HTTPAUTH                                                     A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Standards Track                        October 18, 2013
Expires: April 21,                            July 4, 2014
Expires: January 5, 2015

    Salted Challenge Response (SCRAM) HTTP Authentication Mechanism


   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 HTTP
   Digest challenge response mechanism presently on the standards track
   failed widespread deployment, and have had success only in limited

   This specification describes a family of HTTP 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.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 21, 2014. January 5, 2015.

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Table of Contents

   1.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . . .   3   4
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  SCRAM Algorithm Overview  . . . . . . . . . . . . . . . . . .   5   6
   4.  SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . .   6   7
   5.  SCRAM Authentication Exchange . . . . . . . . . . . . . . . .   7
   5.1.  SCRAM Attributes  . . . . . . . . . . . . . . . . . . . . .   9
   6.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .  11  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   10. Design Motivations  . . . . . . . . . . . . . . . . . . . . .  16  17
   11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  16  17
   12. Internet-Draft Change History . . . . . . . . . . . . . . . .  16  17
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16  17
   13.1.  Normative References . . . . . . . . . . . . . . . . . . .  17
   13.2.  Informative References . . . . . . . . . . . . . . . . . .  17  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18  19

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 [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

   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, the "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
      used as the authentication database.  For network-level protocols
      such as PPP or 802.11x, the use of RADIUS [RFC2865] is more

   o  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.

   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

1.2.  Notation

   The pseudocode description of the algorithm uses the following

   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]).

   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

   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 four-octet encoding of the integer g,
      most significant octet first.

      Hi() 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 (see [RFC5802]).
   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

   SCRAM provides the following protocol features:

   o  The authentication information stored in the authentication
      database is not sufficient by itself (without a dictionary attack)
      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

   o  Mutual authentication is supported, but only the client is named
      (i.e., the server has no name).

3.  SCRAM Algorithm Overview

   The following is a description of a full HTTP SCRAM authentication
   exchange.  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 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 username 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 + "," +
      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

   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

   The AuthMessage is computed by concatenating messages from the
   authentication exchange.  The format of these messages is defined in
   Section 6.

4.  SCRAM Mechanism Names

   A SCRAM mechanism name (authentication scheme) 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:// .

   For interoperability, all HTTP clients and servers supporting SCRAM
   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].

5.  SCRAM Authentication Exchange

   SCRAM is a HTTP Authentication mechanism whose client response
   (<credentials-scram>) and server challenge (<challenge-scram>)
   messages are text-based messages containing one or more attribute-
   value pairs separated by commas.  Each attribute has a one-letter
   name, with the exception of a couple of attributes which are generic
   to HTTP authentication, such as "realm" (and "sid").  The messages
   and their attributes are described in Section 5.1, and defined in
   Section 6.

       challenge-scram   = "SCRAM-SHA-1" 1*SP 1#auth-param scram-name [1*SP 1#auth-param]
             ; Complies with <challenge> ABNF from I-D.ietf-httpbis-p7-auth. RFC 7235.
             ; Included in the WWW-Authenticate header field.

       credentials-scram = "SCRAM-SHA-1" 1*SP 1#auth-param scram-name [1*SP 1#auth-param]
             ; Complies with <credentials> from I-D.ietf-httpbis-p7-auth RFC 7235.
             ; Included in the Authorization header field.

       scram-name = "SCRAM-SHA-1" / other-scram-name
             ; SCRAM-SHA-1 is registered by this RFC
       other-scram-name = "SCRAM-" hash-name
             ; hash-name is a capitalized form of names from IANA
             ; "Hash Function Textual Names" registry.
             ; Additional SCRAM names must be registered in both
             ; the IANA "SASL mechanisms" registry
             ; and the IANA "authentication scheme" registry.

   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):

      [...The server might have returned

   C: GET /resource HTTP/1.1
   C: Host:
   C: [...]

   S: HTTP/1.1 401 Unauthorized first...]
   S: WWW-Authenticate: Digest realm="",
          Digest realm="",
          Digest realm="",
          SCRAM-SHA-1 realm=""
          SCRAM-SHA-1 realm=""
   S: [...]

   C: GET /resource HTTP/1.1
   C: Host:
   C: Authorization: SCRAM-SHA-1 realm="",
   C: [...]

   S: HTTP/1.1 401 Unauthorized
   S: WWW-Authenticate: SCRAM-SHA-1
   S: [...]

   C: GET /resource HTTP/1.1
   C: Host:
   C: Authorization: SCRAM-SHA-1 sid=AAAABBBBCCCCDDDD,
   C: [...]

   S: HTTP/1.1 200 Ok
   S: Authentication-Info: SCRAM-SHA-1
   S: [...Other header fields and resource body...]

   Note that in the example above the client can also initiate SCRAM
   authentication without first being prompted by the server.

   "SCRAM-SHA-1" authentication starts with sending the "Authorization"
   request header field defined by HTTP/1.1, Part 7
   [I-D.ietf-httpbis-p7-auth] [RFC7235] containing
   "SCRAM-SHA-1" authentication scheme and the following attributes:

   o  A "realm" attribute MAY be included to indicate the scope of
      protection in the manner described in HTTP/1.1, Part 7
      [I-D.ietf-httpbis-p7-auth]. [RFC7235].
      As specified in
      [I-D.ietf-httpbis-p7-auth], [RFC7235], the "realm" attribute MUST NOT appear
      more than once.  The realm attribute only appears in the first
      SCRAM message to the server and in the first SCRAM response from
      the server.

   o  The client also includes the "client-first-message" containing:

      *  a header ("g" attribute) consisting of a flag indicating
         whether channel binding is supported-but-not-used, not
         supported, or used .  Note that the "g" attribute always starts
         with "n", "y" or "p", otherwise the message is invalid and
         authentication MUST fail.

      *  SCRAM username and a random, unique nonce attributes.

   In HTTP response, the server sends WWW-Authenticate header field
   containing: a unique session identifier (the "sid" attribute) plus
   the "server-first-message" containing the user's iteration count i,
   the user's salt, and the nonce with a concatenation of the client-
   specified one with a server nonce.

   The client then responds with another HTTP request with the
   Authorization header field, which includes the "sid" attribute
   received in the previous server response, together with "client-
   final-message" data.  The latter has the same nonce and a ClientProof
   computed using the selected hash function (SHA-1) as explained

   The server verifies the nonce and the proof, and, finally, it
   responds with a 200 HTTP response with the Authentication-Info header
   field containing "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 the ABNF section for authoritative

   o  g: This attribute value consist of a flag indicating whether
      channel binding is supported-but-not-used, not supported, or used

   o  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.

         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
         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 which
         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 ).  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.

         For SCRAM-SHA-1 authentication 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.

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 [RFC3629].

   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.

   cb-name         = 1*(ALPHA / DIGIT / "." / "-")
                      ;; See RFC 5056, Section 7.
                      ;; E.g., "tls-server-end-point" or
                      ;; "tls-unique".

   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 ","
                     ;; GS2 header for SCRAM.

   username        = "n=" 1*(value-safe-char / "=2C" / "=3D")
                     ;; Conforms to <value>.
                     ;; 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 =
                     "g=" gs2-header client-first-message-bare
                     ;; Note that this doesn't include "realm" and
                     ;; other generic HTTP directives.

   server-first-message =
                     [reserved-mext ","] nonce "," salt ","
                     iteration-count ["," extensions]
                     ;; Note that this doesn't include "realm", "sid" and
                     ;; other generic HTTP directives.

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

   client-final-message =
                     client-final-message-without-proof "," proof
                     ;; Note that this doesn't include "sid" and
                     ;; other generic HTTP directives.

   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" /
           ; 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

   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".

7.  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 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

   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 posession of "StoredKey"
   and "ServerKey" can't automatic increase the iteration count upon
   successful authentication.  Such increase would require resetting
   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 which protect against this attack are
   available (e.g., the EKE class of mechanisms).  RFC 2945 [RFC2945] is
   an example of such technology.

   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 HTTP authentication 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 most
   preferred of 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).

   SCRAM does not protect against downgrade attacks of channel binding
   types.  The complexities of negotiation a channel binding type, and
   handling down-grade attacks in that negotiation, was 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.

8.  IANA Considerations

   New mechanisms in the SCRAM- family are registered according to the
   IANA procedure specified in [RFC5802].

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

   IANA is requested to add the following entry to the Authentication
   Scheme Registry defined in HTTP/1.1, Part 7
   [I-D.ietf-httpbis-p7-auth]: [RFC7235]:

   Authentication Scheme Name: SCRAM-SHA-1
   Pointer to specification text: [[ this document ]]
   Notes (optional): (none)

9.  Acknowledgements

   This document benefited from discussions on the HTTPAuth, SASL and
   Kitten WG mailing lists.  The authors would like to specially thank
   co-authors of [RFC5802] from which lots of text was copied.

   Special thank you to Tony Hansen for doing an early implementation
   and providing extensive comments on the draft.

10.  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 HTTP authentication mechanism has all modern features: support
      for internationalized usernames and passwords, support for channel

   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 overengineered in
      this respect.

   o  Easier to implement than HTTP Digest in both clients and servers.

11.  Open Issues

   It should be possible to construct a quick reauthentication version
   of the mechanism that uses fewer round-trips (similar to what Digest

12.  Internet-Draft Change History

   (RFC Editor: Please delete this section and all subsections.)

   Changes since -00


13.  References

13.1.  Normative References

              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", draft-ietf-httpbis-p7-auth-22
              (work in progress), March 2012.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February

   [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.

   [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.

   [RFC7235]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", RFC 7235, June 2014.

13.2.  Informative References

   [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

   [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.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.

   [RFC5803]  Melnikov, A., "Lightweight Directory Access Protocol
              (LDAP) Schema for Storing Salted Challenge Response
              Authentication Mechanism (SCRAM) Secrets", RFC 5803, July

              Zhu, L., ., , "Registration of TLS server end-point channel
              bindings", IANA
              channel-binding-types/tls-server-end-point, July 2008.

Author's Address

   Alexey Melnikov
   Isode Ltd