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rfc:rfc5802

Internet Engineering Task Force (IETF) C. Newman Request for Comments: 5802 Oracle Category: Standards Track A. Menon-Sen ISSN: 2070-1721 Oryx Mail Systems GmbH

                                                           A. Melnikov
                                                           Isode, Ltd.
                                                           N. Williams
                                                                Oracle
                                                             July 2010
     Salted Challenge Response Authentication Mechanism (SCRAM)
                    SASL and GSS-API Mechanisms

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 Simple Authentication and
 Security Layer (SASL; RFC 4422) 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.

Newman, et al. Standards Track [Page 1] RFC 5802 SCRAM July 2010

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5802.

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Newman, et al. Standards Track [Page 2] RFC 5802 SCRAM July 2010

Table of Contents

 1. Introduction ....................................................4
 2. Conventions Used in This Document ...............................5
    2.1. Terminology ................................................5
    2.2. Notation ...................................................6
 3. SCRAM Algorithm Overview ........................................7
 4. SCRAM Mechanism Names ...........................................8
 5. SCRAM Authentication Exchange ...................................9
    5.1. SCRAM Attributes ..........................................10
    5.2. Compliance with SASL Mechanism Requirements ...............13
 6. Channel Binding ................................................14
    6.1. Default Channel Binding ...................................15
 7. Formal Syntax ..................................................15
 8. SCRAM as a GSS-API Mechanism ...................................19
    8.1. GSS-API Principal Name Types for SCRAM ....................19
    8.2. GSS-API Per-Message Tokens for SCRAM ......................20
    8.3. GSS_Pseudo_random() for SCRAM .............................20
 9. Security Considerations ........................................20
 10. IANA Considerations ...........................................22
 11. Acknowledgements ..............................................23
 12. References ....................................................24
    12.1. Normative References .....................................24
    12.2. Normative References for GSS-API Implementors ............24
    12.3. Informative References ...................................25
 Appendix A. Other Authentication Mechanisms .......................27
 Appendix B. Design Motivations ....................................27

Newman, et al. Standards Track [Page 3] RFC 5802 SCRAM July 2010

1. 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 (see Appendix A and
 Appendix B).  When used in combination with Transport Layer Security
 (TLS; see [RFC5246]) 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 mechanisms does not presently include
 negotiation of a security layer [RFC4422].  It is intended to be used
 with an external security layer such as that provided by TLS or SSH,
 with optional channel binding [RFC5056] to the external security
 layer.
 SCRAM is specified herein as a pure Simple Authentication and
 Security Layer (SASL) [RFC4422] mechanism, but it conforms to the new
 bridge between SASL and the Generic Security Service Application
 Program Interface (GSS-API) called "GS2" [RFC5801].  This means that
 this document defines both, a SASL mechanism and a GSS-API mechanism.
 SCRAM provides the following protocol features:
 o  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.
 o  The server does not gain the ability to impersonate the client to
    other servers (with an exception for server-authorized proxies).
 o  The mechanism permits the use of a server-authorized proxy without
    requiring that proxy to have super-user rights with the back-end
    server.
 o  Mutual authentication is supported, but only the client is named
    (i.e., the server has no name).
 o  When used as a SASL mechanism, SCRAM is capable of transporting
    authorization identities (see [RFC4422], Section 2) from the
    client to the server.

Newman, et al. Standards Track [Page 4] RFC 5802 SCRAM July 2010

 A separate document defines a standard LDAPv3 [RFC4510] attribute
 that enables storage of the SCRAM authentication information in LDAP.
 See [RFC5803].
 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.

2. 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.

2.1. 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.
 Some clarifications and additional definitions follow:
 o  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, "StoredKey" and
    "ServerKey" (as defined in the algorithm overview) for each
    supported cryptographic hash function.
 o  Authentication database: The database used to look up the
    authentication information associated with a particular identity.
    For application protocols, LDAPv3 (see [RFC4510]) is frequently

Newman, et al. Standards Track [Page 5] RFC 5802 SCRAM July 2010

    used as the authentication database.  For network-level protocols
    such as PPP or 802.11x, the use of RADIUS [RFC2865] is more
    common.
 o  Base64: An encoding mechanism defined in [RFC4648] that converts
    an octet string input to a textual output string that can be
    easily displayed to a human.  The use of base64 in SCRAM is
    restricted to the canonical form with no whitespace.
 o  Octet: An 8-bit byte.
 o  Octet string: A sequence of 8-bit bytes.
 o  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.

2.2. Notation

 The pseudocode description of the algorithm uses the following
 notations:
 o  ":=": The variable on the left-hand side represents the octet
    string resulting from the expression on the right-hand side.
 o  "+": Octet string concatenation.
 o  "[ ]": 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.
 o  Normalize(str): Apply the SASLprep profile [RFC4013] of the
    "stringprep" algorithm [RFC3454] as the normalization algorithm to
    a UTF-8 [RFC3629] encoded "str".  The resulting string is also in
    UTF-8.  When applying SASLprep, "str" is treated as a "stored
    strings", which means that unassigned Unicode codepoints are
    prohibited (see Section 7 of [RFC3454]).  Note that
    implementations MUST either implement SASLprep or disallow use of
    non US-ASCII Unicode codepoints in "str".
 o  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]).

Newman, et al. Standards Track [Page 6] RFC 5802 SCRAM July 2010

 o  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.
 o  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.
 o  Hi(str, salt, i):
   U1   := HMAC(str, salt + INT(1))
   U2   := HMAC(str, U1)
   ...
   Ui-1 := HMAC(str, Ui-2)
   Ui   := HMAC(str, Ui-1)
   Hi := U1 XOR U2 XOR ... XOR Ui
    where "i" is the iteration count, "+" is the string concatenation
    operator, and INT(g) is a 4-octet encoding of the integer g, most
    significant octet first.
    Hi() is, essentially, PBKDF2 [RFC2898] with HMAC() as the
    pseudorandom function (PRF) and with dkLen == output length of
    HMAC() == output length of H().

3. SCRAM Algorithm Overview

 The following is a description of a full, uncompressed SASL SCRAM
 authentication exchange.  Nothing in SCRAM prevents either sending
 the client-first message with the SASL authentication request defined
 by an application protocol ("initial client response"), or sending
 the server-final message as additional data of the SASL outcome of
 authentication exchange defined by an application protocol.  See
 [RFC4422] for more details.
 Note that this section omits some details, such as client and server
 nonces.  See Section 5 for more details.
 To begin with, the SCRAM client is in possession of a username and
 password (*) (or a ClientKey/ServerKey, or SaltedPassword).  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 choose

Newman, et al. Standards Track [Page 7] RFC 5802 SCRAM July 2010

 to use the same iteration count for all accounts.)  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:
 (*) Note that both the username and the password MUST be encoded in
 UTF-8 [RFC3629].
 Informative Note: Implementors are encouraged to create test cases
 that use both usernames and passwords with non-ASCII codepoints.  In
 particular, it's useful to test codepoints whose "Unicode
 Normalization Form C" and "Unicode Normalization Form KC" are
 different.  Some examples of such codepoints include Vulgar Fraction
 One Half (U+00BD) and Acute Accent (U+00B4).
   SaltedPassword  := Hi(Normalize(password), salt, i)
   ClientKey       := HMAC(SaltedPassword, "Client Key")
   StoredKey       := H(ClientKey)
   AuthMessage     := client-first-message-bare + "," +
                      server-first-message + "," +
                      client-final-message-without-proof
   ClientSignature := HMAC(StoredKey, AuthMessage)
   ClientProof     := ClientKey XOR ClientSignature
   ServerKey       := HMAC(SaltedPassword, "Server Key")
   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
 Section 7.

4. SCRAM Mechanism Names

 A SCRAM mechanism name is a string "SCRAM-" followed by the
 uppercased name of the underlying hash function taken from the IANA
 "Hash Function Textual Names" registry (see http://www.iana.org),
 optionally followed by the suffix "-PLUS" (see below).  Note that
 SASL mechanism names are limited to 20 octets, which means that only

Newman, et al. Standards Track [Page 8] RFC 5802 SCRAM July 2010

 hash function names with lengths shorter or equal to 9 octets
 (20-length("SCRAM-")-length("-PLUS") can be used.  For cases when the
 underlying hash function name is longer than 9 octets, an alternative
 9-octet (or shorter) name can be used to construct the corresponding
 SCRAM mechanism name, as long as this alternative name doesn't
 conflict with any other hash function name from the IANA "Hash
 Function Textual Names" registry.  In order to prevent future
 conflict, such alternative names SHOULD be registered in the IANA
 "Hash Function Textual Names" registry.
 For interoperability, all SCRAM clients and servers MUST implement
 the SCRAM-SHA-1 authentication mechanism, i.e., an authentication
 mechanism from the SCRAM family that uses the SHA-1 hash function as
 defined in [RFC3174].
 The "-PLUS" suffix is used only when the server supports channel
 binding to the external channel.  If the server supports channel
 binding, it will advertise both the "bare" and "plus" versions of
 whatever mechanisms it supports (e.g., if the server supports only
 SCRAM with SHA-1, then it will advertise support for both SCRAM-SHA-1
 and SCRAM-SHA-1-PLUS).  If the server does not support channel
 binding, then it will advertise only the "bare" version of the
 mechanism (e.g., only SCRAM-SHA-1).  The "-PLUS" exists to allow
 negotiation of the use of channel binding.  See Section 6.

5. SCRAM Authentication Exchange

 SCRAM is a SASL mechanism whose client response and server challenge
 messages are text-based 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 Section 7.
 SCRAM is a client-first SASL mechanism (see [RFC4422], Section 5,
 item 2a), and returns additional data together with a server's
 indication of a successful outcome.
 This is a simple example of a SCRAM-SHA-1 authentication exchange
 when the client doesn't support channel bindings (username 'user' and
 password 'pencil' are used):
 C: n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL
 S: r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,s=QSXCR+Q6sek8bf92,
    i=4096
 C: c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
    p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts=
 S: v=rmF9pqV8S7suAoZWja4dJRkFsKQ=

Newman, et al. Standards Track [Page 9] RFC 5802 SCRAM July 2010

 First, the client sends the "client-first-message" containing:
 o  a GS2 header consisting of a flag indicating whether channel
    binding is supported-but-not-used, not supported, or used, and an
    optional SASL authorization identity;
 o  SCRAM username and a random, unique nonce attributes.
 Note that the client's first message will always start with "n", "y",
 or "p"; otherwise, the message is invalid and authentication MUST
 fail.  This is important, as it allows for GS2 extensibility (e.g.,
 to add support for security layers).
 In response, the server sends a "server-first-message" containing the
 user's iteration count i and the user's salt, and appends its own
 nonce to the client-specified one.
 The client then responds by sending a "client-final-message" 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 first
 message) is authorized to act as the authentication identity, and,
 finally, it responds with a "server-final-message", 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.
 Note that the order of attributes in client or server messages is
 fixed, with the exception of extension attributes (described by the
 "extensions" ABNF production), which can appear in any order in the
 designated positions.  See Section 7 for authoritative reference.
 o  a: This is an optional attribute, and is part of the GS2 [RFC5801]
    bridge between the GSS-API and SASL.  This 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

Newman, et al. Standards Track [Page 10] RFC 5802 SCRAM July 2010

    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), 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 ','.
 o  n: This attribute specifies the name of the user whose password is
    used for authentication (a.k.a. "authentication identity"
    [RFC4422]).  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 that
    will be associated with the connection subsequent to
    authentication and authorization.
       Before sending the username to the server, the client SHOULD
       prepare the username using the "SASLprep" profile [RFC4013] of
       the "stringprep" algorithm [RFC3454] treating it as a query
       string (i.e., unassigned Unicode code points are allowed).  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 MUST
       either prepare it using the "SASLprep" profile [RFC4013] of the
       "stringprep" algorithm [RFC3454] treating it as a query string
       (i.e., unassigned Unicode codepoints are allowed) or otherwise
       be prepared to do SASLprep-aware string comparisons and/or
       index lookups.  If the preparation of the username fails or
       results in an empty string, the server SHOULD abort the

Newman, et al. Standards Track [Page 11] RFC 5802 SCRAM July 2010

       authentication exchange.  Whether or not the server prepares
       the username using "SASLprep", it MUST use it as received in
       hash calculations.
       The characters ',' or '=' in usernames are sent as '=2C' and
       '=3D' respectively.  If the server receives a username that
       contains '=' not followed by either '2C' or '3D', then the
       server MUST fail the authentication.
 o  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.
 o  r: This attribute specifies a sequence of random printable ASCII
    characters excluding ',' (which forms the nonce used as input to
    the hash function).  No quoting is applied to this string.  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 value be
    different for each authentication (see [RFC4086] for more details
    on how to achieve this).  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.
 o  c: This REQUIRED attribute specifies the base64-encoded GS2 header
    and channel binding data.  It is sent by the client in its second
    authentication message.  The attribute data consist of:
  • the GS2 header from the client's first message (recall that the

GS2 header contains a channel binding flag and an optional

       authzid).  This header is going to include channel binding type
       prefix (see [RFC5056]), if and only if the client is using
       channel binding;
  • followed by the external channel's channel binding data, if and

only if the client is using channel binding.

 o  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.
 o  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.

Newman, et al. Standards Track [Page 12] RFC 5802 SCRAM July 2010

       For the SCRAM-SHA-1/SCRAM-SHA-1-PLUS SASL mechanism, servers
       SHOULD announce a hash iteration-count of at least 4096.  Note
       that a client implementation MAY cache ClientKey&ServerKey (or
       just SaltedPassword) for later reauthentication to the same
       service, as it is likely that the server is going to advertise
       the same salt value upon reauthentication.  This might be
       useful for mobile clients where CPU usage is a concern.
 o  p: This attribute specifies a base64-encoded ClientProof.  The
    client computes this value as described in the overview and sends
    it to the server.
 o  v: This attribute specifies a base64-encoded ServerSignature.  It
    is sent by the server in its final message, and is 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.
 o  e: This attribute specifies an error that occurred during
    authentication exchange.  It is sent by the server in its final
    message and can help diagnose the reason for the authentication
    exchange failure.  On failed authentication, the entire server-
    final-message is OPTIONAL; specifically, a server implementation
    MAY conclude the SASL exchange with a failure without sending the
    server-final-message.  This results in an application-level error
    response without an extra round-trip.  If the server-final-message
    is sent on authentication failure, then the "e" attribute MUST be
    included.
 o  As-yet unspecified mandatory and optional extensions.  Mandatory
    extensions are encoded as values of the 'm' attribute (see ABNF
    for reserved-mext in section 7).  Optional extensions use as-yet
    unassigned attribute names.
    Mandatory extensions sent by one peer but not understood by the
    other MUST cause authentication failure (the server SHOULD send
    the "extensions-not-supported" server-error-value).
    Unknown optional extensions MUST be ignored upon receipt.

5.2. Compliance with SASL Mechanism Requirements

 This section describes compliance with SASL mechanism requirements
 specified in Section 5 of [RFC4422].
 1)  "SCRAM-SHA-1" and "SCRAM-SHA-1-PLUS".
 2a) SCRAM is a client-first mechanism.

Newman, et al. Standards Track [Page 13] RFC 5802 SCRAM July 2010

 2b) SCRAM sends additional data with success.
 3)  SCRAM is capable of transferring authorization identities from
     the client to the server.
 4)  SCRAM does not offer any security layers (SCRAM offers channel
     binding instead).
 5)  SCRAM has a hash protecting the authorization identity.

6. Channel Binding

 SCRAM supports channel binding to external secure channels, such as
 TLS.  Clients and servers may or may not support channel binding,
 therefore the use of channel binding is negotiable.  SCRAM does not
 provide security layers, however, therefore it is imperative that
 SCRAM provide integrity protection for the negotiation of channel
 binding.
 Use of channel binding is negotiated as follows:
 o  Servers that support the use of channel binding SHOULD advertise
    both the non-PLUS (SCRAM-<hash-function>) and PLUS-variant (SCRAM-
    <hash-function>-PLUS) mechanism name.  If the server cannot
    support channel binding, it SHOULD advertise only the non-PLUS-
    variant.  If the server would never succeed in the authentication
    of the non-PLUS-variant due to policy reasons, it MUST advertise
    only the PLUS-variant.
 o  If the client supports channel binding and the server does not
    appear to (i.e., the client did not see the -PLUS name advertised
    by the server), then the client MUST NOT use an "n" gs2-cbind-
    flag.
 o  Clients that support mechanism negotiation and channel binding
    MUST use a "p" gs2-cbind-flag when the server offers the PLUS-
    variant of the desired GS2 mechanism.
 o  If the client does not support channel binding, then it MUST use
    an "n" gs2-cbind-flag.  Conversely, if the client requires the use
    of channel binding then it MUST use a "p" gs2-cbind-flag.  Clients
    that do not support mechanism negotiation never use a "y" gs2-
    cbind-flag, they use either "p" or "n" according to whether they
    require and support the use of channel binding or whether they do
    not, respectively.
 o  Upon receipt of the client-first message, the server checks the
    channel binding flag (gs2-cbind-flag).

Newman, et al. Standards Track [Page 14] RFC 5802 SCRAM July 2010

  • If the flag is set to "y" and the server supports channel

binding, the server MUST fail authentication. This is because

       if the client sets the channel binding flag to "y", then the
       client must have believed that the server did not support
       channel binding -- if the server did in fact support channel
       binding, then this is an indication that there has been a
       downgrade attack (e.g., an attacker changed the server's
       mechanism list to exclude the -PLUS suffixed SCRAM mechanism
       name(s)).
  • If the channel binding flag was "p" and the server does not

support the indicated channel binding type, then the server

       MUST fail authentication.
 The server MUST always validate the client's "c=" field.  The server
 does this by constructing the value of the "c=" attribute and then
 checking that it matches the client's c= attribute value.
 For more discussions of channel bindings, and the syntax of channel
 binding data for various security protocols, see [RFC5056].

6.1. Default Channel Binding

 A default channel binding type agreement process for all SASL
 application protocols that do not provide their own channel binding
 type agreement is provided as follows.
 'tls-unique' is the default channel binding type for any application
 that doesn't specify one.
 Servers MUST implement the "tls-unique" [RFC5929] channel binding
 type, if they implement any channel binding.  Clients SHOULD
 implement the "tls-unique" [RFC5929] channel binding type, if they
 implement any channel binding.  Clients and servers SHOULD choose the
 highest-layer/innermost end-to-end TLS channel as the channel to
 which to bind.
 Servers MUST choose the channel binding type indicated by the client,
 or fail authentication if they don't support it.

