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Versions: (RFC 2222) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 4422

INTERNET-DRAFT                                A. Melnikov, Ed.
Intended Category: Standards Track            ISODE Limited
Expires in six months                         K. Zeilenga, Ed.
Obsoletes: RFC 2222                           OpenLDAP Project
                                              17 November 2005



             Simple Authentication and Security Layer (SASL)
                   <draft-ietf-sasl-rfc2222bis-14.txt>


Status of this Memo

  By submitting this Internet-Draft, each author represents that any
  applicable patent or other IPR claims of which he or she is aware have
  been or will be disclosed, and any of which he or she becomes aware
  will be disclosed, in accordance with Section 6 of BCP 79.

  Internet-Drafts are working documents of the Internet Engineering Task
  Force (IETF), its areas, and its working groups. Note that other
  groups may also distribute working documents as Internet-Drafts.

  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time. It is inappropriate to use Internet-Drafts as reference material
  or to cite them other than as "work in progress."

  The list of current Internet-Drafts can be accessed at
  http://www.ietf.org/1id-abstracts.html

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  http://www.ietf.org/shadow.html


  Copyright (C) The Internet Society (2005).  All Rights Reserved.

  Please see the Full Copyright section near the end of this document
  for more information.


Abstract

  The Simple Authentication and Security Layer (SASL) is a framework for
  providing authentication and data security services in
  connection-oriented protocols via replaceable mechanisms.  It provides
  a structured interface between protocols and mechanisms.  The
  resulting framework allows new protocols to reuse existing mechanisms



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  and allows old protocols to make use of new mechanisms.  The framework
  also provides a protocol for securing subsequent protocol exchanges
  within a data security layer.

  This document describes how a SASL mechanism is structured, describes
  how protocols include support for SASL, and defines the protocol for
  carrying a data security layer over a connection.  Additionally, this
  document defines one SASL mechanism, the EXTERNAL mechanism.

  This document obsoletes RFC 2222.

Table of Contents

  [[Page numbers to be filled in by RFC-Editor]]

  Status of this Memo
  Abstract
  1. Introduction
  1.1. Document Audiences
  1.2. Relationship to Other Documents
  1.3. Conventions
  2. Identity Concepts
  3. The Authentication Exchange
  3.1. Mechanism Naming
  3.2. Mechanism Negotiation
  3.3. Request Authentication Exchange
  3.4. Challenges and Responses
  3.4.1. Authorization Identity String
  3.5. Aborting Authentication Exchanges
  3.6. Authentication Outcome
  3.7. Security Layers
  3.8. Multiple Authentications
  4. Protocol Requirements
  5. Mechanism Requirements
  6. Security Considerations
  6.1. Active Attacks
  6.1.1. Man-in-the-middle Attacks
  6.1.2. Replay Attacks
  6.1.3. Truncation Attacks
  6.2. Passive Attacks
  6.3. Re-keying
  6.4. Other considerations
  7. IANA Considerations
  8. References
  9. Editors' Address
  10. Acknowledgments
  A. The SASL EXTERNAL Mechanism
  B. Changes since RFC 2222



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1.    Introduction

  The Simple Authentication and Security Layer (SASL) is a framework for
  providing authentication and data security services in
  connection-oriented protocols via replaceable mechanisms.  SASL
  provides a structured interface between protocols and mechanisms.
  SASL also provides a protocol for securing subsequent protocol
  exchanges within a data security layer.  The data security layer can
  provide data integrity, data confidentiality, and other services.

  SASL's design is intended to allow new protocols to reuse existing
  mechanisms without requiring redesign of the mechanisms and allows
  existing protocols to make use of new mechanisms without redesign of
  protocols.

  SASL is conceptually a framework which provides an abstraction layer
  between protocols and mechanisms as illustrated in the following
  diagram.

            SMTP    LDAP    XMPP   Other protocols ...
               \       |    |      /
                \      |    |     /
               SASL abstraction layer
                /      |    |     \
               /       |    |      \
        EXTERNAL   GSSAPI  PLAIN   Other mechanisms ...

  It is through the interfaces of this abstraction layer that the
  framework allows any protocol to utilize any mechanism.  While this
  layer does generally hide the particulars of protocols from mechanisms
  and the particulars of mechanisms from protocols, this layer does not
  generally hide the particulars of mechanisms from protocol
  implementations.  For example, different mechanisms require different
  information to operate, some of them use password-based
  authentication, some of then require realm information, others make
  use of Kerberos tickets, certificates, etc..  Also, in order to
  perform authorization, server implementations generally have to
  implement identity mapping between authentication identities, whose
  form is mechanism-specific, and authorization identities, whose form
  is application protocol specific.  Section 2 discusses identity
  concepts.

  It is possible to design and implement this framework in ways which do
  abstract away particulars of similar mechanisms.  Such a framework
  implementation, as well as mechanisms implementations, could be
  designed not only to be shared by multiple implementations of a
  particular protocol, but be shared by implementations of multiple
  protocols.



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  The framework incorporates interfaces with both protocols and
  mechanisms in which authentication exchanges are carried out.  Section
  3 discusses SASL authentication exchanges.

  To use SASL, each protocol (amongst other items) provides a method for
  identifying which mechanism is to be used, provides a method for
  exchange of mechanism-specific server-challenges and client-responses,
  and a method for communicating the outcome of the authentication
  exchange.  Section 4 discusses SASL protocol requirements.

  Each SASL mechanism defines (amongst other items) a series of server
  challenges and client responses which provide authentication services
  and negotiate data security services.  Section 5 discusses SASL
  mechanism requirements.

  Section 6 discusses security considerations.  Section 7 discusses IANA
  considerations.  Appendix A defines the SASL EXTERNAL mechanism.


1.1.  Document Audiences

  This document is written to serve several different audiences:

    - protocol designers using this specification to support
      authentication in their protocol,

    - mechanism designers that define new SASL mechanisms, and

    - implementors of clients or servers for those protocols which
      support SASL.

  While the document organization is intended to allow readers to focus
  on details relevant to their engineering, readers are encouraged to
  read and understand all aspects of this document.


