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

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.

Copyright Notice

Copyright © 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.

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:

  • 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.
  • 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.
  • The mechanism permits the use of a server-authorized proxy without requiring that proxy to have super-user rights with the back-end server.
  • 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.

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:

  • 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.
  • 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.
  • 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.
  • Octet: An 8-bit byte.
  • Octet string: A sequence of 8-bit bytes.
  • Salt: A random octet string that is combined with a password before applying a one-way encryption function. This value is used to protect passwords that are stored in an authentication database.

2.2. Notation

The pseudocode description of the algorithm uses the following notation:

  • ":=": The variable on the left-hand side represents the octet string resulting from the expression on the right-hand side.
  • "+": Octet string concatenation.
  • "[ ]": A portion of an expression enclosed in "[" and "]" is optional in the result under some circumstances. See the associated text for a description of those circumstances.
  • 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]).
  • 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.
  • XOR: Apply the exclusive-or operation to combine the octet string on the left of this operator with the octet string on the right of this operator. The length of the output and each of the two inputs will be the same for this use.
  • Hi(str, salt, 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().

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.

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.

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...]

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:

  • 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.
  • 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.

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.

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.

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.

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>

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.

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.

  • The HTTP authentication mechanism has all modern features: support for internationalized usernames and passwords.
  • The protocol supports mutual authentication.
  • The authentication information stored in the authentication database is not sufficient by itself to impersonate the client.
  • 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.
  • The mechanism is extensible, but (hopefully) not over-engineered in this respect.
  • The mechanism is easier to implement than HTTP Digest in both clients and servers.
  • 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>.
   
   [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>.
   
   [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