7. 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 [RFC3629].

Newman, et al. Standards Track [Page 15] RFC 5802 SCRAM July 2010

 ALPHA = <as defined in RFC 5234 appendix B.1>
 DIGIT = <as defined in RFC 5234 appendix B.1>
 UTF8-2 = <as defined in RFC 3629 (STD 63)>
 UTF8-3 = <as defined in RFC 3629 (STD 63)>
 UTF8-4 = <as defined in RFC 3629 (STD 63)>
 attr-val        = ALPHA "=" value
                   ;; Generic syntax of any attribute sent
                   ;; by server or client
 value           = 1*value-char
 value-safe-char = %x01-2B / %x2D-3C / %x3E-7F /
                   UTF8-2 / UTF8-3 / UTF8-4
                   ;; UTF8-char except NUL, "=", and ",".
 value-char      = value-safe-char / "="
 printable       = %x21-2B / %x2D-7E
                   ;; Printable ASCII except ",".
                   ;; Note that any "printable" is also
                   ;; a valid "value".
 base64-char     = ALPHA / DIGIT / "/" / "+"
 base64-4        = 4base64-char
 base64-3        = 3base64-char "="
 base64-2        = 2base64-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.
 cb-name         = 1*(ALPHA / DIGIT / "." / "-")
                    ;; See RFC 5056, Section 7.
                    ;; E.g., "tls-server-end-point" or
                    ;; "tls-unique".

Newman, et al. Standards Track [Page 16] RFC 5802 SCRAM July 2010

 gs2-cbind-flag  = ("p=" cb-name) / "n" / "y"
                   ;; "n" -> client doesn't support channel binding.
                   ;; "y" -> client does support channel binding
                   ;;        but thinks the server does not.
                   ;; "p" -> client requires channel binding.
                   ;; The selected channel binding follows "p=".
 gs2-header      = gs2-cbind-flag "," [ authzid ] ","
                   ;; GS2 header for SCRAM
                   ;; (the actual GS2 header includes an optional
                   ;; flag to indicate that the GSS mechanism is not
                   ;; "standard", but since SCRAM is "standard", we
                   ;; don't include that flag).
 username        = "n=" saslname
                   ;; Usernames are prepared using SASLprep.
 reserved-mext  = "m=" 1*(value-char)
                   ;; Reserved for signaling mandatory extensions.
                   ;; The exact syntax will be defined in
                   ;; the future.
 channel-binding = "c=" base64
                   ;; base64 encoding of cbind-input.
 proof           = "p=" base64
 nonce           = "r=" c-nonce [s-nonce]
                   ;; Second part provided by server.
 c-nonce         = printable
 s-nonce         = printable
 salt            = "s=" base64
 verifier        = "v=" base64
                   ;; base-64 encoded ServerSignature.
 iteration-count = "i=" posit-number
                   ;; A positive number.
 client-first-message-bare =
                   [reserved-mext ","]
                   username "," nonce ["," extensions]
 client-first-message =
                   gs2-header client-first-message-bare

Newman, et al. Standards Track [Page 17] RFC 5802 SCRAM July 2010

 server-first-message =
                   [reserved-mext ","] nonce "," salt ","
                   iteration-count ["," extensions]
 client-final-message-without-proof =
                   channel-binding "," nonce [","
                   extensions]
 client-final-message =
                   client-final-message-without-proof "," proof
 server-error = "e=" server-error-value
 server-error-value = "invalid-encoding" /
                "extensions-not-supported" /  ; unrecognized 'm' value
                "invalid-proof" /
                "channel-bindings-dont-match" /
                "server-does-support-channel-binding" /
                  ; server does not support channel binding
                "channel-binding-not-supported" /
                "unsupported-channel-binding-type" /
                "unknown-user" /
                "invalid-username-encoding" /
                  ; invalid username encoding (invalid UTF-8 or
                  ; SASLprep failed)
                "no-resources" /
                "other-error" /
                server-error-value-ext
         ; Unrecognized errors should be treated as "other-error".
         ; In order to prevent information disclosure, the server
         ; may substitute the real reason with "other-error".
 server-error-value-ext = value
         ; Additional error reasons added by extensions
         ; to this document.
 server-final-message = (server-error / verifier)
                   ["," extensions]
 extensions = attr-val *("," attr-val)
                   ;; All extensions are optional,
                   ;; i.e., unrecognized attributes
                   ;; not defined in this document
                   ;; MUST be ignored.
 cbind-data    = 1*OCTET

Newman, et al. Standards Track [Page 18] RFC 5802 SCRAM July 2010

 cbind-input   = gs2-header [ cbind-data ]
                   ;; cbind-data MUST be present for
                   ;; gs2-cbind-flag of "p" and MUST be absent
                   ;; for "y" or "n".

8. SCRAM as a GSS-API Mechanism

 This section and its sub-sections and all normative references of it
 not referenced elsewhere in this document are INFORMATIONAL for SASL
 implementors, but they are NORMATIVE for GSS-API implementors.
 SCRAM is actually also a GSS-API mechanism.  The messages are the
 same, but a) the GS2 header on the client's first message and channel
 binding data is excluded when SCRAM is used as a GSS-API mechanism,
 and b) the RFC2743 section 3.1 initial context token header is
 prefixed to the client's first authentication message (context
 token).
 The GSS-API mechanism OID for SCRAM-SHA-1 is 1.3.6.1.5.5.14 (see
 Section 10).
 SCRAM security contexts always have the mutual_state flag
 (GSS_C_MUTUAL_FLAG) set to TRUE.  SCRAM does not support credential
 delegation, therefore SCRAM security contexts alway have the
 deleg_state flag (GSS_C_DELEG_FLAG) set to FALSE.

8.1. GSS-API Principal Name Types for SCRAM

 SCRAM does not explicitly name acceptor principals.  However, the use
 of acceptor principal names to find or prompt for passwords is
 useful.  Therefore, SCRAM supports standard generic name syntaxes for
 acceptors such as GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section
 4.1).  Implementations should use the target name passed to
 GSS_Init_sec_context(), if any, to help retrieve or prompt for SCRAM
 passwords.
 SCRAM supports only a single name type for initiators:
 GSS_C_NT_USER_NAME.  GSS_C_NT_USER_NAME is the default name type for
 SCRAM.
 There is no name canonicalization procedure for SCRAM beyond applying
 SASLprep as described in Section 5.1.
 The query, display, and exported name syntaxes for SCRAM principal
 names are all the same.  There are no SCRAM-specific name syntaxes
 (SCRAM initiator principal names are free-form); -- applications
 should use generic GSS-API name types such as GSS_C_NT_USER_NAME and

Newman, et al. Standards Track [Page 19] RFC 5802 SCRAM July 2010

 GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section 4).  The exported
 name token does, of course, conform to [RFC2743], Section 3.2, but
 the "NAME" part of the token is just a SCRAM user name.

8.2. GSS-API Per-Message Tokens for SCRAM

 The per-message tokens for SCRAM as a GSS-API mechanism SHALL be the
 same as those for the Kerberos V GSS-API mechanism [RFC4121] (see
 Section 4.2 and sub-sections), using the Kerberos V "aes128-cts-hmac-
 sha1-96" enctype [RFC3962].
 The replay_det_state (GSS_C_REPLAY_FLAG), sequence_state
 (GSS_C_SEQUENCE_FLAG), conf_avail (GSS_C_CONF_FLAG) and integ_avail
 (GSS_C_CONF_FLAG) security context flags are always set to TRUE.
 The 128-bit session "protocol key" SHALL be derived by using the
 least significant (right-most) 128 bits of HMAC(StoredKey, "GSS-API
 session key" || ClientKey || AuthMessage).  "Specific keys" are then
 derived as usual as described in Section 2 of [RFC4121], [RFC3961],
 and [RFC3962].
 The terms "protocol key" and "specific key" are Kerberos V5 terms
 [RFC3961].
 SCRAM does support PROT_READY, and is PROT_READY on the initiator
 side first upon receipt of the server's reply to the initial security
 context token.

8.3. GSS_Pseudo_random() for SCRAM

 The GSS_Pseudo_random() [RFC4401] for SCRAM SHALL be the same as for
 the Kerberos V GSS-API mechanism [RFC4402].  There is no acceptor-
 asserted sub-session key for SCRAM, thus GSS_C_PRF_KEY_FULL and
 GSS_C_PRF_KEY_PARTIAL are equivalent for SCRAM's GSS_Pseudo_random().
 The protocol key to be used for the GSS_Pseudo_random() SHALL be the
 same as the key defined in Section 8.2.

9. Security Considerations

 If the authentication exchange is performed without a strong security
 layer (such as TLS with data confidentiality), then a passive
 eavesdropper can gain sufficient information to mount an offline
 dictionary or brute-force attack that 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.

