GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc7804

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.

Melnikov Experimental [Page 1] RFC 7804 HTTP SCRAM March 2016

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.

Melnikov Experimental [Page 2] RFC 7804 HTTP SCRAM March 2016

 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.

Melnikov Experimental [Page 3] RFC 7804 HTTP SCRAM March 2016

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.

Melnikov Experimental [Page 4] RFC 7804 HTTP SCRAM March 2016

 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().

Melnikov Experimental [Page 5] RFC 7804 HTTP SCRAM March 2016

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.

Melnikov Experimental [Page 6] RFC 7804 HTTP SCRAM March 2016

 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.

Melnikov Experimental [Page 7] RFC 7804 HTTP SCRAM March 2016

 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...]

Melnikov Experimental [Page 8] RFC 7804 HTTP SCRAM March 2016

 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.

Melnikov Experimental [Page 9] RFC 7804 HTTP SCRAM March 2016

 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.

Melnikov Experimental [Page 10] RFC 7804 HTTP SCRAM March 2016

    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.

Melnikov Experimental [Page 11] RFC 7804 HTTP SCRAM March 2016

 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.

Melnikov Experimental [Page 12] RFC 7804 HTTP SCRAM March 2016

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>

Melnikov Experimental [Page 13] RFC 7804 HTTP SCRAM March 2016

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.

Melnikov Experimental [Page 14] RFC 7804 HTTP SCRAM March 2016

 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.

Melnikov Experimental [Page 15] RFC 7804 HTTP SCRAM March 2016

 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>.

Melnikov Experimental [Page 16] RFC 7804 HTTP SCRAM March 2016

 [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>.

Melnikov Experimental [Page 17] RFC 7804 HTTP SCRAM March 2016

 [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

Melnikov Experimental [Page 18]

/data/webs/external/dokuwiki/data/pages/rfc/rfc7804.txt · Last modified: 2016/03/09 19:00 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki