





Network Working Group                                  Abhijit Menon-Sen
Internet-Draft                                    Oryx Mail Systems GmbH
Intended Status: Proposed Standard                          Chris Newman
Expires: August 2009                                    Sun Microsystems
                                                         Alexey Melnikov
                                                               Isode Ltd
                                                       February 21, 2009


            Salted Challenge Response (SCRAM) SASL Mechanism

                     draft-newman-auth-scram-10.txt


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    This Internet-Draft expires in July 2009.


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Abstract

    The secure authentication mechanism most widely deployed and used by
    Internet application protocols is the transmission of clear-text
    passwords over a channel protected by Transport Layer Security
    (TLS).  There are some significant security concerns with that
    mechanism, which could be addressed by the use of a challenge
    response authentication mechanism protected by TLS. Unfortunately,
    the challenge response mechanisms presently on the standards track
    all fail to meet requirements necessary for widespread deployment,
    and have had success only in limited use.

    This specification describes a family of authentication mechanisms
    called the Salted Challenge Response Authentication Mechanism
    (SCRAM), which addresses the security concerns and meets the
    deployability requirements. When used in combination with TLS or an
    equivalent security layer, a mechanism from this family could
    improve the status-quo for application protocol authentication and
    provide a suitable choice for a mandatory-to-implement mechanism for
    future application protocol standards.


1. Conventions Used in This Document

    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 [RFC2119].

    Formal syntax is defined by [RFC5234] including the core rules
    defined in Appendix B of [RFC5234].

    Example lines prefaced by "C:" are sent by the client and ones
    prefaced by "S:" by the server. If a single "C:" or "S:" label
    applies to multiple lines, then the line breaks between those lines
    are for editorial clarity only, and are not part of the actual
    protocol exchange.


1.1. Terminology

    This document uses several terms defined in [RFC4949] ("Internet
    Security Glossary") including the following: authentication,
    authentication exchange, authentication information, brute force,
    challenge-response, cryptographic hash function, dictionary attack,
    eavesdropping, hash result, keyed hash, man-in-the-middle, nonce,
    one-way encryption function, password, replay attack and salt.
    Readers not familiar with these terms should use that glossary as a
    reference.



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    Some clarifications and additional definitions follow:

    - Authentication information: Information used to verify an identity
      claimed by a SCRAM client. The authentication information for a
      SCRAM identity consists of salt, iteration count, the "StoredKey"
      and "ServerKey" (as defined in the algorithm overview) for each
      supported cryptographic hash function.

    - Authentication database: The database used to look up the
      authentication information associated with a particular identity.
      For application protocols, LDAPv3 (see [RFC4510]) is frequently
      used as the authentication database. For network-level protocols
      such as PPP or 802.11x, the use of RADIUS is more common.

    - Base64: An encoding mechanism defined in [RFC4648] which converts
      an octet string input to a textual output string which can be
      easily displayed to a human. The use of base64 in SCRAM is
      restricted to the canonical form with no whitespace.

    - Octet: An 8-bit byte.

    - Octet string: A sequence of 8-bit bytes.

    - Salt: A random octet string that is combined with a password
      before applying a one-way encryption function. This value is used
      to protect passwords that are stored in an authentication
      database.


1.2. Notation

    The pseudocode description of the algorithm uses the following
    notations:

    - ":=": The variable on the left hand side represents the octet
      string resulting from the expression on the right hand side.

    - "+": Octet string concatenation.

    - "[ ]": A portion of an expression enclosed in "[" and "]" may not
      be included in the result under some circumstances. See the
      associated text for a description of those circumstances.

    - HMAC(key, str): Apply the HMAC keyed hash algorithm (defined in
      [RFC2104]) using the octet string represented by "key" as the key
      and the octet string "str" as the input string. The size of the
      result is the hash result size for the hash function in use. For
      example, it is 20 octets for SHA-1 (see [RFC3174]).



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    - H(str): Apply the cryptographic hash function to the octet string
      "str", producing an octet string as a result. The size of the
      result depends on the hash result size for the hash function in
      use.

    - XOR: Apply the exclusive-or operation to combine the octet string
      on the left of this operator with the octet string on the right of
      this operator. The length of the output and each of the two inputs
      will be the same for this use.