1.2.  Relationship to other documents

  This document obsoletes RFC 2222.  It replaces all portions of RFC
  2222 excepting sections 7.1 (the KERBEROS_IV mechanism), 7.2 (the
  GSSAPI mechanism), 7.3 (the SKEY mechanism).  The KERBEROS_IV and SKEY
  mechanisms are now viewed as obsolete and their specifications
  provided in RFC 2222 are Historic.  The GSSAPI mechanism is now
  separately specified [SASL-GSSAPI].

  Appendix B provides a summary of changes since RFC 2222.





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1.3.  Conventions

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in BCP 14 [RFC2119].

  Character names in this document use the notation for code points and
  names from the Unicode Standard [Unicode].  For example, the letter
  "a" may be represented as either <U+0061> or <LATIN SMALL LETTER A>.

  Note: a glossary of terms used in Unicode can be found in [Glossary].
  Information on the Unicode character encoding model can be found in
  [CharModel].

  In examples, "C:" and "S:" indicate lines of data to be sent by the
  client and server respectively.  Lines have been wrapped for improved
  readability.


2.  Identity Concepts

  In practice, authentication and authorization may involve multiple
  identities, possibly in different forms (simple username, Kerberos
  principal, X.500 Distinguished Name, etc.), possibly with different
  representations (e.g.: ABNF-described UTF-8 encoded Unicode character
  string, BER-encoded Distinguished Name).  While technical
  specifications often prescribe both the identity form and
  representation used on the network, different identity forms and/or
  representations may (and often are) used within implementations.  How
  identities of different forms relate to each other is, generally, a
  local matter.  Additionally, the forms and representations used within
  an implementation is a local matter.

  However, conceptually, SASL framework involves two identities:
    1) an identity associated with the authentication credentials
       (termed the authentication identity), and
    2) an identity to act as (termed the authorization identity).

  SASL mechanism specifications describe the credential form(s) (e.g.,
  X.509 certificates, Kerberos tickets, simple username/password) used
  to authenticate the client, including (where appropriate) the syntax
  and semantics of associated authentication identities.  SASL protocol
  specifications describe the identity form(s) used in authorization
  and, in particular, prescribe the syntax and semantics of the
  authorization identity character string to be transferred by
  mechanisms.

  The client provides its credentials which (implicitly or explicitly)



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  include an authentication identity and, optionally, a character string
  representing the requested authorization identity as part of the SASL
  exchange.  When this character string is omitted or empty, the client
  is requesting to act as the identity associated with the credentials
  (e.g., the user is requesting to act as the authentication identity).

  The server is responsible for verifying the client's credentials and
  verifying that the client is allowed to act as the authorization
  identity.  A SASL exchange fails if either (or both) of these
  verifications fails.  (The SASL exchange may fail for other reasons,
  such as service authorization failure.)

  However, the precise form(s) of the authentication identities (used
  within the server in its verifications, or otherwise) and the precise
  form(s) of the authorization identities (used in making authorization
  decisions, or otherwise) is beyond the scope of SASL and this
  specification.  In some circumstances, the precise identity forms used
  in some context outside of the SASL exchange may be dictated by other
  specifications.  For instance, an identity assumption authorization
  (proxy authorization) policy specification may dictate how
  authentication and authorization identities are represented in policy
  statements.


3.  The Authentication Exchange

  Each authentication exchange consists of a message from the client to
  the server requesting authentication via a particular mechanism,
  followed by one or more pairs of challenges from the server and
  responses from the client, followed by a message from the server
  indicating the outcome of the authentication exchange.  (Note:
  exchanges may also be aborted as discussed in Section 3.5.)

  The following illustration provides a high-level overview of an
  authentication exchange.

      C: Request authentication exchange
      S: Initial challenge
      C: Initial response
      <additional challenge/response messages>
      S: Outcome of authentication exchange

  If the outcome is successful and a security layer was negotiated, this
  layer is then installed (see Section 3.7).  This also applies to the
  following illustrations.

  Some mechanisms specify that the first data sent in the authentication
  exchange is from the client to the server.  Protocols may provide an



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  optional initial response field in the request message to carry this
  data.  Where the mechanism specifies the first data sent in the
  exchange is from the client to the server, the protocol provides an
  optional initial response field, and the client uses this field, the
  exchange is shortened by one round-trip:

      C: Request authentication exchange + Initial response
      <additional challenge/response messages>
      S: Outcome of authentication exchange

  Where the mechanism specifies the first data sent in the exchange is
  from the client to the server and this field is unavailable or unused,
  the client request is followed by an empty challenge.

      C: Request authentication exchange
      S: Empty Challenge
      C: Initial Response
      <additional challenge/response messages>
      S: Outcome of authentication exchange

  Should a client include an initial response in its request where the
  mechanism does not allow the client to send data first, the
  authentication exchange fails.

  Some mechanisms specify that the server is to send additional data to
  the client when indicating a successful outcome.  Protocols may
  provide an optional additional data field in the outcome message to
  carry this data.  Where the mechanism specifies the server is to
  return additional data with the successful outcome, the protocol
  provides an optional additional data field in the outcome message, and
  the server uses this field, the exchange is shortened by one
  round-trip:

      C: Request authentication exchange
      S: Initial challenge
      C: Initial response
      <additional challenge/response messages>
      S: Outcome of authentication exchange with
         additional data with success

  Where the mechanism specifies the server is to return additional data
  to the client with a successful outcome and this field is unavailable
  or unused, the additional data is sent as a challenge whose response
  is empty.  After receiving this response, the server then indicates
  the successful outcome.

      C: Request authentication exchange
      S: Initial challenge



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      C: Initial response
      <additional challenge/response messages>
      S: Additional data challenge
      C: Empty Response
      S: Outcome of authentication exchange


  Where mechanisms specify the first data sent in the exchange is from
  the client to the server and additional data is sent to the client
  along with indicating a successful outcome, and the protocol provides
  fields supporting both, the exchange can be shorted by two
  round-trips:

      C: Request authentication exchange + Initial response
      <additional challenge/response messages>
      S: Outcome of authentication exchange
         with additional data with success

  instead of:

      C: Request authentication exchange
      S: Empty Challenge
      C: Initial Response
      <additional challenge/response messages>
      S: Additional data challenge
      C: Empty Response
      S: Outcome of authentication exchange


3.1. Mechanism Naming

  SASL mechanisms are named by character strings, from 1 to 20
  characters in length, consisting of ASCII [ASCII] uppercase letters,
  digits, hyphens, and/or underscores.  In the following Augmented
  Backus-Naur Form (ABNF) [RFC4234] grammar, the <sasl-mech> production
  defines the syntax of a SASL mechanism name.

      sasl-mech    = 1*20mech-char
      mech-char    = UPPER-ALPHA / DIGIT / HYPHEN / UNDERSCORE
      ; mech-char is restricted to A-Z (uppercase only), 0-9, -, and _
      ; from ASCII character set.

