

   maximum cipher-text buffer size the client is able to receive, and

   the following octets containing the authorization identity.  The

   client must then append from one to eight zero-valued octets so that

   the length of the data is a multiple of eight octets. The client must

   then encrypt using DES PCBC mode the data with the session key and

   respond with the encrypted data.  The server decrypts the data and

   verifies the contained checksum.  The server must verify that the

   principal identified in the Kerberos ticket is authorized to connect

   as that authorization identity.  After this verification, the

   authentication process is complete.







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   The security layers and their corresponding bit-masks are as follows:



      1 No security layer

      2 Integrity (krb_mk_safe) protection

      4 Privacy (krb_mk_priv) protection



   Other bit-masks may be defined in the future; bits which are not

   understood must be negotiated off.



   EXAMPLE: The following are two Kerberos version 4 login scenarios to

   the IMAP4 protocol (note that the line breaks in the sample

   authenticators are for editorial clarity and are not in real

   authenticators)



     S: * OK IMAP4 Server

     C: A001 AUTHENTICATE KERBEROS_V4

     S: + AmFYig==

     C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT

        +nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd

        WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh

     S: + or//EoAADZI=

     C: DiAF5A4gA+oOIALuBkAAmw==

     S: A001 OK Kerberos V4 authentication successful





     S: * OK IMAP4 Server

     C: A001 AUTHENTICATE KERBEROS_V4

     S: + gcfgCA==

     C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT

        +nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd

        WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh

     S: A001 NO Kerberos V4 authentication failed



7.2.  GSSAPI mechanism



   The mechanism name associated with all mechanisms employing the

   GSSAPI [RFC 2078] is "GSSAPI".



7.2.1 Client side of authentication protocol exchange



   The client calls GSS_Init_sec_context, passing in 0 for

   input_context_handle (initially) and a targ_name equal to output_name

   from GSS_Import_Name called with input_name_type of

   GSS_C_NT_HOSTBASED_SERVICE and input_name_string of

   "service@hostname" where "service" is the service name specified in

   the protocol's profile, and "hostname" is the fully qualified host

   name of the server.  The client then responds with the resulting

   output_token.  If GSS_Init_sec_context returns GSS_S_CONTINUE_NEEDED,







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   then the client should expect the server to issue a token in a

   subsequent challenge.  The client must pass the token to another call

   to GSS_Init_sec_context, repeating the actions in this paragraph.



   When GSS_Init_sec_context returns GSS_S_COMPLETE, the client takes

   the following actions: If the last call to GSS_Init_sec_context

   returned an output_token, then the client responds with the

   output_token, otherwise the client responds with no data.  The client

   should then expect the server to issue a token in a subsequent

   challenge.  The client passes this token to GSS_Unwrap and interprets

   the first octet of resulting cleartext as a bit-mask specifying the

   security layers supported by the server and the second through fourth

   octets as the maximum size output_message to send to the server.  The

   client then constructs data, with the first octet containing the

   bit-mask specifying the selected security layer, the second through

   fourth octets containing in network byte order the maximum size

   output_message the client is able to receive, and the remaining

   octets containing the authorization identity.  The client passes the

   data to GSS_Wrap with conf_flag set to FALSE, and responds with the

   generated output_message.  The client can then consider the server

   authenticated.



7.2.2 Server side of authentication protocol exchange



   The server passes the initial client response to

   GSS_Accept_sec_context as input_token, setting input_context_handle

   to 0 (initially).  If GSS_Accept_sec_context returns

   GSS_S_CONTINUE_NEEDED, the server returns the generated output_token

   to the client in challenge and passes the resulting response to

   another call to GSS_Accept_sec_context, repeating the actions in this

   paragraph.



   When GSS_Accept_sec_context returns GSS_S_COMPLETE, the client takes

   the following actions: If the last call to GSS_Accept_sec_context

   returned an output_token, the server returns it to the client in a

   challenge and expects a reply from the client with no data.  Whether

   or not an output_token was returned (and after receipt of any

   response from the client to such an output_token), the server then

   constructs 4 octets of data, with the first octet containing a bit-

   mask specifying the security layers supported by the server and the

   second through fourth octets containing in network byte order the

   maximum size output_token the server is able to receive.  The server

   must then pass the plaintext to GSS_Wrap with conf_flag set to FALSE

   and issue the generated output_message to the client in a challenge.

   The server must then pass the resulting response to GSS_Unwrap and

   interpret the first octet of resulting cleartext as the bit-mask for

   the selected security layer, the second through fourth octets as the

   maximum size output_message to send to the client, and the remaining







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   octets as the authorization identity.  The server must verify that

   the src_name is authorized to authenticate as the authorization

   identity.  After these verifications, the authentication process is

   complete.



7.2.3 Security layer



   The security layers and their corresponding bit-masks are as follows:



     1 No security layer

     2 Integrity protection.

       Sender calls GSS_Wrap with conf_flag set to FALSE

     4 Privacy protection.

       Sender calls GSS_Wrap with conf_flag set to TRUE



   Other bit-masks may be defined in the future; bits which are not

   understood must be negotiated off.



7.3.  S/Key mechanism



   The mechanism name associated with S/Key [RFC 1760] using the MD4

   digest algorithm is "SKEY".



   The client sends an initial response with the authorization identity.



   The server then issues a challenge which contains the decimal

   sequence number followed by a single space and the seed string for

   the indicated authorization identity.  The client responds with the

   one-time-password, as either a 64-bit value in network byte order or

   encoded in the "six English words" format.



   The server must verify the one-time-password.  After this

   verification, the authentication process is complete.



   S/Key authentication does not provide for any security layers.



   EXAMPLE: The following are two S/Key login scenarios in the IMAP4

   protocol.