    - Hi(str, salt):

         U0   := HMAC(str, salt + INT(1))
         U1   := HMAC(str, U0)
         U2   := HMAC(str, U1)
         ...
         Ui-1 := HMAC(str, Ui-2)
         Ui   := HMAC(str, Ui-1)

         Hi := U0 XOR U1 XOR U2 XOR ... XOR Ui
      where "i" is the iteration count, "+" is the string concatenation
      operator and INT(g) is a four-octet encoding of the integer g,
      most significant octet first.

      This is, essentially, PBKDF2 [RFC2898] with HMAC() as the PRF and
      with dkLen == output length of HMAC() == output length of H().



2. Introduction

    This specification describes a family of authentication mechanisms
    called the Salted Challenge Response Authentication Mechanism
    (SCRAM) which addresses the requirements necessary to deploy a
    challenge-response mechanism more widely than past attempts. When
    used in combination with Transport Layer Security (TLS, see [TLS])
    or an equivalent security layer, a mechanism from this family could
    improve the status-quo for application protocol authentication and
    provide a suitable choice for a mandatory-to-implement mechanism for
    future application protocol standards.

    For simplicity, this family of mechanism does not presently include
    negotiation of a security layer. It is intended to be used with an
    external security layer such as that provided by TLS or SSH.

    SCRAM provides the following protocol features:

    - The authentication information stored in the authentication



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      database is not sufficient by itself to impersonate the client.
      The information is salted to prevent a pre-stored dictionary
      attack if the database is stolen.

    - The server does not gain the ability to impersonate the client to
      other servers (with an exception for server-authorized proxies).

    - The mechanism permits the use of a server-authorized proxy without
      requiring that proxy to have super-user rights with the back-end
      server.

    - A standard attribute is defined to enable storage of the
      authentication information in LDAPv3 (see [RFC4510]).

    - Both the client and server can be authenticated by the protocol.

    For an in-depth discussion of why other challenge response
    mechanisms are not considered sufficient, see appendix A. For more
    information about the motivations behind the design of this
    mechanism, see appendix B.

    Comments regarding this draft may be sent either to the ietf-
    sasl@imc.org mailing list or to the authors.


3. SCRAM Algorithm Overview

    Note that this section omits some details, such as client and server
    nonces.  See Section 5 for more details.

    To begin with, the client is in possession of a username and
    password.  It sends the username to the server, which retrieves the
    corresponding authentication information, i.e. a salt, StoredKey,
    ServerKey and the iteration count i. (Note that a server
    implementation may chose to use the same iteration count for all
    account.) The server sends the salt and the iteration count to the
    client, which then computes the following values and sends a
    ClientProof to the server:

        SaltedPassword  := Hi(password, salt)
        ClientKey       := H(SaltedPassword)
        StoredKey       := H(ClientKey)
        AuthMessage     := client-first-message + "," +
                           server-first-message + "," +
                           client-final-message-without-proof
        ClientSignature := HMAC(StoredKey, AuthMessage)
        ClientProof     := ClientKey XOR ClientSignature
        ServerKey       := HMAC(SaltedPassword, salt)



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        ServerSignature := HMAC(ServerKey, AuthMessage)

    The server authenticates the client by computing the
    ClientSignature, exclusive-ORing that with the ClientProof to
    recover the ClientKey and verifying the correctness of the ClientKey
    by applying the hash function and comparing the result to the
    StoredKey. If the ClientKey is correct, this proves that the client
    has access to the user's password.

    Similarly, the client authenticates the server by computing the
    ServerSignature and comparing it to the value sent by the server.
    If the two are equal, it proves that the server had access to the
    user's ServerKey.

    The AuthMessage is computed by concatenating messages from the
    authentication exchange. The format of these messages is defined in
    the Formal Syntax section.


4. SCRAM mechanism names

    A SCRAM mechanism name is a string "SCRAM-HMAC-" followed by the
    uppercased name of the underlying hashed function taken from the
    IANA "Hash Function Textual Names" registry (see
    http://www.iana.org).

    For interoperability, all SCRAM clients and servers MUST implement
    the SCRAM-HMAC-SHA-1 authentication mechanism, i.e. an
    authentication mechanism from the SCRAM family that uses the SHA-1
    hash function as defined in [RFC3174].


5. SCRAM Authentication Exchange

    SCRAM is a text protocol where the client and server exchange
    messages containing one or more attribute-value pairs separated by
    commas. Each attribute has a one-letter name. The messages and their
    attributes are described in section 5.1, and defined in the Formal
    Syntax section.

    This is a simple example of a SCRAM-HMAC-SHA-1 authentication
    exchange:
        C: n=Chris Newman,r=ClientNonce
        S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128
        C: r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
        S: v=WxPv/siO5l+qxN4

    With channel-binding data sent by the client this might look like this:



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        C: n=Chris Newman,r=ClientNonce
        S: r=ClientNonceServerNonce,s=PxR/wv+epq,i=128
        C: c=0123456789ABCDEF,r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
        S: v=WxPv/siO5l+qxN4

    <<Note that the channel-bind data above, as well as all hashes are fake>>

    First, the client sends a message containing the username, and a
    random, unique nonce. In response, the server sends the user's
    iteration count i, the user's salt, and appends its own nonce to the
    client-specified one.  The client then responds with the same nonce
    and a ClientProof computed using the selected hash function as
    explained earlier.  In this step the client can also include an
    optional authorization identity.  The server verifies the nonce and
    the proof, verifies that the authorization identity (if supplied by
    the client in the second message) is authorized to act as the
    authentication identity, and, finally, it responds with a
    ServerSignature, concluding the authentication exchange. The client
    then authenticates the server by computing the ServerSignature and
    comparing it to the value sent by the server.  If the two are
    different, the client MUST consider the authentication exchange to
    be unsuccessful and it might have to drop the connection.


5.1 SCRAM attributes

    This section describes the permissible attributes, their use, and
    the format of their values. All attribute names are single US-ASCII
    letters and are case-sensitive.

    - a: This optional attribute specifies an authorization identity. A
      client may include it in its second message to the server if it
      wants to authenticate as one user, but subsequently act as a
      different user.  This is typically used by an administrator to
      perform some management task on behalf of another user, or by a
      proxy in some situations.

      Upon the receipt of this value the server verifies its correctness
      according to the used SASL protocol profile. Failed verification
      results in failed authentication exchange.

      If this attribute is omitted (as it normally would be), or
      specified with an empty value, the authorization identity is
      assumed to be derived from the username specified with the
      (required) "n" attribute.

      The server always authenticates the user specified by the "n"
      attribute.  If the "a" attribute specifies a different user, the



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      server associates that identity with the connection after
      successful authentication and authorization checks.

      The syntax of this field is the same as that of the "n" field with
      respect to quoting of '=' and ','.

    - n: This attribute specifies the name of the user whose password is
      used for authentication. A client must include it in its first
      message to the server. If the "a" attribute is not specified
      (which would normally be the case), this username is also the
      identity which will be associated with the connection subsequent
      to authentication and authorization.

      Before sending the username to the server, the client MUST prepare
      the username using the "SASLPrep" profile [SASLPrep] of the
      "stringprep" algorithm [RFC3454]. If the preparation of the
      username fails or results in an empty string, the client SHOULD
      abort the authentication exchange (*).

      (*) An interactive client can request a repeated entry of the
      username value.

      Upon receipt of the username by the server, the server SHOULD
      prepare it using the "SASLPrep" profile [SASLPrep] of the
      "stringprep" algorithm [RFC3454]. If the preparation of the
      username fails or results in an empty string, the server SHOULD
      abort the authentication exchange.

      The characters ',' or '=' in usernames are sent as '=2C' and '=3D'
      respectively. If the server receives a username which contains '='
      not followed by either '2C' or '3D', then the server MUST fail the
      authentication.

    - m: This attribute is reserved for future extensibility.  In this
      version of SCRAM, its presence in a client or a server message
      MUST cause authentication failure when the attribute is parsed by
      the other end.

    - r: This attribute specifies a sequence of random printable
      characters excluding ',' which forms the nonce used as input to
      the hash function.  No quoting is applied to this string (<<unless
      the binding of SCRAM to a particular protocol states otherwise>>).
      As described earlier, the client supplies an initial value in its
      first message, and the server augments that value with its own
      nonce in its first response. It is important that this be value
      different for each authentication. The client MUST verify that the
      initial part of the nonce used in subsequent messages is the same
      as the nonce it initially specified. The server MUST verify that



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      the nonce sent by the client in the second message is the same as
      the one sent by the server in its first message.

    - c: This optional attribute specifies base64-encoded channel-
      binding data. It is sent by the client in the second step. If
      specified by the client, if the server supports the specified
      channel binding type and if the server can't verify it, then the
      server MUST fail the authentication exchange.  Whether this
      attribute is included, and the meaning and contents of the
      channel-binding data depends on the external security layer in
      use. This is necessary to detect a man-in-the-middle attack on the
      security layer.

    - s: This attribute specifies the base64-encoded salt used by the
      server for this user. It is sent by the server in its first
      message to the client.

    - i: This attribute specifies an iteration count for the selected
      hash function and user, and must be sent by the server along with
      the user's salt.

      For SCRAM-HMAC-SHA-1 SASL mechanism servers SHOULD announce a hash
      iteration-count of at least 128.

    - p: This attribute specifies a base64-encoded ClientProof. The
      client computes this value as described in the overview and sends
      it to the server.

    - v: This attribute specifies a base64-encoded ServerSignature. It
      is sent by the server in its final message, and may be used by the
      client to verify that the server has access to the user's
      authentication information. This value is computed as explained in
      the overview.


6. Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form (ABNF) notation as specified in [RFC5234].  "UTF8-2", "UTF8-3"
    and "UTF8-4" non-terminal are defined in [UTF-8].

      generic-message = attr-val *("," attr-val)
                        ;; Generic syntax of any server challenge
                        ;; or client response

      attr-val        = ALPHA "=" value

      value           = 1*(value-char)



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      value-safe-char = %01-2B / %2D-3C / %3E-7F /
                        UTF8-2 / UTF-3 / UTF8-4
                        ;; UTF8-char except NUL, "=", and ",".

      value-char      = value-safe-char / "="

      base64-char     = ALPHA / DIGIT / "/" / "+"

      base64-4        = 4*4(base64-char)

      base64-3        = 3*3(base64-char) "="

      base64-2        = 2*2(base64-char) "=="

      base64          = *(base64-4) [base64-3 / base64-2]

      posit-number = (%x31-39) *DIGIT
                        ;; A positive number

      saslname        = 1*(value-safe-char / "=2C" / "=3D")
                        ;; Conforms to <value>

      authzid         = "a=" saslname
                        ;; Protocol specific.

      username        = "n=" saslname
                        ;; Usernames are prepared using SASLPrep.

      reserved-mext  = "m=" 1*(value-char)
                        ;; Reserved for signalling mandatory extensions.
                        ;; The exact syntax will be defined in
                        ;; the future.

      channel-binding = "c=" base64

      proof           = "p=" base64

      nonce           = "r=" c-nonce [s-nonce]
                        ;; Second part provided by server.

      c-nonce         = value

      s-nonce         = value

      salt            = "s=" base64

      verifier        = "v=" base64
                        ;; base-64 encoded ServerSignature.



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      iteration-count = "i=" posit-number
                        ;; A positive number

      client-first-message =
                        [reserved-mext ","] username "," nonce [","
                        extensions]

      server-first-message =
                        [reserved-mext ","] nonce "," salt ","
                        iteration-count ["," extensions]

      client-final-message-without-proof =
                        [authzid ","] [channel-binding ","] nonce [","
                        extensions]

      client-final-message =
                        client-final-message-without-proof "," proof

      server-final-message =
                        verifier ["," extensions]

      extensions = attr-val *("," attr-val)
                        ;; All extensions are optional,
                        ;; i.e. unrecognized attributes
                        ;; not defined in this document
                        ;; MUST be ignored.


7. Security Considerations

    If the authentication exchange is performed without a strong
    security layer, then a passive eavesdropper can gain sufficient
    information to mount an offline dictionary or brute-force attack
    which can be used to recover the user's password. The amount of time
    necessary for this attack depends on the cryptographic hash function
    selected, the strength of the password and the iteration count
    supplied by the server. An external security layer with strong
    encryption will prevent this attack.

    If the external security layer used to protect the SCRAM exchange
    uses an anonymous key exchange, then the SCRAM channel binding
    mechanism can be used to detect a man-in-the-middle attack on the
    security layer and cause the authentication to fail as a result.
    However, the man-in-the-middle attacker will have gained sufficient
    information to mount an offline dictionary or brute-force attack.
    For this reason, SCRAM includes the ability to increase the
    iteration count over time.




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    If the authentication information is stolen from the authentication
    database, then an offline dictionary or brute-force attack can be
    used to recover the user's password. The use of salt mitigates this
    attack somewhat by requiring a separate attack on each password.
    Authentication mechanisms which protect against this attack are
    available (e.g., the EKE class of mechanisms), but the patent
    situation is presently unclear.

