HTTPAUTH A. Melnikov
Internet-Draft Isode Ltd
Intended status: Standards Track November 25, 2015
Expires: May 28, 2016

Salted Challenge Response (SCRAM) HTTP Authentication Mechanism


The secure authentication mechanism most widely deployed and used by Internet application protocols is the transmission of clear-text passwords over a channel protected by Transport Layer Security (TLS). There are some significant security concerns with that mechanism, which could be addressed by the use of a challenge response authentication mechanism protected by TLS. Unfortunately, the HTTP Digest challenge response mechanism presently on the standards track failed widespread deployment, and have had success only in limited use.

This specification describes a family of HTTP authentication mechanisms called the Salted Challenge Response Authentication Mechanism (SCRAM), which addresses security concerns with HTTP Digest and meets the deployability requirements. When used in combination with TLS or an equivalent security layer, a mechanism from this family could improve the status-quo for HTTP authentication.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on May 28, 2016.

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Table of Contents

1. 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.

1.1. Terminology

This document uses several terms defined in [RFC4949] ("Internet Security Glossary") including the following: authentication, authentication exchange, authentication information, brute force, challenge-response, cryptographic hash function, dictionary attack, eavesdropping, hash result, keyed hash, man-in-the-middle, nonce, one-way encryption function, password, replay attack and salt. Readers not familiar with these terms should use that glossary as a reference.

Some clarifications and additional definitions follow:

1.2. Notation

The pseudocode description of the algorithm uses the following notations:

   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

2. Introduction

This specification describes a family of authentication mechanisms called the Salted Challenge Response Authentication Mechanism (SCRAM) which addresses the requirements necessary to deploy a challenge-response mechanism more widely than past attempts (see [RFC5802]). When used in combination with Transport Layer Security (TLS, see [RFC5246]) or an equivalent security layer, a mechanism from this family could improve the status-quo for HTTP authentication.

HTTP SCRAM is adoptation of [RFC5802] for use in HTTP. (SCRAM data exchanged is identical to what is defined in [RFC5802].) It also adds 1 round trip reauthentication mode.

HTTP SCRAM provides the following protocol features:

3. SCRAM Algorithm Overview

The following is a description of a full HTTP SCRAM authentication exchange. Note that this section omits some details, such as client and server nonces. See Section 5 for more details.

To begin with, the SCRAM client is in possession of a username and password (*) (or a ClientKey/ServerKey, or SaltedPassword). It sends the username to the server, which retrieves the corresponding authentication information, i.e. a salt, StoredKey, ServerKey and the iteration count i. (Note that a server implementation may choose to use the same iteration count for all accounts.) The server sends the salt and the iteration count to the client, which then computes the following values and sends a ClientProof to the server:

(*) - Note that both the username and the password MUST be encoded in UTF-8 [RFC3629].

Informative Note: Implementors are encouraged to create test cases that use both username passwords with non-ASCII codepoints. In particular, it's useful to test codepoints whose "Unicode Normalization Form C" and "Unicode Normalization Form KC" are different. Some examples of such codepoints include Vulgar Fraction One Half (U+00BD) and Acute Accent (U+00B4).

   SaltedPassword  := Hi(Normalize(password), salt, i)
   ClientKey       := HMAC(SaltedPassword, "Client Key")
   StoredKey       := H(ClientKey)
   AuthMessage     := client-first-message-bare + "," +
                      server-first-message + "," +
   ClientSignature := HMAC(StoredKey, AuthMessage)
   ClientProof     := ClientKey XOR ClientSignature
   ServerKey       := HMAC(SaltedPassword, "Server Key")
   ServerSignature := HMAC(ServerKey, AuthMessage)

The server authenticates the client by computing the ClientSignature, exclusive-ORing that with the ClientProof to recover the ClientKey and verifying the correctness of the ClientKey by applying the hash function and comparing the result to the StoredKey. If the ClientKey is correct, this proves that the client has access to the user's password.

Similarly, the client authenticates the server by computing the ServerSignature and comparing it to the value sent by the server. If the two are equal, it proves that the server had access to the user's ServerKey.

For initial authentication the AuthMessage is computed by concatenating decoded "data" attribute values from the authentication exchange. The format of these messages 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 .

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 a HTTP Authentication mechanism whose client response (<credentials-scram>) and server challenge (<challenge-scram>) messages are text-based messages containing one or more attribute-value pairs separated by commas. 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 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 "authentication scheme" 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:
   C: [...]
   S: HTTP/1.1 401 Unauthorized
   S: WWW-Authenticate: Digest realm="",
          Digest realm="",
          Digest realm="",
          SCRAM-SHA-256 realm="",
          SCRAM-SHA-256 realm=""
   S: [...]

   C: GET /resource HTTP/1.1
   C: Host:
   C: Authorization: SCRAM-SHA-256 realm="",
   C: [...]

   S: HTTP/1.1 401 Unauthorized
   S: WWW-Authenticate: SCRAM-SHA-256
   S: [...]

   C: GET /resource HTTP/1.1
   C: Host:
   C: Authorization: SCRAM-SHA-256 sid=AAAABBBBCCCCDDDD,
   C: [...]

