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rfc:rfc4422

Network Working Group A. Melnikov, Ed. Request for Comments: 4422 Isode Limited Obsoletes: 2222 K. Zeilenga, Ed. Category: Standards Track OpenLDAP Foundation

                                                             June 2006
          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 (2006).

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 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.  In addition, this
 document defines one SASL mechanism, the EXTERNAL mechanism.
 This document obsoletes RFC 2222.

Melnikov & Zeilenga Standards Track [Page 1] RFC 4422 SASL June 2006

Table of Contents

 1. Introduction ....................................................3
    1.1. Document Audiences .........................................4
    1.2. Relationship to Other Documents ............................4
    1.3. Conventions ................................................5
 2. Identity Concepts ...............................................5
 3. The Authentication Exchange .....................................6
    3.1. Mechanism Naming ...........................................8
    3.2. Mechanism Negotiation ......................................9
    3.3. Request Authentication Exchange ............................9
    3.4. Challenges and Responses ...................................9
         3.4.1. Authorization Identity String ......................10
    3.5. Aborting Authentication Exchanges .........................10
    3.6. Authentication Outcome ....................................11
    3.7. Security Layers ...........................................12
    3.8. Multiple Authentications ..................................12
 4. Protocol Requirements ..........................................13
 5. Mechanism Requirements .........................................16
 6. Security Considerations ........................................18
    6.1. Active Attacks ............................................19
         6.1.1. Hijack Attacks .....................................19
         6.1.2. Downgrade Attacks ..................................19
         6.1.3. Replay Attacks .....................................20
         6.1.4. Truncation Attacks .................................20
         6.1.5. Other Active Attacks ...............................20
    6.2. Passive Attacks ...........................................20
    6.3. Re-keying .................................................21
    6.4. Other Considerations ......................................21
 7. IANA Considerations ............................................22
    7.1. SASL Mechanism Registry ...................................22
    7.2. Registration Changes ......................................26
 8. References .....................................................26
    8.1. Normative References ......................................26
    8.2. Informative References ....................................27
 9. Acknowledgements ...............................................28
 Appendix A.  The SASL EXTERNAL Mechanism ..........................29
    A.1. EXTERNAL Technical Specification ..........................29
    A.2. SASL EXTERNAL Examples ....................................30
    A.3. Security Considerations ...................................31
 Appendix B.  Changes since RFC 2222 ...............................31

Melnikov & Zeilenga Standards Track [Page 2] RFC 4422 SASL June 2006

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 that 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 that 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 to be shared by implementations of multiple
 protocols.

Melnikov & Zeilenga Standards Track [Page 3] RFC 4422 SASL June 2006

 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, 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 that 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:
  1. protocol designers using this specification to support

authentication in their protocol,

  1. mechanism designers that define new SASL mechanisms, and
  1. implementors of clients or servers for those protocols that

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.

Melnikov & Zeilenga Standards Track [Page 4] RFC 4422 SASL June 2006

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 be (and often are) used within implementations.
 How identities of different forms relate to each other is, generally,
 a local matter.  In addition, the forms and representations used
 within an implementation are a local matter.
 However, conceptually, the 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 authentication identities carried in the
 credentials.  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.

Melnikov & Zeilenga Standards Track [Page 5] RFC 4422 SASL June 2006

 The client provides its credentials (which include or imply 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 identity it associates with the client's
 credentials (e.g., the authentication identity) 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) are 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.

Melnikov & Zeilenga Standards Track [Page 6] RFC 4422 SASL June 2006

 Some mechanisms specify that the first data sent in the
 authentication exchange is from the client to the server.  Protocols
 may provide an optional initial response field in the request message
 to carry this data.  Where the mechanism specifies that 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 that 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 that 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 that 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.

Melnikov & Zeilenga Standards Track [Page 7] RFC 4422 SASL June 2006

    C: Request authentication exchange
    S: Initial challenge
    C: Initial response
    <additional challenge/response messages>
    S: Additional data challenge
    C: Empty Response
    S: Outcome of authentication exchange
 Where mechanisms specify that 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, then the exchange takes two fewer
 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 mechanism names are registered as discussed in Section 7.1.

Melnikov & Zeilenga Standards Track [Page 8] RFC 4422 SASL June 2006

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 that it supports and finds suitable.
 Note 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.
 Note that the name of the mechanism is not protected by the
 mechanism, and hence is 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:
  1. authenticate the client to the server,
  1. authenticate the server to the client,

Melnikov & Zeilenga Standards Track [Page 9] RFC 4422 SASL June 2006

  1. transfer an authorization identity string,
  1. negotiate a security layer, and
  1. 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.
 A non-empty authorization identity string indicates that 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 that the exchange is aborted.  The server may be required
 by the protocol to return a message in response to the client's abort
 message.

Melnikov & Zeilenga Standards Track [Page 10] RFC 4422 SASL June 2006

 Likewise, a server may abort the authentication exchange by sending a
 message, the particulars of which are protocol specific, to the
 client, indicating that 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
  1. the authentication exchange failed for any reason,
  1. the client's credentials could not be verified,
  1. the server cannot associate an identity with the client's

credentials,

  1. the client-provided authorization identity string is malformed,
  1. the identity associated with the client's credentials is not

authorized to act as the requested authorization identity,

  1. the negotiated security layer (or lack thereof) is not

suitable, or

  1. 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 that are best dealt with by re-
 prompting the user for her credentials, errors that 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

Melnikov & Zeilenga Standards Track [Page 11] RFC 4422 SASL June 2006

 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.  Typical services include data integrity and data
 confidentiality.  SASL mechanisms that 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 of the outcome indication.  In both cases,
 the layer is installed before transfer of further protocol data.  The
 precise position upon which 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 that the other side expects.  Upon the receipt
 of a length field whose value is greater than the maximum size, the
 receiver SHOULD close the connection, as this might be a sign of an
 attack.
 The maximum size that 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

Melnikov & Zeilenga Standards Track [Page 12] RFC 4422 SASL June 2006

 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
 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 was affected.  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 Generic Security Service Application Program Interface
    (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 that 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 that 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 the following:

Melnikov & Zeilenga Standards Track [Page 13] RFC 4422 SASL June 2006

    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,
       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
    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

Melnikov & Zeilenga Standards Track [Page 14] RFC 4422 SASL June 2006

    may be appropriate for the protocol to provide a facility by which
    the client can discover information (such as the representation of
    the identity used in making access control decisions) about
    established identities for these uses.
 5) Detail any facility the protocol provides that allows the client
    and/or server to abort authentication exchange (see Section 3.5).
    Protocols that support multiple authentications typically allow a
    client to abort an ongoing authentication exchange by initiating a
    new authentication exchange.  Protocols that do not support
    multiple authentications may require the client to close the
    connection and start over to abort an ongoing authentication
    exchange.
    Protocols typically allow the server to abort ongoing
    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 layers to start taking
    effect on the first octet following the outcome message in data
    being sent by the server and on the first octet sent after receipt
    of the outcome message in data being sent by the client.
 7) If the protocol supports other layered security services, such as
    Transport Layer Security (TLS) [RFC4346], 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.

Melnikov & Zeilenga Standards Track [Page 15] RFC 4422 SASL June 2006

 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 a previously
    established authentication and authorization state.
 Protocol specifications SHOULD avoid stating implementation
 requirements that would hinder replacement of applicable mechanisms.
 In general, protocol specifications SHOULD be mechanism neutral.
 There are a number of reasonable exceptions to this recommendation,
 including
  1. detailing how credentials (which are mechanism specific) are

managed in the protocol,

  1. detailing how authentication identities (which are mechanism

specific) and authorization identities (which are protocol

       specific) relate to each other, and
  1. 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 the following:
    a) An indication of whether the 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
       in the authentication request, 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 in the authentication request 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 in the
       authentication request (the CRAM-MD5 mechanism [CRAM-MD5] is an
       example of this case).
    b) An indication of 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

Melnikov & Zeilenga Standards Track [Page 16] RFC 4422 SASL June 2006

       specification MUST indicate that 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
    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 that are capable of transferring an authorization
    identity string MUST be capable of transferring arbitrary non-
    empty sequences of Unicode characters, excluding those that
    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 that
    might appear in the authorization identity string are escaped to
    avoid ambiguity during decoding of the authorization identity
    string.  Typically, mechanisms that have special characters
    require these special characters to be escaped or encoded in the
    character string (after encoding it in a particular Unicode
    transformation format) using a data encoding scheme such as Base64
    [RFC3548].
 4) The specification MUST detail whether 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 the UTF-8 transformation format [RFC3629]
 for encoding Unicode [Unicode] code points for transfer.

Melnikov & Zeilenga Standards Track [Page 17] RFC 4422 SASL June 2006

 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.
 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.  That is, as
 different clients (of the same or different protocol) may provide
 different authorization identity strings that 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 susceptible 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) 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.

Melnikov & Zeilenga Standards Track [Page 18] RFC 4422 SASL June 2006

6.1. Active Attacks

6.1.1. Hijack Attacks

 When the client selects a SASL security layer with at least integrity
 protection, this protection 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 close the connection when the security services in a 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 a 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 the client-advertised security feature list within
 a mechanism response.  To protect against this sort of attack,
 implementations SHOULD NOT advertise mechanisms and/or features that
 cannot meet their minimum security requirements, SHOULD NOT enter
 into or continue authentication exchanges that 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 can discover the SASL mechanisms that
 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 that 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

Melnikov & Zeilenga Standards Track [Page 19] RFC 4422 SASL June 2006

 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
 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.

Melnikov & Zeilenga Standards Track [Page 20] RFC 4422 SASL June 2006

6.3. Re-keying

 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, in 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 an unpleasant user experience.
 Re-keying (key renegotiation process) is a way of addressing the
 weakening of cryptographic keys.  The SASL framework does not itself
 provide for re-keying; SASL mechanisms may.  Designers of future SASL
 mechanisms should consider providing re-keying services.
 Implementations 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 that 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 a manner acceptable to the 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.

Melnikov & Zeilenga Standards Track [Page 21] RFC 4422 SASL June 2006

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

7. IANA Considerations

7.1. SASL Mechanism Registry

 The 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 also to provide a definitive
 reference 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.
 The SASL mechanism registry has been updated to reflect that this
 document provides the definitive technical specification for SASL and
 that this section provides the registration procedures for this
 registry.

Melnikov & Zeilenga Standards Track [Page 22] RFC 4422 SASL June 2006

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):
    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 procedure does 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.

Melnikov & Zeilenga Standards Track [Page 23] RFC 4422 SASL June 2006

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.)
 and sending it via electronic mail to the IETF SASL mailing list at
 <ietf-sasl@imc.org> and carbon copying 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 disapprove the request
 with notice to the 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.

Melnikov & Zeilenga Standards Track [Page 24] RFC 4422 SASL June 2006

 Authors are encouraged to pursue 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, has 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 that 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 that
 are on the IETF standards track.

Melnikov & Zeilenga Standards Track [Page 25] RFC 4422 SASL June 2006

7.2. Registration Changes

 The IANA has updated the SASL mechanisms registry as follows:
 1) Changed the "Intended usage" of the KERBEROS_V4 and SKEY mechanism
    registrations to OBSOLETE.
 2) Changed 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 4422
    Published specification (optional, recommended): RFC 4422
    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

8.1. Normative References

 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2244]     Newman, C. and J. G. Myers, "ACAP -- Application
               Configuration Access Protocol", RFC 2244, November
               1997.
 [RFC2434]     Narten, T. and H. Alvestrand, "Guidelines for Writing
               an IANA Considerations Section in RFCs", BCP 26, RFC
               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", STD 63, RFC 3629, November 2003.
 [RFC4013]     Zeilenga, K., "SASLprep: Stringprep Profile for User
               Names and Passwords", RFC 4013, February 2005.

Melnikov & Zeilenga Standards Track [Page 26] RFC 4422 SASL June 2006

 [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

 [RFC3206]     Gellens, R., "The SYS and AUTH POP Response Codes", RFC
               3206, February 2002.
 [RFC3548]     Josefsson, S., "The Base16, Base32, and Base64 Data
               Encodings", RFC 3548, July 2003.
 [RFC4301]     Kent, S. and K. Seo, "Security Architecture for the
               Internet Protocol", RFC 4301, December 2005.
 [RFC4346]     Dierks, T. and E. Rescorla, "The Transport Layer
               Security (TLS) Protocol Version 1.1", RFC 4346, April
               2006.
 [SASL-GSSAPI] Melnikov, A. (Editor), "The Kerberos V5 ("GSSAPI") SASL
               Mechanism", Work in Progress, May 2006.
 [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", Work in
               Progress.

Melnikov & Zeilenga Standards Track [Page 27] RFC 4422 SASL June 2006

 [DIGEST-MD5]  Leach, P., C. Newman, and A. Melnikov, "Using Digest
               Authentication as a SASL Mechanism", Work in Progress,
               March 2006.

9. Acknowledgements

 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.
 The following individuals contributed significantly to this revision:
 Abhijit Menon-Sen, Hallvard 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.

Melnikov & Zeilenga Standards Track [Page 28] RFC 4422 SASL June 2006

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 [RFC4301] or TLS [RFC4346] services.  In absence of some a
 priori 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 that 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
 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

Melnikov & Zeilenga Standards Track [Page 29] RFC 4422 SASL June 2006

 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
  1. the client has not established its credentials via external means,
  1. the client's credentials are inadequate,
  1. 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,
  1. the client provided a non-empty authorization identity string that

is invalid per the syntax requirements of the applicable

    application protocol specification,
  1. the client provided a non-empty authorization identity string

representing an identity that the client is not allowed to act as,

    or
  1. 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 understand 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 the authentication exchange.
    S: * ACAP (SASL "DIGEST-MD5")
    C: a001 STARTTLS
    S: a001 OK "Begin TLS negotiation now"
    <TLS negotiation, further commands are under TLS layer>

Melnikov & Zeilenga Standards Track [Page 30] RFC 4422 SASL June 2006

    S: * ACAP (SASL "DIGEST-MD5" "EXTERNAL")
    C: a002 AUTHENTICATE "EXTERNAL"
    S: + ""
    C: + ""
    S: a002 OK "Authenticated"
 The 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 that the authorization identity string was a
 string of Unicode characters, let alone character data, implied that
 the authorization identity string was a string of octets.
  1. 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 exchange.
  1. Deleted "If so, when the client does not send data first, the

initial challenge MUST be specified as being an empty challenge."

Melnikov & Zeilenga Standards Track [Page 31] RFC 4422 SASL June 2006

 The following technical change was made to the EXTERNAL mechanism:
  1. The authorization identity string is to be UTF-8 encoded.
    Note that protocol and mechanism specification requirements have
    been significantly tightened.  Existing protocol and mechanism
    specifications will need to be updated to meet these requirements.

Editors' Addresses

 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

Melnikov & Zeilenga Standards Track [Page 32] RFC 4422 SASL June 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 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.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Melnikov & Zeilenga Standards Track [Page 33]

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