    If an attacker obtains the authentication information from the
    authentication repository and either eavesdrops on one
    authentication exchange or impersonates a server, the attacker gains
    the ability to impersonate that user to all servers providing SCRAM
    access using the same hash function, password, iteration count and
    salt.  For this reason, it is important to use randomly-generated
    salt values.

    If the server detects (from the value of the client-specified "h"
    attribute) that both endpoints support a stronger hash function that
    the one the client actually chooses to use, then it SHOULD treat
    this as a downgrade attack and reject the authentication attempt.

    A hostile server can perform a computational denial-of-service
    attack on clients by sending a big iteration count value.




























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8. IANA considerations

    IANA is requested to add the following entry to the SASL Mechanism
    registry established by [RFC4422]:

    To: iana@iana.org
    Subject: Registration of a new SASL mechanism SCRAM-HMAC-SHA-1

    SASL mechanism name (or prefix for the family): SCRAM-HMAC-SHA-1
    Security considerations: Section 7 of [RFCXXXX]
    Published specification (optional, recommended): [RFCXXXX]
    Person & email address to contact for further information:
     IETF SASL WG <ietf-sasl@imc.org>
    Intended usage: COMMON
    Owner/Change controller: IESG <iesg@ietf.org>
    Note:

    Note that even though this document defines a family of SCRAM-HMAC
    mechanisms, it doesn't register a family of SCRAM-HMAC mechanisms in
    the SASL Mechanisms registry. IANA is requested to prevent future
    registrations of SASL mechanisms starting with SCRAM-HMAC- without
    consulting the SASL mailing list <ietf-sasl@imc.org> first.

    Note to future SCRAM-HMAC mechanism designers: each new SCRAM-HMAC
    SASL mechanism MUST be explicitly registered with IANA and MUST
    comply with SCRAM-HMAC mechanism naming convention defined in
    Section 4 of this document.



9. Acknowedgements

    The authors would like to thank Dave Cridland for his contributions
    to this document.


10. Normative References

    [RFC4648]  Josefsson, "The Base16, Base32, and Base64 Data
               Encodings", RFC 4648, SJD, October 2006.

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

    [RFC2104]  Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for
               Message Authentication", IBM, February 1997.

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



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               Requirement Levels", RFC 2119, Harvard University, March
               1997.

    [RFC3174]  Eastlake, Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC
               3174, Motorola, September 2001

    [RFC5234]  Crocker, Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 5234, January 2008.

    [RFC4422]  Melnikov, Zeilenga, "Simple Authentication and Security
               Layer (SASL)", RFC 4422, Isode Limited, June 2006.

    [SASLPrep] Zeilenga, K., "SASLprep: Stringprep profile for user
               names and passwords", RFC 4013, February 2005.

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



11. Informative References

    [RFC2195]  Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension
               for Simple Challenge/Response", RFC 2195, MCI, September
               1997.

    [RFC2202]  Cheng, Glenn, "Test Cases for HMAC-MD5 and HMAC-SHA-1",
               RFC 2202, IBM, September 1997

    [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
               Specification Version 2.0", RFC 2898, September 2000.

    [TLS]  Dierks, Rescorla, "The Transport Layer Security (TLS)
               Protocol, Version 1.2", RFC 5246, August 2008.

    [RFC4949]  Shirey, "Internet Security Glossary, Version 2", RFC
               4949, FYI 0036, August 2007.

    [RFC4086]  Eastlake, Schiller, Crocker, "Randomness Requirements for
               Security", RFC 4086, BCP 0106, Motorola Laboratories,
               June 2005.

    [RFC4510]  Zeilenga, "Lightweight Directory Access Protocol (LDAP):
               Technical Specification Road Map", RFC 4510, June 2006.

    [DIGEST-MD5] Leach, P. and C. Newman , "Using Digest Authentication
               as a SASL Mechanism", RFC 2831, May 2000.  <<Also draft-



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               ietf-sasl-rfc2831bis-12.txt>>

    [DIGEST-HISTORIC] Melnikov, "Moving DIGEST-MD5 to Historic", work in
               progress, draft-ietf-sasl-digest-to-historic-00.txt, July
               2008

    [CRAM-HISTORIC] Zeilenga, "CRAM-MD5 to Historic", work in progress,
               draft-ietf-sasl-crammd5-to-historic-00.txt, November
               2008.