      UPPER-ALPHA  = %x41-5A  ; A-Z (uppercase only)
      DIGIT        = %x30-39  ; 0-9
      HYPHEN       = %x2D ; hyphen (-)
      UNDERSCORE   = %x5F ; underscore (_)

  SASL mechanisms names are registered as discussed in Section 7.1.



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3.2. Mechanism Negotiation

  Mechanism negotiation is protocol-specific.

  Commonly, a protocol will specify that the server advertises supported
  and available mechanisms to the client via some facility provided by
  the protocol and the client will then select the "best" mechanism from
  this list which its supports and finds suitable.

  It is noted that the mechanism negotiation is not protected by the
  subsequent authentication exchange and hence is subject to downgrade
  attacks if not protected by other means.

  To detect downgrade attacks, a protocol can allow the client to
  discover available mechanisms subsequent to the authentication
  exchange and installation of data security layers with at least data
  integrity protection.  This allows the client to detect changes to the
  list of mechanisms supported by the server.


3.3. Request Authentication Exchange

  The authentication exchange is initiated by the client by requesting
  authentication via a mechanism it specifies.  The client sends a
  message that contains the name of the mechanism to the server.  The
  particulars of the message are protocol specific.

  It is noted that the name of the mechanism is not protected by the
  mechanism, and hence subject to alteration by an attacker if not
  integrity protected by other means.

  Where the mechanism is defined to allow the client to send data first,
  and the protocol's request message includes an optional initial
  response field, the client may include the response to the initial
  challenge in the authentication request message.


3.4. Challenges and Responses

  The authentication exchange involves one or more pairs of
  server-challenges and client-responses, the particulars of which are
  mechanism specific.  These challenges and responses are enclosed in
  protocol messages, the particulars of which are protocol specific.

  Through these challenges and responses, the mechanism may:
    - authenticate the client to the server,
    - authenticate the server to the client,
    - transfer an authorization identity string,



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    - negotiate a security layer, and
    - provide other services.

  The negotiation of the security layer may involve negotiation of the
  security services to be provided in the layer, how these services will
  be provided, and negotiation of a maximum cipher-text buffer size each
  side is able to receive in the layer (see Section 3.6).

  After receiving an authentication request or any client response, the
  server may issue a challenge, abort the exchange, or indicate the
  outcome of an exchange.  After receiving a challenge, a client
  mechanism may issue a response or abort the exchange.


3.4.1.  Authorization Identity String

  The authorization identity string is a sequence of zero or more
  Unicode [Unicode] characters, excluding the NUL (U+0000) character,
  representing the identity to act as.

  If the authorization identity string is absent, the client is
  requesting to act as the identity the server associates with the
  client's credentials.  An empty string is equivalent to an absent
  authorization identity.

  Non-empty authorization identity string indicates the client wishes to
  act as the identity represented by the string.  In this case, the form
  of identity represented by the string, as well as the precise syntax
  and semantics of the string, is protocol specific.

  While the character encoding schema used to transfer the authorization
  identity string in the authentication exchange is mechanism specific,
  mechanisms are expected to be capable of carrying the entire Unicode
  repertoire (with the exception of the NUL character).


3.5. Aborting Authentication Exchanges

  A client or server may desire to abort an authentication exchange if
  it is unwilling or unable to continue (or enter into).

  A client may abort the authentication exchange by sending a message,
  the particulars of which are protocol-specific, to the server,
  indicating the exchange is aborted.  The server may be required by the
  protocol to return a message in response to the client's abort
  message.

  Likewise, a server may abort the authentication exchange by sending a



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  message, the particulars of which are protocol-specific, to the
  client, indicating the exchange is aborted.


3.6.  Authentication Outcome

  At the conclusion of the authentication exchange, the server sends a
  message, the particulars of which are protocol specific, to the client
  indicating the outcome of the exchange.

  The outcome is not successful if
    - the authentication exchange failed for any reason,
    - the clients credentials could not be verified,
    - the server cannot associate an identity with the client's
      credentials,
    - the client-provided authorization identity string is malformed,
    - the identity associated with the client's credentials is not
      authorized to act as the requested authorization identity,
    - the negotiated security layer (or lack thereof) is not suitable,
      or
    - the server is not willing to provide service to the client for any
      reason.

  The protocol may include an optional additional data field in this
  outcome message.  This field can only include additional data when the
  outcome is successful.

  If the outcome is successful and a security layer was negotiated, this
  layer is then installed.  If the outcome is unsuccessful, or a
  security layer was not negotiated, any existing security is left in
  place.

  The outcome message provided by the server can provide a way for the
  client to distinguish between errors which are best dealt with by re-
  prompting the user for her credentials, errors which are best dealt
  with by telling the user to try again later, and errors where the user
  must contact a system administrator for resolution (see The SYS and
  AUTH POP Response Codes [RFC3206] specification for an example).  This
  distinction is particularly useful during scheduled server maintenance
  periods as it reduces support costs.  It is also important that the
  server can be configured such that the outcome message will not
  distinguish between a valid user with invalid credentials and an
  invalid user.


3.7.  Security Layers

  SASL mechanisms may offer a wide range of services in security layers.



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  Typical services include data integrity and data confidentiality.
  SASL mechanisms which do not provide a security layer are treated as
  negotiating no security layer.

  If use of a security layer is negotiated in the authentication
  protocol exchange, the layer is installed by the server after
  indicating the outcome of the authentication exchange and installed by
  the client upon receipt the outcome indication.  In both cases, the
  layer is installed before transfer of further protocol data.  The
  precise position that the layer takes effect in the protocol data
  stream is protocol specific.

  Once the security layer is in effect in the protocol data stream, it
  remains in effect until either a subsequently negotiated security
  layer is installed, or the underlying transport connection is closed.

  When in effect, the security layer processes protocol data into
  buffers of protected data.  If at any time the security layer is
  unable or unwilling to continue producing buffers protecting protocol
  data, the underlying transport connection MUST be closed.  If the
  security layer is not able to decode a received buffer, the underlying
  connection MUST be closed.  In both cases the underlying transport
  connection SHOULD be closed gracefully.

  Each buffer of protected data is transferred over the underlying
  transport connection as a sequence of octets prepended with a four
  octet field in network byte order that represents the length of the
  buffer.  The length of the protected data buffer MUST be no larger
  than the maximum size the other side expects.  Upon the receipt of a
  length field whose value is greater than maximum size, the receiver
  SHOULD close the connection, as this might be a sign of an attack.

  The maximum size each side expects is fixed by the mechanism, either
  through negotiation or by its specification.


3.8.  Multiple Authentications

  Unless explicitly permitted in the protocol (as stated in the
  protocol's technical specification), only one successful SASL
  authentication exchange may occur in a protocol session.  In this
  case, once an authentication exchange has successfully completed,
  further attempts to initiate an authentication exchange fail.

  Where multiple successful SASL authentication exchanges are permitted
  in the protocol, then in no case may multiple SASL security layers be
  simultaneously in effect.  If a security layer is in effect and a
  subsequent SASL negotiation selects a second security layer, then the



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  second security layer replaces the first.  If a security layer is in
  effect and a subsequent SASL negotiation selects no security layer,
  the original security layer remains in effect.

  Where multiple successful SASL negotiations are permitted in the
  protocol, the effect of a failed SASL authentication exchange upon the
  previously established authentication and authorization state is
  protocol specific.  The protocol's technical specification should be
  consulted to determine whether the previous authentication and
  authorization state remains in force, or changed to an anonymous
  state, or otherwise effected.  Regardless of the protocol-specific
  effect upon previously established authentication and authorization
  state, the previously negotiated security layer remains in effect.


4.  Protocol Requirements

  In order for a protocol to offer SASL services, its specification MUST
  supply the following information:

  1) A service name, to be selected from registry of "service" elements
     for the GSSAPI host-based service name form, as described in
     Section 4.1 of [RFC2743].  Note that this registry is shared by all
     GSSAPI and SASL mechanisms.

  2) Detail any mechanism negotiation facility the protocol provides
     (see Section 3.2).

     A protocol SHOULD specify a facility through which the client may
     discover, both before initiation of the SASL exchange and after
     installing security layers negotiated by the exchange, the names of
     the SASL mechanisms the server makes available to the client.  The
     latter is important to allow the client to detect downgrade
     attacks.  This facility is typically provided through the
     protocol's extensions or capabilities discovery facility.


  3) Definition of the messages necessary for authentication exchange,
     including:

     a) A message to initiate the authentication exchange (see Section
        3.3).

        This message MUST contain a field for carrying the name of the
        mechanism selected by the client.

        This message SHOULD contain an optional field for carrying an
        initial response.  If the message is defined with this field,



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        the specification MUST describe how messages with an empty
        initial response are distinguished from messages with no initial
        response.  This field MUST be capable of carrying arbitrary
        sequences of octets (including zero length sequences and
        sequences containing zero-valued octets).

     b) Messages to transfer server challenges and client responses.
        (see Section 3.4).

        Each of these messages MUST be capable of carrying arbitrary
        sequences of octets (including zero length sequences and
        sequences containing zero-valued octets).

     c) A message to indicate the outcome of the authentication exchange
        (see Section 3.6).

        This message SHOULD contain an optional field for carrying
        additional data with a successful outcome.  If the message is
        defined with this field, the specification MUST describe how
        messages with an empty additional data are distinguished from
        messages with no additional data.  This field MUST be capable of
        carrying arbitrary sequences of octets (including zero length
        sequences and sequences containing zero-valued octets).


  4) Prescribe the syntax and semantics of non-empty authorization
     identity strings (see Section 3.4.1).

     In order to avoid interoperability problems due to differing
     normalizations, the protocol specification MUST detail precisely
     the how and where (client or server) non-empty authorization
     identity strings are prepared, including all normalizations, for
     comparison and other applicable functions to ensure proper
     function.

     Specifications are encouraged to prescribe use of existing
     authorization identity forms as well as existing string
     representations, such as simple user names [RFC4013].

     Where the specification does not precisely prescribe how identities
     in SASL relate to identities used elsewhere in the protocol, for
     instance in access control policy statements, it may be appropriate
     for the protocol to provide a facility by which the client can
     discover information (such as the representation of the
     authentication identity used in making access control decisions)
     about established identities for these uses.





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  5) Detail any facility the protocol provides that allows the client
     and/or server to abort authentication exchange (see Section 3.5).

     Protocols which support multiple authentications typically allow a
     client to abort an on-going authentication exchange by initiating a
     new authentication exchange.  Protocols which do not support
     multiple authentications may require the client to close the
     connection and start over to abort an on-going authentication
     exchange.

     Protocols typically allow the server to abort on-going
     authentication exchanges by returning a non-successful outcome
     message.


  6) Identify precisely where newly negotiated security layers start to
     take effect, in both directions (see Section 3.7).

     Typically, specifications require security layer to start taking
     effect, in data being sent by the server, on the first octet
     following the outcome message and, in data being sent by the
     client, on the first octet sent after receipt of the outcome
     message.


  7) If the protocol supports other layered security services, such as
     Transport Layer Security (TLS) [RFC2246], the specification MUST
     prescribe the order in which security layers are applied to
     protocol data.

     For instance, where a protocol supports both TLS and SASL security
     layers, the specification could prescribe any of the following:
     a) SASL security layer is always applied first to data being sent
        and, hence, applied last to received data,
     b) SASL security layer is always applied last to data being sent
        and, hence, applied first to received data,
     c) Layers are applied in the order in which they were installed,
     d) Layers are applied in the reverse order in which they were
        installed, or
     e) Both TLS and SASL security layers cannot be installed.


  8) Indicate whether the protocol supports multiple authentications
     (see Section 3.8).  If so, the protocol MUST detail the effect a
     failed SASL authentication exchange will have upon previously
     established authentication and authorization state.