Newman, et al. Standards Track [Page 20] RFC 5802 SCRAM July 2010

 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 allows to increase the iteration count over
 time.  (Note that a server that is only in possession of "StoredKey"
 and "ServerKey" can't automatically increase the iteration count upon
 successful authentication.  Such an increase would require resetting
 the user's password.)
 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 that protect against this attack are
 available (e.g., the EKE class of mechanisms).  RFC 2945 [RFC2945] is
 an example of such technology.  The WG elected not to use EKE like
 mechanisms as a basis for SCRAM.
 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.
 SCRAM does not negotiate a hash function to use.  Hash function
 negotiation is left to the SASL mechanism negotiation.  It is
 important that clients be able to sort a locally available list of
 mechanisms by preference so that the client may pick the appropriate
 mechanism to use from a server's advertised mechanism list.  This
 preference order is not specified here as it is a local matter.  The
 preference order should include objective and subjective notions of
 mechanism cryptographic strength (e.g., SCRAM with a successor to
 SHA-1 may be preferred over SCRAM with SHA-1).
 Note that to protect the SASL mechanism negotiation applications
 normally must list the server mechanisms twice: once before and once
 after authentication, the latter using security layers.  Since SCRAM
 does not provide security layers, the only ways to protect the
 mechanism negotiation are a) use channel binding to an external
 channel, or b) use an external channel that authenticates a user-
 provided server name.

Newman, et al. Standards Track [Page 21] RFC 5802 SCRAM July 2010

 SCRAM does not protect against downgrade attacks of channel binding
 types.  The complexities of negotiating a channel binding type, and
 handling down-grade attacks in that negotiation, were intentionally
 left out of scope for this document.
 A hostile server can perform a computational denial-of-service attack
 on clients by sending a big iteration count value.
 See [RFC4086] for more information about generating randomness.

10. IANA Considerations

 IANA has added the following family of SASL mechanisms to the SASL
 Mechanism registry established by [RFC4422]:
 To: iana@iana.org
 Subject: Registration of a new SASL family SCRAM
 SASL mechanism name (or prefix for the family): SCRAM-*
 Security considerations: Section 7 of [RFC5802]
 Published specification (optional, recommended): [RFC5802]
 Person & email address to contact for further information:
 IETF SASL WG <sasl@ietf.org>
 Intended usage: COMMON
 Owner/Change controller: IESG <iesg@ietf.org>
 Note: Members of this family MUST be explicitly registered
 using the "IETF Review" [RFC5226] registration procedure.
 Reviews MUST be requested on the SASL mailing list
 <sasl@ietf.org> (or a successor designated by the responsible
 Security AD).
 Note to future SCRAM-mechanism designers: each new SCRAM-SASL
 mechanism MUST be explicitly registered with IANA and MUST comply
 with SCRAM-mechanism naming convention defined in Section 4 of this
 document.

Newman, et al. Standards Track [Page 22] RFC 5802 SCRAM July 2010

 IANA has added the following entries to the SASL Mechanism registry
 established by [RFC4422]:
 To: iana@iana.org
 Subject: Registration of a new SASL mechanism SCRAM-SHA-1
 SASL mechanism name (or prefix for the family): SCRAM-SHA-1
 Security considerations: Section 7 of [RFC5802]
 Published specification (optional, recommended): [RFC5802]
 Person & email address to contact for further information:
 IETF SASL WG <sasl@ietf.org>
 Intended usage: COMMON
 Owner/Change controller: IESG <iesg@ietf.org>
 Note:
 To: iana@iana.org
 Subject: Registration of a new SASL mechanism SCRAM-SHA-1-PLUS
 SASL mechanism name (or prefix for the family): SCRAM-SHA-1-PLUS
 Security considerations: Section 7 of [RFC5802]
 Published specification (optional, recommended): [RFC5802]
 Person & email address to contact for further information:
 IETF SASL WG <sasl@ietf.org>
 Intended usage: COMMON
 Owner/Change controller: IESG <iesg@ietf.org>
 Note:
 Per this document, IANA has assigned a GSS-API mechanism OID for
 SCRAM-SHA-1 from the iso.org.dod.internet.security.mechanisms prefix
 (see "SMI Security for Mechanism Codes" registry).

11. Acknowledgements

 This document benefited from discussions on the SASL WG mailing list.
 The authors would like to specially thank Dave Cridland, Simon
 Josefsson, Jeffrey Hutzelman, Kurt Zeilenga, Pasi Eronen, Ben
 Campbell, Peter Saint-Andre, and Tobias Markmann for their
 contributions to this document.  A special thank you to Simon
 Josefsson for shepherding this document and for doing one of the
 first implementations of this specification.

Newman, et al. Standards Track [Page 23] RFC 5802 SCRAM July 2010

12. References

12.1. Normative References

 [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
            Hashing for Message Authentication", RFC 2104,
            February 1997.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
            (SHA1)", RFC 3174, September 2001.
 [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.
 [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
            Security Layer (SASL)", RFC 4422, June 2006.
 [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 4648, October 2006.
 [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
            Channels", RFC 5056, November 2007.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
            for TLS", RFC 5929, July 2010.

12.2. Normative References for GSS-API Implementors

 [RFC2743]  Linn, J., "Generic Security Service Application Program
            Interface Version 2, Update 1", RFC 2743, January 2000.
 [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
            Kerberos 5", RFC 3961, February 2005.

Newman, et al. Standards Track [Page 24] RFC 5802 SCRAM July 2010

 [RFC3962]  Raeburn, K., "Advanced Encryption Standard (AES)
            Encryption for Kerberos 5", RFC 3962, February 2005.
 [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
            Version 5 Generic Security Service Application Program
            Interface (GSS-API) Mechanism: Version 2", RFC 4121,
            July 2005.
 [RFC4401]  Williams, N., "A Pseudo-Random Function (PRF) API
            Extension for the Generic Security Service Application
            Program Interface (GSS-API)", RFC 4401, February 2006.
 [RFC4402]  Williams, N., "A Pseudo-Random Function (PRF) for the
            Kerberos V Generic Security Service Application Program
            Interface (GSS-API) Mechanism", RFC 4402, February 2006.
 [RFC5801]  Josefsson, S. and N. Williams, "Using Generic Security
            Service Application Program Interface (GSS-API) Mechanisms
            in Simple Authentication and Security Layer (SASL): The
            GS2 Mechanism Family", RFC 5801, July 2010.

12.3. Informative References

 [CRAMHISTORIC]
            Zeilenga, K., "CRAM-MD5 to Historic", Work in Progress,
            November 2008.
 [DIGESTHISTORIC]
            Melnikov, A., "Moving DIGEST-MD5 to Historic", Work
            in Progress, July 2008.
 [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
            "Remote Authentication Dial In User Service (RADIUS)",
            RFC 2865, June 2000.
 [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
            Specification Version 2.0", RFC 2898, September 2000.
 [RFC2945]  Wu, T., "The SRP Authentication and Key Exchange System",
            RFC 2945, September 2000.
 [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
            Requirements for Security", BCP 106, RFC 4086, June 2005.
 [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol
            (LDAP): Technical Specification Road Map", RFC 4510,
            June 2006.

Newman, et al. Standards Track [Page 25] RFC 5802 SCRAM July 2010

 [RFC4616]  Zeilenga, K., "The PLAIN Simple Authentication and
            Security Layer (SASL) Mechanism", RFC 4616, August 2006.
 [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
            RFC 4949, August 2007.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.
 [RFC5803]  Melnikov, A., "Lightweight Directory Access Protocol
            (LDAP) Schema for Storing Salted Challenge Response
            Authentication Mechanism (SCRAM) Secrets", RFC 5803,
            July 2010.
 [tls-server-end-point]
            IANA, "Registration of TLS server end-point channel
            bindings", available from http://www.iana.org, June 2008.

Newman, et al. Standards Track [Page 26] RFC 5802 SCRAM July 2010

Appendix A. Other Authentication Mechanisms

 The DIGEST-MD5 [DIGESTHISTORIC] 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 that led to the creation of SCRAM, see [DIGESTHISTORIC].
 The CRAM-MD5 SASL mechanism, while widely deployed, also has some
 problems.  In particular, it is missing some modern SASL features
 such as support for internationalized usernames and passwords,
 support for passing of authorization identity, and support for
 channel bindings.  It also doesn't support server authentication.
 For a more complete list of problems with CRAM-MD5, see
 [CRAMHISTORIC].
 The PLAIN [RFC4616] 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.
 o  The SASL mechanism has all modern SASL features: support for
    internationalized usernames and passwords, support for passing of
    authorization identity, and support for channel bindings.
 o  The protocol supports mutual authentication.
 o  The authentication information stored in the authentication
    database is not sufficient by itself to impersonate the client.
 o  The server does not gain the ability to impersonate the client to
    other servers (with an exception for server-authorized proxies),
    unless such other servers allow SCRAM authentication and use the
    same salt and iteration count for the user.
 o  The mechanism is extensible, but (hopefully) not over-engineered
    in this respect.
 o  The mechanism is easier to implement than DIGEST-MD5 in both
    clients and servers.

Newman, et al. Standards Track [Page 27] RFC 5802 SCRAM July 2010

Authors' Addresses

 Chris Newman
 Oracle
 800 Royal Oaks
 Monrovia, CA  91016
 USA
 EMail: chris.newman@oracle.com
 Abhijit Menon-Sen
 Oryx Mail Systems GmbH
 EMail: ams@toroid.org
 Alexey Melnikov
 Isode, Ltd.
 EMail: Alexey.Melnikov@isode.com
 Nicolas Williams
 Oracle
 5300 Riata Trace Ct
 Austin, TX  78727
 USA
 EMail: Nicolas.Williams@oracle.com

Newman, et al. Standards Track [Page 28]

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