    [PLAIN] Zeilenga, "The PLAIN Simple Authentication and Security
               Layer (SASL) Mechanism" RFC 4616, August 2006.


12. Authors' Addresses

    Abhijit Menon-Sen
    Oryx Mail Systems GmbH

    Email: ams@oryx.com


    Alexey Melnikov
    Isode Ltd

    EMail: Alexey.Melnikov@isode.com


    Chris Newman
    Sun Microsystems
    1050 Lakes Drive
    West Covina, CA 91790
    USA

    Email: chris.newman@sun.com


Appendix A: Other Authentication Mechanisms

    The DIGEST-MD5 [DIGEST-MD5] mechanism has proved to be too complex
    to implement and test, and thus has poor interoperability. The
    security layer is often not implemented, and almost never used;
    everyone uses TLS instead.  For a more complete list of problems
    with DIGEST-MD5 which lead to the creation of SCRAM see [DIGEST-
    HISTORIC].

    The CRAM-MD5 SASL mechanism, while widely deployed has also some
    problems, in particular it is missing some modern SASL features such



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    as support for internationalized usernames and passwords, support
    for passing of authorization identity, support for channel bindings.
    It also doesn't support server authentication.  For a more complete
    list of problems with CRAM-MD5 see [CRAM-HISTORIC].

    The PLAIN [PLAIN] SASL mechanism allows a malicious server or
    eavesdropper to impersonate the authenticating user to any other
    server for which the user has the same password. It also sends the
    password in the clear over the network, unless TLS is used. Server
    authentication is not supported.


Appendix B: Design Motivations

    The following design goals shaped this document. Note that some of
    the goals have changed since the initial version of the document.

      The SASL mechanism has all modern SASL features: support for
      internationalized usernames and passwords, support for passing of
      authorization identity, support for channel bindings.

      Both the client and server can be authenticated by the protocol.

      The authentication information stored in the authentication
      database is not sufficient by itself to impersonate the client.

      <<The server does not gain the ability to impersonate the client
      to other servers (with an exception for server-authorized
      proxies).>>

      The mechanism is extensible, but [hopefully] not overengineered in
      this respect.

      Easier to implement than DIGEST-MD5 in both clients and servers.


Appendix C: SCRAM Examples

    <<To be written.>>












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        (RFC Editor: Please delete everything after this point)


Open Issues

    - The appendices need to be written.

    - Should the server send a base64-encoded ServerSignature for the
      value of the "v" attribute, or should it compute a ServerProof the
      way the client computes a ClientProof?


Changes since -07

    Updated References.

    Clarified purpose of the m= attribute.

    Fixed a problem with authentication/authorization identity's ABNF
      not allowing for some characters.

    Updated ABNF for nonce to show client-generated and server-generated
      parts.

    Only register SCRAM-HMAC-SHA-1 with IANA and require explicit
      registrations of all other SCRAM-HMAC- mechanisms.



Changes since -06

    Removed hash negotiation from SCRAM and turned it into a family of
      SASL mechanisms.

    Start using "Hash Function Textual Names" IANA registry for SCRAM
      mechanism naming.

    Fixed definition of Hi(str, salt) to be consistent with [RFC2898].

    Clarified extensibility of SCRAM: added m= attribute (for future
      mandatory extensions) and specified that all unrecognized
      attributes must be ignored.



Changes since -05

    Changed the mandatory to implement hash algorithm to SHA-1 (as per



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      WG consensus).

    Added text about use of SASLPrep for username
      canonicalization/validation.

    Clarified that authorization identity is canonicalized/verified
      according to SASL protocol profile.

    Clarified that iteration count is per-user.

    Clarified how clients select the authentication function.

    Added IANA registration for the new mechanism.

    Added missing normative references (UTF-8, SASLPrep).

    Various editorial changes based on comments from Hallvard B
      Furuseth, Nico William and Simon Josefsson.



Changes since -04

    - Update Base64 and Security Glossary references.

    - Add Formal Syntax section.

    - Don't bother with "v=".

    - Make MD5 mandatory to implement. Suggest i=128.



Changes since -03

    - Seven years have passed, in which it became clear that DIGEST-MD5
      suffered from unacceptably bad interoperability, so SCRAM-MD5 is
      now back from the dead.

    - Be hash agnostic, so MD5 can be replaced more easily.

    - General simplification.









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