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  Protocol specifications SHOULD avoid stating implementation
  requirements which would hinder replacement of applicable mechanisms.
  In general, protocol specification SHOULD be mechanism neutral.  There
  are a number reasonable exceptions to this recommendation, including:
    - detailing how credentials (which are mechanism-specific) are
      managed in the protocol,
    - detailing how authentication identities (which are
      mechanism-specific) and authorization identities (which are
      protocol-specific) relate to each other, and
    - detailing which mechanisms are applicable to the protocol.


5.  Mechanism Requirements

  SASL mechanism specifications MUST supply the following information:

  1) The name of the mechanism (see Section 3.1).  This name MUST be
     registered as discussed in Section 7.1.


  2) A definition of the server-challenges and client-responses of the
     authentication exchange, as well as:

     a) An indication whether mechanism is client-first, variable, or
        server-first.  If a SASL mechanism is defined as client-first
        and the client does not send an initial response, then the first
        server challenge MUST be empty (the EXTERNAL mechanism is an
        example of this case).  If a SASL mechanism is defined as
        variable, then the specification needs to state how the server
        behaves when the initial client response is omitted (the
        DIGEST-MD5 mechanism [DIGEST-MD5] is an example of this case).
        If a SASL mechanism is defined as server-first then the client
        MUST NOT send an initial client response (the CRAM-MD5 mechanism
        [CRAM-MD5] is an example of this case).


     b) An indication whether the server is expected to provide
        additional data when indicating a successful outcome.  If so, if
        the server sends the additional data as a challenge, the
        specification MUST indicate the response to this challenge is an
        empty response.

     SASL mechanisms SHOULD be designed to minimize the number of
     challenges and responses necessary to complete the exchange.


  3) An indication of whether the mechanism is capable of transferring
     authorization identity strings (see Section 3.4.1).  While some



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     legacy mechanisms are incapable of transmitting an authorization
     identity (which means that for these mechanisms the authorization
     identity is always the empty string), newly defined mechanisms
     SHOULD be capable of transferring authorization identity strings.
     The mechanism SHOULD NOT be capable of transferring both no
     authorization identity string and an empty authorization identity.

     Mechanisms which are capable of transferring an authorization
     identity string MUST be capable of transferring arbitrary non-empty
     sequences of Unicode characters, excluding those which contain the
     NUL (U+0000) character.  Mechanisms SHOULD use the UTF-8 [RFC3629]
     transformation format.  The specification MUST detail how any
     Unicode code points special to the mechanism which might appear in
     the authorization identity string are escaped to avoid ambiguity
     during decoding of the authorization identity string.  Typically,
     mechanisms which have special characters require these special
     characters to be escaped or encoded in the character string (after
     encoding it a particular Unicode transformation format) using a
     data encoding scheme such as Base64 [RFC3548].


  4) The specification MUST detail whether or not the mechanism offers a
     security layer.  If the mechanism does, the specification MUST
     detail the security and other services offered in the layer as well
     as how these services are to be implemented.


  5) If the underlying cryptographic technology used by a mechanism
     supports data integrity, then the mechanism specification MUST
     integrity protect the transmission of an authorization identity and
     the negotiation of the security layer.


  SASL mechanisms SHOULD be protocol neutral.

  SASL mechanisms SHOULD reuse existing credential and identity forms,
  as well as associated syntaxes and semantics.

  SASL mechanisms SHOULD use UTF-8 transformation format [RFC3629] for
  encoding Unicode [Unicode] code points for transfer.

  In order to avoid interoperability problems due to differing
  normalizations, when a mechanism calls for character data (other than
  the authorization identity string) to be used as input to a
  cryptographic and/or comparison function, the specification MUST
  detail precisely how and where (client or server) the character data
  is to be prepared, including all normalizations, for input into the
  function to ensure proper operation.



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  For simple user names and/or passwords in authentication credentials,
  SASLprep [RFC4013] (a profile of the StringPrep [RFC3454] preparation
  algorithm), SHOULD be specified as the preparation algorithm.

  The mechanism SHOULD NOT use the authorization identity string in
  generation of any long-term cryptographic keys or hashes as there is
  no requirement that the authorization identity string be canonical.
  Long-term, here, means a term longer than the duration of the
  authentication exchange in which they were generated in.  That is, as
  different clients (of the same or different protocol) may provide
  different authorization identity strings which are semantically
  equivalent, use of authorization identity strings in generation of
  cryptographic keys and hashes will likely lead to interoperability and
  other problems.


6.  Security Considerations

  Security issues are discussed throughout this memo.

  Many existing SASL mechanisms do not provide adequate protection
  against passive attacks, let alone active attacks, in the
  authentication exchange.  Many existing SASL mechanisms do not offer
  security layers.  It is hoped that future SASL mechanisms will provide
  strong protection against passive and active attacks in the
  authentication exchange, as well as security layers with strong basic
  data security features (e.g., data integrity and data confidentiality)
  services.  It is also hoped that future mechanisms will provide more
  advanced data security services like re-keying (see Section 6.3).

  Regardless, the SASL framework is suspectable to downgrade attacks.
  Section 6.1.2 offers a variety of approaches for preventing or
  detecting these attacks.  In some cases, it is appropriate to use data
  integrity protective services external to SASL (e.g., TLS [TLS]) to
  protect against downgrade attacks in SASL.  Use of external protective
  security services is also important when the mechanisms available do
  not themselves offer adequate integrity and/or confidentiality
  protection of the authentication exchange and/or protocol data.


6.1. Active Attacks

6.1.1. Hijack Attacks

  When the client selects a SASL security layer with at least integrity
  protection, this protect serves as a counter-measure against an active
  attacker hijacking the connection and modifying protocol data sent
  after establishment of the security layer.  Implementations should



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  close the connection when the security services in an SASL security
  layer report protocol data report lack of data integrity.


6.1.2. Downgrade Attacks

  It is important that any security-sensitive protocol negotiations be
  performed after installation of security layer with data integrity
  protection.  Protocols should be designed such that negotiations
  performed prior to this installation should be revalidated after
  installation is complete.  Negotiation of the SASL mechanism is
  security-sensitive.

  When a client negotiates the authentication mechanism with the server
  and/or other security features, it is possible for an active attacker
  to cause a party to use the least secure security services available.
  For instance, an attacker can modify the server-advertised mechanism
  list or can modify client-advertised security feature list within a
  mechanism response.  To protect against this sort of attack,
  implementations should not advertise mechanisms and/or features which
  cannot meet their minimum security requirements, should not enter into
  or continue authentication exchanges which cannot meet their minimum
  security requirements, and should verify that completed authentication
  exchanges result in security services that meet their minimum security
  requirements.  Note that each endpoint needs to independently verify
  that its security requirements are met.

