Subject: draft SASL base spec

From: John Gardiner Myers <[email-masked]>
Date: Tue, 20 Mar 2001 19:36:20 -0800

Network Working Group                                           J. Myers
Internet Draft                                                March 2001
Document: draft-myers-saslrev-XX.txt

            Simple Authentication and Security Layer (SASL)

Status of this Memo

   This document is an Internet Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet Drafts are working documents of the Internet Engineering
   Task Force (IETF), its Areas, and its Working Groups.  Note that
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   To learn the current status of any Internet-Draft, please check the
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   A revised version of this draft document will be submitted to the RFC
   editor as a Draft Standard for the Internet Community.  Discussion
   and suggestions for improvement are requested.  Distribution of this
   draft is unlimited.

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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 "Overview", "Authentication Mechanisms", "Protocol
   Profile Requirements", "Specific Issues", 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" [KEYWORDS].

2.3.  Examples

   Examples in this document are for the IMAP profile [IMAP4] 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.

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3.    Overview

   The Simple Authentication and Security Layer (SASL) is a method for
   adding authentication support to connection-based protocols.

   The SASL specification has three layers, as indicated in the diagram
   below.  At the top, a protocol definition using SASL specifies a
   profile, including a command for identifying and authenticating a
   user to a server and for optionally negotiating a security layer for
   subsequent protocol interactions.  At the bottom, a SASL mechanism
   definition specifies an authentication mechanism.  The SASL
   framework, specified by this document, constrains the behavior of
   protocol profiles and mechanisms, separating protocol from mechanism
   and defining how they interact.

                SMTP Protocol     LDAP Protocol          Etc
                   Profile           Profile            . . .
                          \-----        |       -----/
                                \       |      /
                                 SASL framework
                                /       |      \
                          /-----        |       -----\
                   EXTERNAL         DIGEST-MD5           Etc
                SASL mechanism    SASL mechanism        . . .

   This separation between the definition of protocols and the
   definition of authentication mechanisms is crucial.  It permits an
   authentication mechanism to be defined once, making it usable by any
   SASL protocol profiles.  In many implementations, the same SASL
   mechanism code is used for multiple protocols.

4.    Authentication mechanisms

   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"

4.1.  Authentication protocol exchange

   A SASL mechanism is responsible for conducting an authentication
   protocol exchange.  This consists of a series of server challenges
   and client responses, the contents of which are specific to and
   defined by the mechanism.  To the protocol, the challenges and
   responses are opaque binary tokens of arbitrary length.  The
   protocol's profile then specifies how these binary tokens are then
   encoded for transfer over the connection.

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   After receiving an authentication command or any client response, a
   server mechanism may issue a challenge, indicate failure, or indicate
   completion.  The protocol's profile specifies how each of these is
   then represented over the connection.

   After receiving a challenge, a client mechanism may issue a response
   or abort the exchange.  The protocol's profile specifies how each of
   these is then represented over the connection.

   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 security layer buffer size that each side is able to receive.

4.2.  Proxy authentication

   The authorization identity transmitted during the authentication
   protocol exchange may be different than the identity derived from the
   client's authentication credentials.  When the two are different,
   this is typically referred to as "proxy authentication".  The
   identity derived from the client's authentication credentials is
   known as the "authentication identity".

   This feature 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 identity.

4.3.  Security layers

   If use of a security layer is negotiated by the authentication
   protocol exchange, the security layer 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 security encoded
   data.  Each buffer of security encoded data 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 security encoded data buffer must be no
   larger than the maximum size that was either defined in the mechanism

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   specification or negotiated by the other side during the
   authentication protocol exchange.

5.    Protocol profile 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. [GSSAPI]
      This service name is made available to the authentication
      mechanism.

   2. A definition of the command to initiate the authentication
      protocol exchange.  This command must have as a parameter the name
      of the mechanism 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
      6.1 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.

   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.

6.    Specific issues

6.1.  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

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   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.)

6.2.  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.

6.3.  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.

   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.

7.    Registration procedures

   Registration of a SASL mechanism is done by filling in the template
   in section 7.4 and sending it in to [email-masked].  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.

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   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.

7.1.  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.

7.2.  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 RFC-1700].  The SASL
   mechanism description and other supporting material may also be
   published as an Informational RFC by sending it to
   "[email-masked]" (please follow the instructions to RFC authors
   [RFC-1543]).

7.3.  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.

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   The IESG is considered to be the owner of all SASL mechanisms which
   are on the IETF standards track.

7.4.  Registration template

     To: [email-masked]
     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)

     Owner/Change controller:

     (Any other information that the author deems interesting may be
     added below this line.)

8.    The 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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9.    References

   [IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4",
   RFC 1730, University of Washington, December 1994.

   [GSSAPI] Linn, J., "Generic Security Service Application Program
   Interface, Version 2", RFC 2078, January 1997

   [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, March 1997

   [RFC-1543] Postel, J., "Instructions to RFC Authors", RFC 1543,
   October 1993

   [SASL-GSSAPI] Myers, J., "SASL GSSAPI mechanisms", draft-ietf-cat-
   sasl-gssapi-XX.txt, September 2000

   [SASL-OTP] Newman, C., "The One-Time-Password SASL Mechanism", RFC
   2444, October 1998

10.   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.

   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.

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   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.

11.   Author's address

   John G. Myers
   Netscape Communications
   501 E. Middlefield Road
   Mail Stop SCA 15:201
   Mountain View, CA 94043-4042

   Email: [email-masked]

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

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   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.

Appendix B. IANA considerations

   The IANA is directed to modify the SASL mechanisms registry as
   follows:

   1. Change the "Intended usage" of the KERBEROS_V4 and SKEY mechanism
      registrations to OBSOLETE.

   2. Change the "Published specification" of the EXTERNAL mechanism to
      this document.

Appendix C. Changes since RFC 2222

   The GSSAPI mechanism was removed.  It is now specified in a separate
   document [SASL-GSSAPI].

   The "KERBEROS_V4" mechanism defined in RFC 2222 is obsolete and has
   been removed.

   The "SKEY" mechanism described in RFC 2222 is obsolete and has been
   removed.  It has been replaced by the OTP mechanism [SASL-OTP].

   The overview has been substantially reorganized and clarified.

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                           Table of Contents

Status of this Memo ...............................................    i
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 ....................................................    2
3.    Overview ....................................................    3
4.    Authentication mechanisms ...................................    3
4.1.  Authentication protocol exchange ............................    3
4.2.  Proxy authentication ........................................    4
4.3.  Security layers .............................................    4
5.    Protocol profile requirements ...............................    5
6.    Specific issues .............................................    5
6.1.  Client sends data first .....................................    5
6.2.  Server returns success with additional data .................    6
6.3.  Multiple authentications ....................................    6
7.    Registration procedures .....................................    6
7.1.  Comments on SASL mechanism registrations ....................    7
7.2.  Location of registered SASL mechanism list ..................    7
7.3.  Change control ..............................................    7
7.4.  Registration template .......................................    8
8.    The external mechanism ......................................    8
9.    References ..................................................    9
10.   Security considerations .....................................    9
11.   Author's address ............................................   10
Appendix A. Relation of SASL to transport security ................   10
Appendix B. IANA considerations ...................................   11
Appendix C. Changes since RFC 2222 ................................   11

J. Myers                                                       [Page ii]