Network Working Group                                  Abhijit Menon-Sen
Internet-Draft                                    Oryx Mail Systems GmbH
Intended Status: Proposed Standard                          Chris Newman
Expires: December 13, 2008                              Sun Microsystems
                                                           July 13, 2008

       Salted Challenge Response Authentication Mechanism (SCRAM)


Status of this Memo

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Copyright Notice

    Copyright (C) The IETF Trust (2008).


    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

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    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 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, this mechanism
    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",
    document are to be interpreted as described in [RFC2119].

    Formal syntax is defined by [RFC4234] including the core rules
    defined in Appendix B of [RFC4234].

    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

    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

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      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

1.2. Notation

    The pseudocode description of the algorithm uses the following

    - ":=": 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]).

    - 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

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    - 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)
         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 counter.  <<This is, essentially,
      PBKDF2 from RFC2898 with HMAC() as the PRF and with dkLen ==
      output length of HMAC() == output length of H().>>

2. Introduction

    This specification describes 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 [RFC4346]) or an equivalent security layer, this
    mechanism 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 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
      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

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

    - Bindings to several authentication frameworks are provided so the
      mechanism is not limited to a small subset of protocols. <<This
      doesn't seem to be true anymore.>>

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

    - The cryptographic hash function used to authenticate can be
      upgraded gracefully without breaking backwards compatibility or
      risking downgrade attacks. <<This will not be true, if hash
      negotiation is removed.>>

    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- mailing list or to the authors.

3. SCRAM Algorithm Overview

    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,
    and ServerKey and the iteration count. The server sends the salt and
    an 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 + "," +
        ClientSignature := HMAC(StoredKey, AuthMessage)
        ClientProof     := ClientKey XOR ClientSignature
        ServerKey       := HMAC(SaltedPassword, salt)
        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

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    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 SaltedPassword.

    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 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 4.1, and defined in the Formal
    Syntax section.

    This is a simple example of a SCRAM authentication exchange:

        C: n=Chris Newman,h=sha1,r=ClientNonce
        S: r=ClientNonceServerNonce,h=sha1,s=PxR/wv+epq,i=128
        C: r=ClientNonceServerNonce,p=WxPv/siO5l+qxN4
        S: v=WxPv/siO5l+qxN4

    First, the client sends a message containing the username, an
    optional authorization identity, a list of the hash functions it
    supports, and a random, unique nonce. In response, the server sends
    its list of supported hash functions, an iteration count i, the
    user's salt, and appends its own nonce to the client-specified one.
    The client then picks a hash function common to both the client and
    the server (*), and then it responds with the same nonce and a
    ClientProof computed using the selected hash function as explained
    earlier. The server verifies the nonce and the proof, verifies that
    the authorization identity (if supplied by the client in the initial
    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.

    (*) The client might retry SCRAM authentication using different hash
    functions. It MUST start with the strongest hash function (common to
    both the client and the server) it supports.

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4.1 SCRAM attributes

    This section describes the permissible attributes, their use, and
    the format of their values.

    - a: This optional attribute specifies an authorization identity. A
      client may include it in its first 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 (see appendix A for more details).

      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
      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 username

      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

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      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

    - h: This attribute is a colon-separated list of supported hash
      function names, as defined in the IANA "Hash Function Textual
      Names" registry.

    - 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
      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 may be sent by either the client or the server.
      If specified, the authentication MUST fail unless the value is
      successfully verified.  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 must be sent by the server along with the
      user's salt.

    - 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

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      authentication information. This value is computed as explained in
      the overview.

5. Hash functions

    SCRAM can use hash functions defined by the IANA "Hash Function
    Textual Names" registry.

    For interoperability, all SCRAM clients and servers MUST implement
    the SHA-1 hash function as defined in [RFC3174].

    Servers SHOULD announce a hash iteration-count of at least 128.

6. Formal Syntax

    The following syntax specification uses the Augmented Backus-Naur
    Form (ABNF) notation as specified in [RFC4234].  "UTF8-2", "UTF8-3"
    and "UTF8-4" non-terminal are defined in [UTF-8].

      generic-message = attr-val *("," attr-val)

      attr-val        = ALPHA "=" value

      value           = *(value-char)

      value-safe-char = %20-2B / %2D-3C / %3E-7E /
                        UTF8-2 / UTF-3 / UTF8-4
                        ;; UTF8-char except CTL, "=", 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]

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

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

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

      channel-binding = "c=" base64

      proof           = "p=" base64

      nonce           = "r=" value [value]
                        ;; Second part provided by server.

      salt            = "s=" base64

      verifier        = "v=" base64

      hash-list       = "h=" hash-name *(":" hash-name)

      hash-name       = value
                        ;; Hash Function Textual Name, from

      iteration-count = "i=" 1*DIGIT

      client-first-message =
                        [authzid ","] username "," hash-list "," nonce

      server-first-message =
                        nonce "," hash-list "," salt "," iteration-count

      client-final-message-without-proof =
                        nonce "," channel-binding

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

      server-final-message =

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

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    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.

    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 <<also iteration count?>>,
    password 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

    (To be done: Hash function names registry.)

    IANA is requested to add the following entry to the SASL Mechanism
    registry [IANA-SASL]:

    Subject: Registration of a new SASL mechanism SCRAM

    SASL mechanism name (or prefix for the family): SCRAM
    Security considerations: Section 7 of [RFCXXXX]
    Published specification (optional, recommended): [RFCXXXX]
    Person & email address to contact for further information:
     IETF SASL WG <>
    Intended usage: COMMON
    Owner/Change controller: IESG <>

9. Acknowedgements

    The authors would like to thank Alexey Melnikov and Dave Cridland
    for their 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
               Requirement Levels", RFC 2119, Harvard University, March

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

    [RFC4234]  Crocker, Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 4234, Brandenburg
               Internetworking, Demon Internet Ltd, October 2005.

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    [RFC4346]  Dierks, Rescorla, "The Transport Layer Security (TLS)
               Protocol, Version 1.1", RFC 4346, Brandenburg
               Internetworking, April 2006.

    [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

    [RFC1939]  Myers, Rose, "Post Office Protocol - Version 3", RFC
               1939, Carnegie Mellon, May 1996.

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

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

    [RFC2289]  Haller, Metz, Nesser, Straw, "A One-Time Password
               System", RFC 2289, STD0061, February 1998.

    [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.

    [RFC4120]  Neuman, Yo, Hartman, Raebun, "The Kerberos Network
               Authentication Service (V5)", RFC 4120, USC-ISI, July

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

    [DIGEST-MD5] Melnikov, "Using Digest Authentication as a SASL
               Mechanism", draft-ietf-sasl-rfc2831bis-12.txt, Isode
               Ltd., March 2007

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12. Authors' Addresses

    Abhijit Menon-Sen
    Oryx Mail Systems GmbH


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


Appendix A: Other Authentication Mechanisms

    The 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

    The 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.

    (To be completed.)

Appendix B: Design Motivations

    (To be written.)

Appendix C: SCRAM Examples

    (To be written.)

Appendix D: SCRAM Interoperability Testing

    (To be written.)

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Full Copyright Statement

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    This document and the information contained herein are provided on


    Funding for the RFC Editor function is currently provided by the
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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?

    - What about this LDAP attribute to store authentication

    - It is entirely unclear to me how best to handle channel bindings.
      Should the channel binding data be sent directly at all?

    Should hash algorithm be negotiated inside SCRAM, or should it be a
      part of the mechanism name (e.g. SCRAM-HMAC-SHA1)?

    Need to clarify extensibility of authentication exchange: should
      unrecognized attributes be ignored? If yes, need to update ABNF

Changes since -05
    Changed the mandatory to implement hash algorithm to SHA1 (as per WG

    Added text about use of SASLPrep for username

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

    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

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    - 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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