  In order to detect downgrade attacks to the least (or less) secure
  mechanism supported, the client may discover the SASL mechanisms the
  server makes available both before the SASL authentication exchange
  and after the negotiated SASL security layer (with at least data
  integrity protection) has been installed through the protocol's
  mechanism discovery facility.  If the client finds that the integrity
  protected list (the list obtained after the security layer was
  installed) contains a stronger mechanism than those in the previously
  obtained list, the client should assume the previously obtained list
  was modified by an attacker and should close the underlying transport
  connection.

  The client's initiation of the SASL exchange, including the 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.

  Multi-level negotiation of security features is prone to downgrade
  attack.  Protocol designers should avoid offering higher level



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  negotiation of security features in protocols (e.g., above SASL
  mechanism negotiation) and mechanism designers should avoid lower
  level negotiation of security features in mechanisms (e.g., below SASL
  mechanism negotiation).


6.1.3. Replay Attacks

  Some mechanisms may be subject to replay attacks unless protected by
  external data security services (e.g., TLS).


6.1.4.  Truncation Attacks

  Most existing SASL security layers do not themselves offer protection
  against truncation attack.   In a truncation attack, the active
  attacker causes the protocol session to be closed, causing a
  truncation of the possibly integrity protected data stream that leads
  to behavior of one or both the protocol peers that inappropriately
  benefits the attacker.  Truncation attacks are fairly easy to defend
  against in connection-oriented application-level protocols.  A
  protocol can defend against these attacks by ensuring that each
  information exchange has a clear final result and that each protocol
  session has a graceful closure mechanism, and that these are integrity
  protected.


6.1.5.  Other Active Attacks

  When use of a security layer is negotiated by the authentication
  protocol exchange, the receiver should handle gracefully any protected
  data buffer larger than the defined/negotiated maximal size.  In
  particular, it must not blindly allocate the amount of memory
  specified in the buffer size field, as this might cause the "out of
  memory" condition.  If the receiver detects a large block, it should
  close the connection.


6.2.  Passive Attacks

  Many mechanisms are subject to various passive attacks, including
  simple eavesdropping of unprotected credential information as well as
  online and offline dictionary attacks of protected credential
  information.


6.3.  Re-keying




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  The secure or administratively permitted lifetimes of SASL mechanisms'
  security layers are finite.  Cryptographic keys weaken as they are
  used and as time passes; the more time and/or cipher-text that a
  cryptanalyst has after the first use of the a key, the easier it is
  for the cryptanalyst to mount attacks on the key.

  Administrative limits on a security layer's lifetime may take the form
  of time limits expressed in X.509 certificates, Kerberos V tickets, or
  in directories, and are often desired.  In practice one likely effect
  of administrative lifetime limits is that applications may find that
  security layers stop working in the middle of application protocol
  operation, such as, perhaps, during large data transfers.  As the
  result of this the connection will be closed (see Section 3.7), which
  will result in unpleasant user experience.

  Re-keying (key renegotiation process) is a way of addressing the
  weakening of cryptographic keys.  SASL framework does not itself
  provide for re-keying, SASL mechanisms may.  Designers of future SASL
  mechanisms should consider providing re-keying services.

  Applications that wish to re-key SASL security layers where the
  mechanism does not provide for re-keying should reauthenticate the
  same IDs and replace the expired or soon-to-expire security layers.
  This approach requires support for reauthentication in the application
  protocols (see Section 3.8).


6.4. Other Considerations

  Protocol designers and implementors should understand the security
  considerations of mechanisms so they may select mechanisms which are
  applicable to their needs.

  Distributed server implementations need to be careful in how they
  trust other parties.  In particular, authentication secrets should
  only be disclosed to other parties that are trusted to manage and use
  those secrets in manner acceptable to disclosing party.  Applications
  using SASL assume that SASL security layers providing data
  confidentiality are secure even when an attacker chooses the text to
  be protected by the security layer.  Similarly applications assume
  that the SASL security layer is secure even if the attacker can
  manipulate the cipher-text output of the security layer.  New SASL
  mechanisms are expected to meet these assumptions.

  Unicode security considerations [UTR36] apply to authorization
  identity strings, and well as UTF-8 [RFC3629] security considerations
  where UTF-8 is used.  SASLprep [RFC4013] and StringPrep [RFC3454]
  security considerations also apply where used.



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7.    IANA Considerations

7.1.  SASL Mechanism Registry

  SASL mechanism registry is maintained by IANA.  The registry is
  currently available at
  <http://www.iana.org/assignments/sasl-mechanisms>.

  The purpose of this registry is not only to ensure uniqueness of
  values used to name SASL mechanisms, but to provide a definitive
  references to technical specifications detailing each SASL mechanism
  available for use on the Internet.

  There is no naming convention for SASL mechanisms; any name that
  conforms to the syntax of a SASL mechanism name can be registered.

  The procedure detailed in Section 7.1.1 is to be used for registration
  of a value naming a specific individual mechanism.

  The procedure detailed in Section 7.1.2 is to be used for registration
  of a value naming a family of related mechanisms.

  Comments may be included in the registry as discussed in Section 7.1.3
  and may be changed as discussed in Section 7.1.4.

  It is requested that the SASL mechanism registry be updated to reflect
  that this document provides the definitive technical specification for
  SASL and that this section provides the registration procedures for
  this registry.


7.1.1.  Mechanism Name Registration Procedure

  IANA will register new SASL mechanism names on a First Come First
  Served basis, as defined in BCP 26 [RFC2434].  IANA has the right to
  reject obviously bogus registration requests, but will perform no
  review of claims made in the registration form.

  Registration of a SASL mechanism is requested by filling in the
  following template:

      Subject: Registration of SASL mechanism X

      SASL mechanism name (or prefix for the family):

      Security considerations:

      Published specification (recommended):



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      Person & email address to contact for further information:

      Intended usage: (One of COMMON, LIMITED USE or OBSOLETE)

      Owner/Change controller:

      Note: (Any other information that the author deems relevant may be
      added here .)

  and sending it via electronic mail to IANA at <iana@iana.org>.

  While this registration procedures do not require expert review,
  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.


7.1.2.  Family Name Registration Procedure

  As noted above, there is no general naming convention for SASL
  mechanisms.  However, specifications may reserve a portion of the SASL
  mechanism namespace for a set of related SASL mechanisms, a "family"
  of SASL mechanisms.  Each family of SASL mechanisms is identified by a
  unique prefix, such as X-.  Registration of new SASL mechanism family
  names requires Expert Review as defined in BCP 26 [RFC2434].

  Registration of a SASL family name is requested by filling in the
  following template:

      Subject: Registration of SASL mechanism family X

      SASL family name (or prefix for the family):

      Security considerations:

      Published specification (recommended):

      Person & email address to contact for further information:

      Intended usage: (One of COMMON, LIMITED USE or OBSOLETE)

      Owner/Change controller:

      Note: (Any other information that the author deems relevant may be
      added here .)




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  and sending it via electronic mail to the IETF SASL mailing list at
  <ietf-sasl@imc.org> with copy to IANA at <iana@iana.org>.  After
  allowing two weeks for community input on the IETF SASL mailing list,
  the expert will determine the appropriateness of the registration
  request and either approve or disappove the request with notice to
  requestor, the mailing list, and IANA.

  The review should focus on the appropriateness of the requested family
  name for the proposed use and the appropriateness of the proposed
  naming and registration plan for existing and future mechanism names
  in the family.  The scope of this request review may entail
  consideration of relevant aspects of any provided technical
  specification, such as their IANA Considerations section.  However
  this review is narrowly focused on the appropriateness of the
  requested registration and not on the overall soundness of any
  provided technical specification.

  Authors are encouraged to community review by posting the technical
  specification as an Internet-Draft and soliciting comment by posting
  to appropriate IETF mailing lists.


7.1.3.  Comments on SASL Mechanism Registrations

  Comments on a registered SASL mechanism/family should first be sent to
  the "owner" of the mechanism/family and/or to the <ietf-sasl@imc.org>
  mailing list.

  Submitters of comments may, after a reasonable attempt to contact the
  owner, request IANA to attach their comment to the SASL mechanism
  registration itself by sending mail to <iana@iana.org>.  At IANA's
  sole discretion, IANA may attach the comment to the SASL mechanism's
  registration.


7.1.4.  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



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  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 usage" 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.


7.2.  Registration Changes

  It is requested that IANA update 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 as indicated below:

      Subject: Updated Registration of SASL mechanism EXTERNAL
      Family of SASL mechanisms: NO
      SASL mechanism name: EXTERNAL
      Security considerations: See A.3 of RFC XXXX
      Published specification (optional, recommended): RFC XXXX
      Person & email address to contact for further information:
          Alexey Melnikov <Alexey.Melnikov@isode.com>
      Intended usage: COMMON
      Owner/Change controller: IESG <iesg@ietf.org>
      Note: Updates existing entry for EXTERNAL


8.    References

  [[Note to the RFC Editor: please replace the citation tags used in
  referencing Internet-Drafts with tags of the form RFCnnnn where
  possible.]]

8.1.  Normative References

  [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14 (also RFC 2119), March 1997.

  [RFC2434]     Narten, T. and H. Alvestrand, "Guidelines for Writing an
                IANA Considerations Section in RFCs", BCP 26 (also RFC



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                2434), October 1998.

                [RFC2743]     Linn, J., "Generic Security Service
                Application Program Interface, Version 2, Update 1", RFC
                2743, January 2000.

  [RFC3454]     Hoffman, P. and M. Blanchet, "Preparation of
                Internationalized Strings ('stringprep')", RFC 3454,
                December 2002.

  [RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 3629 (also STD 63), November 2003.

  [RFC4013]     Zeilenga, K., "SASLprep: Stringprep Profile for User
                Names and Passwords", RFC 4013, February 2005.

  [RFC4234]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 4234, October 2005.

  [ASCII]       Coded Character Set--7-bit American Standard Code for
                Information Interchange, ANSI X3.4-1986.

  [Unicode]     The Unicode Consortium, "The Unicode Standard, Version
                3.2.0" is defined by "The Unicode Standard, Version 3.0"
                (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
                as amended by the "Unicode Standard Annex #27: Unicode
                3.1" (http://www.unicode.org/reports/tr27/) and by the
                "Unicode Standard Annex #28: Unicode 3.2"
                (http://www.unicode.org/reports/tr28/).

  [CharModel]   Whistler, K. and M. Davis, "Unicode Technical Report
                #17, Character Encoding Model", UTR17,
                <http://www.unicode.org/unicode/reports/tr17/>, August
                2000.

  [Glossary]    The Unicode Consortium, "Unicode Glossary",
                <http://www.unicode.org/glossary/>.


8.2.  Informative References

  [RFC2246]     Dierks, T. and, C. Allen, "The TLS Protocol Version
                1.0", RFC 2246, January 1999.

  [RFC2401]     Kent, S., and R.  Atkinson, "Security Architecture for
                the Internet Protocol", RFC 2401, November 1998.

  [RFC3548]     Josefsson, S., "The Base16, Base32, and Base64 Data



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                Encodings", RFC 3548, July 2003.

  [TLS]         Dierks, T. and, E. Rescorla, "The TLS Protocol Version
                1.1", draft-ietf-tls-rfc2246-bis-xx.txt, a work in
                progress.

  [SASL-GSSAPI] Melnikov, A. (Editor), "SASL GSSAPI mechanisms",
                draft-ietf-sasl-gssapi-XX.txt, a work in progress.

                [UTR36]       Davis, M., "(Draft) Unicode Technical
                Report #36, Character Encoding Model", UTR17,
                <http://www.unicode.org/unicode/reports/tr36/>, February
                2005.

  [CRAM-MD5]    Nerenberg, L., "The CRAM-MD5 SASL Mechanism",
                draft-ietf-sasl-crammd5-xx.txt, a work in progress.

  [DIGEST-MD5]  Leach, P., C. Newman, and A. Melnikov, "Using Digest
                Authentication as a SASL Mechanism",
                draft-ietf-sasl-rfc2831bis-xx.txt, a work in progress.



9.   Editors' Address

  Alexey Melnikov
  Isode Limited
  5 Castle Business Village
  36 Station Road
  Hampton, Middlesex,
  TW12 2BX, United Kingdom

  Email: Alexey.Melnikov@isode.com
  URI:   http://www.melnikov.ca/


  Kurt D. Zeilenga
  OpenLDAP Foundation

  Email: Kurt@OpenLDAP.org


10.   Acknowledgments

  This document is a revision of RFC 2222 written by John Myers.

  This revision is a product of the IETF Simple Authentication and
  Security Layer (SASL) Working Group.



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  The following individuals contributed significantly to this revision:
  Abhijit Menon-Sen, Hallvard B Furuseth, Jeffrey Hutzelman, John Myers,
  Luke Howard, Magnus Nystrom, Nicolas Williams, Peter Saint-Andre, RL
  'Bob' Morgan, Rob Siemborski, Sam Hartman, Simon Josefsson, Tim Alsop,
  and Tony Hansen.


Appendix A.  The SASL EXTERNAL Mechanism

  This appendix is normative.

  The EXTERNAL mechanism allows a client to request the server to use
  credentials established by means external to the mechanism to
  authenticate the client.  The external means may be, for instance, IP
  Security [RFC2401] or TLS [RFC2246] services.  In absence of some
  apriori agreement between the client and the server, the client cannot
  make any assumption as to what external means the server has used to
  obtain the client's credentials, nor make an assumption as to the form
  of credentials.  For example, the client cannot assume the server will
  use the credentials the client has established via TLS.

A.1.  EXTERNAL Technical Specification

  The name of this mechanism is "EXTERNAL".

  The mechanism does not provide a security layer.

  The mechanism is capable of transferring an authorization identity
  string.  If empty, the client is requesting to act as the identity the
  server has associated with the client's credentials.  If non-empty,
  the client is requesting to act as the identity represented by the
  string.

  The client is expected to send data first in the authentication
  exchange.  Where the client does not provide an initial response data
  in its request to initiate the authentication exchange, the server is
  to respond to the request with an empty initial challenge and then the
  client is to provide its initial response.

  The client sends the initial response containing the UTF-8 [RFC3629]
  encoding of the requested authorization identity string.  This
  response is non-empty when the client is requesting to act as the
  identity represented by the (non-empty) string.  This response is
  empty when the client is requesting to act as the identity the server
  associated with its authentication credentials.

  The syntax of the initial response is specified as a value of the
  <extern-initial-resp> production detailed below using the Augmented



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  Backus-Naur Form (ABNF) [RFC4234] notation.

      external-initial-resp = authz-id-string
      authz-id-string           = *( UTF8-char-no-nul )
      UTF8-char-no-nul      = UTF8-1-no-nul / UTF8-2 / UTF8-3 / UTF8-4
      UTF8-1-no-nul         = %x01-7F

  where the <UTF8-2>, <UTF8-3>, and <UTF8-4> productions are as defined
  in [RFC3629].

  There are no additional challenges and responses.

  Hence, the server is to return the outcome of the authentication
  exchange.

  The exchange fails if
  - the client has not established its credentials via external means,
  - the client's credentials are inadequate,
  - The client provided an empty authorization identity string and the
    server is unwilling or unable to associate an authorization identity
    with the client's credentials,
  - The client provided a non-empty authorization identity string which
    is invalid per the syntax requirements of the applicable application
    protocol specification,
  - The client provided a non-empty authorization identity string
    representing an identity which the client is not allowed to act as,
    or
  - the server is unwilling or unable to provide service to the client
    for any other reason.

    Otherwise the exchange is successful.  When indicating a successful
    outcome, additional data is not provided.


A.2.  SASL EXTERNAL Examples

    This section provides examples of EXTERNAL authentication exchanges.
    The examples are intended to help the readers under the above text.
    The examples are not definitive.  The Application Configuration
    Access Protocol (ACAP) [RFC2244] is used in the examples.

    The first example shows use of EXTERNAL with an empty authorization
    identity.  In this example, the initial response is not sent in the
    client's request to initiate authentication exchange.

      S: * ACAP (SASL "DIGEST-MD5")
      C: a001 STARTTLS
      S: a001 OK "Begin TLS negotiation now"



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      <TLS negotiation, further commands are under TLS layer>
      S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL")
      C: a002 AUTHENTICATE "EXTERNAL"
      S: + ""
      C: + ""
      S: a002 OK "Authenticated"

  In second example shows use of EXTERNAL with an authorization identity
  of "fred@example.com".  In this example, the initial response is sent
  with the client's request to initiate the authentication exchange.
  This saves a round-trip.

      S: * ACAP (SASL "DIGEST-MD5")
      C: a001 STARTTLS
      S: a001 OK "Begin TLS negotiation now"
      <TLS negotiation, further commands are under TLS layer>
      S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL")
      C: a002 AUTHENTICATE "EXTERNAL" {16+}
      C: fred@example.com
      S: a002 NO "Cannot assume requested authorization identity"


A.3.  Security Considerations

  The EXTERNAL mechanism provides no security protection; it is
  vulnerable to spoofing by either client or server, active attack, and
  eavesdropping.  It should only be used when adequate security services
  have been established.



Appendix B.  Changes since RFC 2222

  This appendix is non-normative.

  The material in RFC 2222 was significantly rewritten in the production
  of this document.

  RFC 2222, by not stating the authorization identity string was a
  string of Unicode characters, let alone character data, implied the
  authorization identity string was a string of octets.

  - The authorization identity string is now defined as a string of
    Unicode characters.  The NUL (U+0000) character is prohibited.
    While protocol specifications are responsible for defining the
    authorization identity form, as well as the Unicode string syntax
    and related semantics, mechanism specifications are responsible for
    defining how the Unicode string is carried in the authentication



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    exchange.
  - Deleted "If so, when the client does not send data first, the
    initial challenge MUST be specified as being an empty challenge."

  The following technical change was made to the EXTERNAL mechanism:

  - The authorization identity string is to be UTF-8 encoded.

  It is noted that protocol and mechanism specification requirements
  have been significant tightened.  Existing protocol and mechanism
  specifications will need to be updated to meet these requirements.




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Full Copyright

  Copyright (C) The Internet Society (2005).

  This document is subject to the rights, licenses and restrictions
  contained in BCP 78, and except as set forth therein, the authors
  retain all their rights.



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  This document and the information contained herein are provided on an
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