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


In the above example the first client request contains data attribute which base64 decodes as follows: "n,,n=user,r=rOprNGfwEbeRWgbNEkqO" (with no quotes). Server then responds with data attribute which base64 decodes as follows: "r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,s=W22ZaJ0SNY7soEsUEjb6gQ==,i=4096". The next client request contains data attribute which base64 decodes as follows: "c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF,p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=". The final server response contains a data attribute which 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 "SCRAM-SHA-256" authentication scheme and the following attributes:

In HTTP response, the server sends WWW-Authenticate header field containing: a unique session identifier (the "sid" attribute) plus the "data" attribute containing 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 with a server nonce.

The client then responds with another HTTP request with the Authorization header field, which includes the "sid" attribute received in the previous server response, together with the "data" attribute containing base64-encoded "client-final-message" data. The latter has the same nonce 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 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 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 that is equal to the user's "i" attribute on first reauthentication and is incremented by 1 on each successful re-authentication.

If the server considers the s-nonce part of the nonce attribute (the "r" attribute) to be still 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 user for the password.

When constructing AuthMessage 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:
   C: [...]
   S: HTTP/1.1 401 Unauthorized
   S: WWW-Authenticate: Digest realm="",
          Digest realm="",
          Digest realm="",
          SCRAM-SHA-256 realm="",
          SCRAM-SHA-256 realm="", sr=%hvYDpWUa2RaTCAfuxFIlj)hNlF
          SCRAM-SHA-256 realm="", sr=AAABBBCCCDDD, ttl=120
   S: [...]

   [Client authenticates as usual to realm ""]
   [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:
   C: Authorization: SCRAM-SHA-256 realm="",

   C: [...]

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


6. Use of 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]. The "UTF8-2", "UTF8-3" and "UTF8-4" non-terminals are defined in [RFC3629].

   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 re-authentication.

   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 security layer (such as TLS with data confidentiality), then a passive eavesdropper can gain sufficient information to mount an offline dictionary or brute-force attack which can be used to recover the user's password. The amount of time necessary for this attack depends on the cryptographic hash function selected, the strength of the password and the iteration count supplied by the server. 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 posession of "StoredKey" and "ServerKey" can't automatic increase the iteration count upon successful authentication. Such increase would require resetting user's password.) An external security layer with strong encryption will prevent these attack.

If the authentication information is stolen from the authentication database, then an offline dictionary or brute-force attack can be used to recover the user's password. The use of salt mitigates this attack somewhat by requiring a separate attack on each password. Authentication mechanisms which protect against this attack are available (e.g., the EKE class of mechanisms). RFC 2945 [RFC2945] is an example of such technology.

If an attacker obtains the authentication information from the authentication repository and either eavesdrops on one authentication exchange or impersonates a server, the attacker gains the ability to impersonate that user to all servers providing SCRAM access using the same hash function, password, iteration count and salt. For this reason, it is important to use randomly-generated salt values.

SCRAM does not negotiate a hash function to use. Hash function negotiation is left to the HTTP authentication mechanism negotiation. It is important that clients be able to sort a locally available list of mechanisms by preference so that the client may pick the most preferred of a server's advertised mechanism list. This preference order is not specified here as it is a local matter. The preference order should include objective and subjective notions of mechanism cryptographic strength (e.g., SCRAM with a successor to SHA-1 may be preferred over SCRAM with SHA-1).

SCRAM does not protect against downgrade attacks of channel binding types. The complexities of negotiation a channel binding type, and handling down-grade attacks in that negotiation, was intentionally left out of scope for this document.

A hostile server can perform a computational denial-of-service attack on clients by sending a big iteration count value.

See [RFC4086] for more information about generating randomness.

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 is requested to add the following entry to the Authentication Scheme Registry defined in HTTP/1.1, Part 7 [RFC7235]:

Authentication Scheme Name: SCRAM-SHA-256
Pointer to specification text: [[ this document ]]
Notes (optional): (none)
Authentication Scheme Name: SCRAM-SHA-1
Pointer to specification text: [[ this document ]]
Notes (optional): (none)

10. Acknowledgements

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

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

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

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

11. Design Motivations

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

12. References

12.1. Normative References

[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, DOI 10.17487/RFC3454, December 2002.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, DOI 10.17487/RFC4013, February 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A. and N. Williams, "Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, DOI 10.17487/RFC5802, July 2010.
[RFC5929] Altman, J., Williams, N. and L. Zhu, "Channel Bindings for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010.
[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.
[RFC7235] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC 7235, DOI 10.17487/RFC7235, June 2014.
[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.
[RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields", RFC 7615, DOI 10.17487/RFC7615, September 2015.
[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.

12.2. 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.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography Specification Version 2.0", RFC 2898, DOI 10.17487/RFC2898, September 2000.
[RFC2945] Wu, T., "The SRP Authentication and Key Exchange System", RFC 2945, DOI 10.17487/RFC2945, September 2000.
[RFC4086] Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol (LDAP): Technical Specification Road Map", RFC 4510, DOI 10.17487/RFC4510, June 2006.
[RFC4616] Zeilenga, K., "The PLAIN Simple Authentication and Security Layer (SASL) Mechanism", RFC 4616, DOI 10.17487/RFC4616, August 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[tls-server-end-point] Zhu, L., , "Registration of TLS server end-point channel bindings", IANA, July 2008.

Author's Address

Alexey Melnikov Isode Ltd EMail: