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RFC 7804

Experimental
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Salted Challenge Response HTTP Authentication Mechanism

 


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Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 7804                                     Isode Ltd
Category: Experimental                                        March 2016
ISSN: 2070-1721


        Salted Challenge Response HTTP Authentication Mechanism

Abstract

   This specification describes a family of HTTP authentication
   mechanisms called the Salted Challenge Response Authentication
   Mechanism (SCRAM), which provides a more robust authentication
   mechanism than a plaintext password protected by Transport Layer
   Security (TLS) and avoids the deployment obstacles presented by
   earlier TLS-protected challenge response authentication mechanisms.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see 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/rfc7804.

Page 2 
Copyright Notice

   Copyright (c) 2016 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  SCRAM Algorithm Overview  . . . . . . . . . . . . . . . . . .   6
   4.  SCRAM Mechanism Names . . . . . . . . . . . . . . . . . . . .   7
   5.  SCRAM Authentication Exchange . . . . . . . . . . . . . . . .   7
     5.1.  One Round-Trip Reauthentication . . . . . . . . . . . . .  10
   6.  Use of the Authentication-Info Header Field with SCRAM  . . .  12
   7.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   10. Design Motivations  . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The 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 has had only limited success.

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   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]).  In particular, it addresses some of the issues
   identified with HTTP Digest, as described in [RFC6331], such as the
   complexity of implementation and protection of the whole
   authentication exchange in order to protect against certain man-in-
   the-middle attacks.

   HTTP SCRAM is an adaptation of [RFC5802] for use in HTTP.  The SCRAM
   data exchanged is identical to what is defined in [RFC5802].  This
   document also adds a 1 round-trip reauthentication mode.

   HTTP 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 make it
      harder to do 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),
      unless it performs a dictionary attack.

   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).

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.

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2.1.  Terminology

   This document uses several terms defined in the "Internet Security
   Glossary" [RFC4949], 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, the StoredKey,
      and the 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 lower-layer protocols
      such as PPP or 802.11x, the use of RADIUS [RFC2865] is more
      common.

   o  Base64: An encoding mechanism defined in Section 4 of [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
   notation:

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

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   o  "+": Octet string concatenation.

   o  "[ ]": A portion of an expression enclosed in "[" and "]" is
      optional in the result under some circumstances.  See the
      associated text for a description of those circumstances.

   o  Normalize(str): Apply the Preparation and Enforcement steps
      according to the OpaqueString profile (see [RFC7613]) to a UTF-8
      [RFC3629] encoded "str".  The resulting string is also in UTF-8.
      Note that implementations MUST either implement OpaqueString
      profile operations from [RFC7613] or disallow the use of non
      US-ASCII Unicode codepoints in "str".  The latter is a particular
      case of compliance with [RFC7613].

   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 32 octets for SHA-256 and 20 octets for SHA-1 (see
      [RFC6234]).

   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 four-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().

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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, both encoded in UTF-8 [RFC3629] (or a ClientKey/ServerKey,
   or SaltedPassword).  It sends the username to the server, which
   retrieves the corresponding authentication information: a salt, a
   StoredKey, a ServerKey, and an 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:

   Informative Note: Implementors are encouraged to create test cases
   that use both usernames and passwords with non-ASCII codepoints.  In
   particular, it is useful to test codepoints whose Unicode
   Normalization Canonical Composition (NFC) and Unicode Normalization
   Form Compatibility Composition (NFKC) are different (see
   [Unicode-UAX15]).  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, this proves that the server had access to the
   user's ServerKey.

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   For initial authentication, the AuthMessage is computed by
   concatenating decoded "data" attribute values from the authentication
   exchange.  The format of each of these 3 decoded "data" attributes is
   defined in [RFC5802].

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://www.iana.org/assignments/hash-function-text-names>).

   For interoperability, all HTTP clients and servers supporting SCRAM
   MUST implement the SCRAM-SHA-256 authentication mechanism, i.e., an
   authentication mechanism from the SCRAM family that uses the SHA-256
   hash function as defined in [RFC7677].

5.  SCRAM Authentication Exchange

   HTTP SCRAM is an 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.  The messages and their attributes
   are described below and defined in Section 7.

    challenge-scram   = scram-name [1*SP 1#auth-param]
          ; Complies with <challenge> ABNF from RFC 7235.
          ; Included in the WWW-Authenticate header field.

    credentials-scram = scram-name [1*SP 1#auth-param]
          ; Complies with <credentials> from RFC 7235.
          ; Included in the Authorization header field.

    scram-name = "SCRAM-SHA-256" / "SCRAM-SHA-1" / other-scram-name
          ; SCRAM-SHA-256 and SCRAM-SHA-1 are registered by this RFC.
          ;
          ; SCRAM-SHA-1 is registered for database compatibility
          ; with implementations of RFC 5802 (such as IMAP or Extensible
            Messaging and Presence Protocol (XMPP)
          ; servers), but it is not recommended for new deployments.

    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 "HTTP Authentication Schemes" registry.

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   This is a simple example of a SCRAM-SHA-256 authentication exchange
   (no support for channel bindings, as this feature is not currently
   supported by HTTP).  Username 'user' and password 'pencil' are used.
   Note that long lines are folded for readability.

   C: GET /resource HTTP/1.1
   C: Host: server.example.com
   C: [...]

   S: HTTP/1.1 401 Unauthorized
   S: WWW-Authenticate: Digest realm="realm1@example.com",
          Digest realm="realm2@example.com",
          Digest realm="realm3@example.com",
          SCRAM-SHA-256 realm="realm3@example.com",
          SCRAM-SHA-256 realm="testrealm@example.com"
   S: [...]

   C: GET /resource HTTP/1.1
   C: Host: server.example.com
   C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com",
          data=biwsbj11c2VyLHI9ck9wck5HZndFYmVSV2diTkVrcU8K
   C: [...]

   S: HTTP/1.1 401 Unauthorized
   S: WWW-Authenticate: SCRAM-SHA-256
           sid=AAAABBBBCCCCDDDD,
           data=cj1yT3ByTkdmd0ViZVJXZ2JORWtxTyVodllEcFdVYTJSYVRDQWZ1eEZJ
              bGopaE5sRixzPVcyMlphSjBTTlk3c29Fc1VFamI2Z1E9PSxpPTQwOTYK
   S: [...]

   C: GET /resource HTTP/1.1
   C: Host: server.example.com
   C: Authorization: SCRAM-SHA-256 sid=AAAABBBBCCCCDDDD,
          data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ
             0FmdXhGSWxqKWhObEYscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empm
             TUhnc3FtbWl6N0FuZFZRPQo=
   C: [...]

   S: HTTP/1.1 200 Ok
   S: Authentication-Info: sid=AAAABBBBCCCCDDDD,
          data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5N
             Uc0PQo=
   S: [...Other header fields and resource body...]

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   In the above example, the first client request contains a "data"
   attribute that base64 decodes as follows:

      n,,n=user,r=rOprNGfwEbeRWgbNEkqO

   The server then responds with a "data" attribute that base64 decodes
   as follows:

      r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,s=W22ZaJ0SNY7soE
      sUEjb6gQ==,i=4096

   The next client request contains a "data" attribute that base64
   decodes as follows:

      c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,p=dHzbZap
      WIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=

   The final server response contains a "data" attribute that base64
   decodes as follows:

      v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=

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

   Initial "SCRAM-SHA-256" authentication starts with sending the
   Authorization request header field (defined by HTTP/1.1, Part 7
   [RFC7235]) containing the "SCRAM-SHA-256" 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 [RFC7235].
      As specified in [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 "data" attribute that contains the
      base64-encoded "client-first-message" [RFC5802] containing:

      *  a header consisting of a flag indicating whether channel
         binding is supported-but-not-used, not supported, or used.
         Note that this version of SCRAM doesn't support HTTP channel
         bindings, so this header always starts with "n"; otherwise, the
         message is invalid and authentication MUST fail.

      *  SCRAM username and a random, unique "nonce" attribute.

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   In an HTTP response, the server sends the WWW-Authenticate header
   field containing a unique session identifier (the "sid" attribute)
   plus the "data" attribute containing the base64-encoded "server-
   first-message" [RFC5802].  The "server-first-message" contains the
   user's iteration count i, the user's salt, and the nonce with a
   concatenation of the client-specified one (taken from the "client-
   first-message") with a freshly generated 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 the "data"
   attribute containing base64-encoded "client-final-message" data.  The
   latter has the same nonce as in "server-first-message" and a
   ClientProof computed using the selected hash function (e.g., SHA-256)
   as explained earlier.

   The server verifies the nonce and the proof, and, finally, it
   responds with a 200 HTTP response with the Authentication-Info header
   field [RFC7615] containing the "sid" attribute (as received from the
   client) and the "data" attribute containing the base64-encoded
   "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.  One Round-Trip Reauthentication

   If the server supports SCRAM reauthentication, the server sends in
   its initial HTTP response a WWW-Authenticate header field containing
   the "realm" attribute (as defined earlier), the "sr" attribute that
   contains the server part of the "r" attribute (see s-nonce in
   [RFC5802]), and an optional "ttl" attribute (which contains the "sr"
   value validity in seconds).

   If the client has authenticated to the same realm before (i.e., it
   remembers "i" and "s" attributes for the user from earlier
   authentication exchanges with the server), it can respond to that
   with "client-final-message".  When constructing the "client-final-
   message", the client constructs the c-nonce part of the "r" attribute
   as on initial authentication and the s-nonce part as follows: s-nonce
   is a concatenation of nonce-count and the "sr" attribute (in that
   order).  The nonce-count is a positive integer that is equal to the
   user's "i" attribute on first reauthentication and is incremented by
   1 on each successful reauthentication.

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      The purpose of the nonce-count is to allow the server to detect
      request replays by maintaining its own copy of this count -- if
      the same nonce-count value is seen twice, then the request is a
      replay.

   If the server considers the s-nonce part of the "nonce" attribute
   (the "r" attribute) to still be valid (i.e., the nonce-count part is
   as expected (see above) and the "sr" part is still fresh), it will
   provide access to the requested resource (assuming the client hash
   verifies correctly, of course).  However, if the server considers
   that the server part of the nonce is stale (for example, if the "sr"
   value is used after the "ttl" seconds), the server returns "401
   Unauthorized" containing the SCRAM mechanism name with the following
   attributes: a new "sr", "stale=true", and an optional "ttl".  The
   "stale" attribute signals to the client that there is no need to ask
   the user for the password.

      Formally, the "stale" attribute is defined as a flag, indicating
      that the previous request from the client was rejected because the
      nonce value was stale.  If stale is TRUE (case-insensitive), the
      client may wish to simply retry the request with a new encrypted
      response without reprompting the user for a new username and
      password.  The server should only set stale to TRUE if it receives
      a request for which the nonce is invalid but with a valid digest
      for that nonce (indicating that the client knows the correct
      username/password).  If stale is FALSE or anything other than
      TRUE, or the stale directive is not present, the username and/or
      password are invalid, and new values must be obtained.

   When constructing AuthMessage (see Section 3) to be used for
   calculating client and server proofs, "client-first-message-bare" and
   "server-first-message" are reconstructed from data known to the
   client and the server.

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   Reauthentication can look like this:

   C: GET /resource HTTP/1.1
   C: Host: server.example.com
   C: [...]

   S: HTTP/1.1 401 Unauthorized
   S: WWW-Authenticate: Digest realm="realm1@example.com",
          Digest realm="realm2@example.com",
          Digest realm="realm3@example.com",
          SCRAM-SHA-256 realm="realm3@example.com",
          SCRAM-SHA-256 realm="testrealm@example.com", sr=%hvYDpWUa2RaTC
           AfuxFIlj)hNlF
          SCRAM-SHA-256 realm="testrealm2@example.com", sr=AAABBBCCCDDD,
           ttl=120
   S: [...]

   [The client authenticates as usual to realm "testrealm@example.com"]
   [Some time later, client decides to reauthenticate.
   It will use the cached "i" (4096) and "s" (W22ZaJ0SNY7soEsUEjb6gQ==)
   from earlier exchanges.  It will use the nonce-value of 4096 together
   with the server advertised "sr" value as the server part of the "r".]

   C: GET /resource HTTP/1.1
   C: Host: server.example.com
   C: Authorization: SCRAM-SHA-256 realm="testrealm@example.com",
          data=Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU80MDk2JWh2WURwV1VhM
           lJhVENBZnV4RklsailoTmxGLHA9ZEh6YlphcFdJazRqVWhOK1V0ZTl5dGFnOX
           pqZk1IZ3NxbW1pejdBbmRWUT0K

   C: [...]

   S: HTTP/1.1 200 Ok
   S: Authentication-Info: sid=AAAABBBBCCCCDDDD,
          data=dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5N
           Uc0PQo=
   S: [...Other header fields and resource body...]

6.  Use of the Authentication-Info Header Field with SCRAM

   When used with SCRAM, the Authentication-Info header field is allowed
   in the trailer of an HTTP message transferred via chunked transfer-
   coding.

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7.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [RFC5234].

      ALPHA = <as defined in RFC 5234 Appendix B.1>
      DIGIT = <as defined in RFC 5234 Appendix B.1>

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

      base64-4        = 4base64-char

      base64-3        = 3base64-char "="

      base64-2        = 2base64-char "=="

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

      sr              = "sr=" s-nonce
                        ;; s-nonce is defined in RFC 5802.

      data            = "data=" base64
                        ;; The "data" attribute value is base64-encoded
                        ;; SCRAM challenge or response defined in
                        ;; RFC 5802.

      ttl             = "ttl=" 1*DIGIT
                        ;; "sr" value validity in seconds.
                        ;; No leading 0s.

      reauth-s-nonce  = nonce-count s-nonce

      nonce-count     = posit-number
                        ;; posit-number is defined in RFC 5802.
                        ;; The initial value is taken from the "i"
                        ;; attribute for the user and is incremented
                        ;; by 1 on each successful reauthentication.

      sid             = "sid=" token
                        ;; See token definition in RFC 7235.

      stale           = "stale=" ( "true" / "false" )

      realm           = "realm=" <as defined in RFC 7235>

Top      ToC       Page 14 
8.  Security Considerations

   If the authentication exchange is performed without a strong session
   encryption (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.  SCRAM
   allows the server/server administrator to increase the iteration
   count over time in order to slow down the above attacks.  (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.)  An external security layer with strong encryption will
   prevent these attacks.

   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 Encrypted Key Exchange (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 which 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 SHA-256 should be preferred
   over SCRAM with SHA-1).

   This document recommends use of SCRAM with SHA-256 hash.  SCRAM-SHA-1
   is registered for database compatibility with implementations of RFC
   5802 (such as IMAP or XMPP servers) that want to also expose HTTP
   access to a related service, but it is not recommended for new
   deployments.

Top      ToC       Page 15 
   A hostile server can perform a computational denial-of-service attack
   on clients by sending a big iteration count value.  In order to
   defend against that, a client implementation can pick a maximum
   iteration count that it is willing to use and reject any values that
   exceed that threshold (in such cases, the client, of course, has to
   fail the authentication).

   See [RFC4086] for more information about generating randomness.

9.  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 has added the following entries to the "HTTP Authentication
   Schemes" registry defined in HTTP/1.1, Part 7 [RFC7235]:

      Authentication Scheme Name: SCRAM-SHA-256
      Pointer to specification text: RFC 7804
      Notes (optional): (none)

      Authentication Scheme Name: SCRAM-SHA-1
      Pointer to specification text: RFC 7804
      Notes (optional): (none)

10.  Design Motivations

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

   o  The HTTP authentication mechanism has all modern features: support
      for internationalized usernames and passwords.

   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.

Top      ToC       Page 16 
   o  The mechanism is extensible, but (hopefully) not over-engineered
      in this respect.

   o  The mechanism is easier to implement than HTTP Digest in both
      clients and servers.

   o  The protocol supports 1 round-trip reauthentication.

11.  References

11.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
              DOI 10.17487/RFC5802, July 2010,
              <http://www.rfc-editor.org/info/rfc5802>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <http://www.rfc-editor.org/info/rfc6234>.

Top      ToC       Page 17 
   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <http://www.rfc-editor.org/info/rfc7235>.

   [RFC7613]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 7613,
              DOI 10.17487/RFC7613, August 2015,
              <http://www.rfc-editor.org/info/rfc7613>.

   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,
              <http://www.rfc-editor.org/info/rfc7615>.

   [RFC7677]  Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
              Authentication and Security Layer (SASL) Mechanisms",
              RFC 7677, DOI 10.17487/RFC7677, November 2015,
              <http://www.rfc-editor.org/info/rfc7677>.

11.2.  Informative References

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,
              <http://www.rfc-editor.org/info/rfc2865>.

   [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
              Specification Version 2.0", RFC 2898,
              DOI 10.17487/RFC2898, September 2000,
              <http://www.rfc-editor.org/info/rfc2898>.

   [RFC2945]  Wu, T., "The SRP Authentication and Key Exchange System",
              RFC 2945, DOI 10.17487/RFC2945, September 2000,
              <http://www.rfc-editor.org/info/rfc2945>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC4510]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510,
              DOI 10.17487/RFC4510, June 2006,
              <http://www.rfc-editor.org/info/rfc4510>.

Top      ToC       Page 18 
   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <http://www.rfc-editor.org/info/rfc4949>.

   [RFC6331]  Melnikov, A., "Moving DIGEST-MD5 to Historic", RFC 6331,
              DOI 10.17487/RFC6331, July 2011,
              <http://www.rfc-editor.org/info/rfc6331>.

   [Unicode-UAX15]
              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", June 2015,
              <http://www.unicode.org/reports/tr15/>.

Acknowledgements

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

   Thank you to Martin Thomson for the idea of adding the "ttl"
   attribute.

   Thank you to Julian F. Reschke for corrections regarding use of the
   Authentication-Info header field.

   A special thank you to Tony Hansen for doing an early implementation
   and providing extensive comments on the document.

   Thank you to Russ Housley, Stephen Farrell, Barry Leiba, and Tim
   Chown for doing detailed reviews of the document.

Author's Address

   Alexey Melnikov
   Isode Ltd

   Email: Alexey.Melnikov@isode.com