     S: * OK IMAP4 Server

     C: A001 AUTHENTICATE SKEY

     S: +

     C: bW9yZ2Fu

     S: + OTUgUWE1ODMwOA==

     C: Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==

     S: A001 OK S/Key authentication successful











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     S: * OK IMAP4 Server

     C: A001 AUTHENTICATE SKEY

     S: +

     C: c21pdGg=

     S: + OTUgUWE1ODMwOA==

     C: BsAY3g4gBNo=

     S: A001 NO S/Key authentication failed



   The following is an S/Key login scenario in an IMAP4-like protocol

   which has an optional "initial response" argument to the AUTHENTICATE

   command.



     S: * OK IMAP4-Like Server

     C: A001 AUTHENTICATE SKEY bW9yZ2Fu

     S: + OTUgUWE1ODMwOA==

     C: Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==

     S: A001 OK S/Key authentication successful



7.4.  External mechanism



   The mechanism name associated with external authentication is

   "EXTERNAL".



   The client sends an initial response with the authorization identity.



   The server uses information, external to SASL, to determine whether

   the client is authorized to authenticate as the authorization

   identity.  If the client is so authorized, the server indicates

   successful completion of the authentication exchange; otherwise the

   server indicates failure.



   The system providing this external information may be, for example,

   IPsec or TLS.



   If the client sends the empty string as the authorization identity

   (thus requesting the authorization identity be derived from the

   client's authentication credentials), the authorization identity is

   to be derived from authentication credentials which exist in the

   system which is providing the external authentication.

























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8.    References



   [RFC 2060] Crispin, M., "Internet Message Access Protocol - Version

              4rev1", RFC 2060, December 1996.



   [RFC 2078] Linn, J., "Generic Security Service Application Program

              Interface, Version 2", RFC 2078, January 1997.



   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate

              Requirement Levels", RFC 2119, March 1997.



   [RFC 2223] Postel, J., and J. Reynolds, "Instructions to RFC

              Authors", RFC 2223, October 1997.



   [RFC 1760] Haller, N., "The S/Key One-Time Password System", RFC

              1760, February 1995.



   [RFC 1700] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,

              RFC 1700, October 1994.



9.    Security Considerations



   Security issues are discussed throughout this memo.



   The mechanisms that support integrity protection are designed such

   that the negotiation of the security layer and authorization identity

   is integrity protected.  When the client selects a security layer

   with at least integrity protection, this protects against an active

   attacker hijacking the connection and modifying the authentication

   exchange to negotiate a plaintext connection.



   When a server or client supports multiple authentication mechanisms,

   each of which has a different security strength, it is possible for

   an active attacker to cause a party to use the least secure mechanism

   supported.  To protect against this sort of attack, a client or

   server which supports mechanisms of different strengths should have a

   configurable minimum strength that it will use.  It is not sufficient

   for this minimum strength check to only be on the server, since an

   active attacker can change which mechanisms the client sees as being

   supported, causing the client to send authentication credentials for

   its weakest supported mechanism.





















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   The client's selection of a SASL mechanism is done in the clear and

   may be modified by an active attacker.  It is important for any new

   SASL mechanisms to be designed such that an active attacker cannot

   obtain an authentication with weaker security properties by modifying

   the SASL mechanism name and/or the challenges and responses.



   Any protocol interactions prior to authentication are performed in

   the clear and may be modified by an active attacker.  In the case

   where a client selects integrity protection, it is important that any

   security-sensitive protocol negotiations be performed after

   authentication is complete.  Protocols should be designed such that

   negotiations performed prior to authentication should be either

   ignored or revalidated once authentication is complete.



10.   Author's Address



   John G. Myers

   Netscape Communications

   501 E. Middlefield Road

   Mail Stop MV-029

   Mountain View, CA 94043-4042



   EMail: jgmyers@netscape.com

























































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Appendix A. Relation of SASL to Transport Security



   Questions have been raised about the relationship between SASL and

   various services (such as IPsec and TLS) which provide a secured

   connection.



   Two of the key features of SASL are:



   1. The separation of the authorization identity from the identity in

      the client's credentials.  This permits agents such as proxy

      servers to authenticate using their own credentials, yet request

      the access privileges of the identity for which they are proxying.



   2. Upon successful completion of an authentication exchange, the

      server knows the authorization identity the client wishes to use.

      This allows servers to move to a "user is authenticated" state in

      the protocol.



   These features are extremely important to some application protocols,

   yet Transport Security services do not always provide them.  To

   define SASL mechanisms based on these services would be a very messy

   task, as the framing of these services would be redundant with the

   framing of SASL and some method of providing these important SASL

   features would have to be devised.



   Sometimes it is desired to enable within an existing connection the

   use of a security service which does not fit the SASL model.  (TLS is

   an example of such a service.)  This can be done by adding a command,

   for example "STARTTLS", to the protocol.  Such a command is outside

   the scope of SASL, and should be different from the command which

   starts a SASL authentication protocol exchange.



   In certain situations, it is reasonable to use SASL underneath one of

   these Transport Security services.  The transport service would

   secure the connection, either service would authenticate the client,

   and SASL would negotiate the authorization identity.  The SASL

   negotiation would be what moves the protocol from "unauthenticated"

   to "authenticated" state.  The "EXTERNAL" SASL mechanism is

   explicitly intended to handle the case where the transport service

   secures the connection and authenticates the client and SASL

   negotiates the authorization identity.



   When using SASL underneath a sufficiently strong Transport Security

   service, a SASL security layer would most likely be redundant.  The

   client and server would thus probably want to negotiate off the use

   of a SASL security layer.











Myers                       Standards Track                    [Page 15]



RFC 2222                          SASL                      October 1997





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Myers                       Standards Track                    [Page 16]



