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The Session Initiation Protocol (SIP) Digest Access Authentication Scheme

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8760.
Author Rifaat Shekh-Yusef
Last updated 2020-03-19 (Latest revision 2019-11-04)
Replaces draft-yusef-sipcore-digest-scheme
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Jean Mahoney
Shepherd write-up Show Last changed 2019-07-03
IESG IESG state RFC 8760 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Adam Roach
Send notices to Jean Mahoney <>
IANA IANA review state IANA OK - No Actions Needed
IANA action state No IANA Actions
RFC Editor RFC Editor state AUTH48-DONE
SIP Core                                                  R. Shekh-Yusef
Internet-Draft                                                     Avaya
Updates: 3261 (if approved)                             November 3, 2019
Intended status: Standards Track
Expires: May 6, 2020

   The Session Initiation Protocol (SIP) Digest Authentication Scheme


   This document updates RFC 3261 by updating the Digest Access
   Authentication scheme used by the Session Initiation Protocol (SIP)
   to add support for more secure digest algorithms, e.g., SHA-256 and
   SHA-512-256, to replace the obsolete MD5 algorithm.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 6, 2020.

Copyright Notice

   Copyright (c) 2019 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
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  SIP Digest Authentication Scheme Updates  . . . . . . . . . .   3
     2.1.  Hash Algorithms . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Representation of Digest Values . . . . . . . . . . . . .   4
     2.3.  UAS Behavior  . . . . . . . . . . . . . . . . . . . . . .   4
     2.4.  UAC Behavior  . . . . . . . . . . . . . . . . . . . . . .   4
     2.5.  Forking . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.6.  HTTP Digest Authentication Scheme Modifications . . . . .   5
     2.7.  Augmented BNF for SIP . . . . . . . . . . . . . . . . . .   7
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Session Initiation Protocol [RFC3261] uses the same mechanism
   that the Hypertext Transfer Protocol (HTTP) uses for authenticating
   users.  This mechanism is called Digest Access Authentication, and it
   is a simple challenge-response mechanism that allows a server to
   challenge a client request and allows a client to provide
   authentication information in response to that challenge.  The
   version of Digest Access Authentication that [RFC3261] references is
   specified in [RFC2617].

   The default hash algorithm for Digest Access Authentication is MD5.
   However, it has been demonstrated that the MD5 algorithm is not
   collision resistant, and is now considered a bad choice for a hash
   function [RFC6151].

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   The HTTP Digest Access Authentication [RFC7616] document obsoletes
   [RFC2617] and adds stronger algorithms that can be used with the
   Digest Authentication scheme, and establishes a registry for these
   algorithms, known as the "Hash Algorithms for HTTP Digest
   Authentication" registry, so that algorithms can be added in the

   This document updates the Digest Access Authentication scheme used by
   SIP to support the algorithms listed in the "Hash Algorithms for HTTP
   Digest Authentication" registry defined by [RFC7616].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  SIP Digest Authentication Scheme Updates

   This section describes the modifications to the operation of the
   Digest mechanism as specified in [RFC3261] in order to support the
   algorithms defined in the "Hash Algorithms for HTTP Digest
   Authentication" registry described in [RFC7616].

   It replaces the reference used in [RFC3261] for Digest Access
   Authentication, substituting [RFC7616] for the obsolete [RFC2617],
   and describes the modifications to the usage of the Digest mechanism
   in [RFC3261] resulting from that reference update.  It adds support
   for the SHA-256 and SHA-512-256 algorithms [SHA2].  It adds required
   support for the "qop" parameter.  It provides additional User Agent
   Client (UAC) and User Agent Server (UAS) procedures regarding usage
   of multiple SIP Authorization, WWW-Authenticate and Proxy-
   Authenticate header fields, including in which order to insert and
   process them.  It provides guidance regarding forking.  Finally, it
   updates the SIP BNF as required by the updates.

2.1.  Hash Algorithms

   The Digest scheme has an 'algorithm' parameter that specifies the
   algorithm to be used to compute the digest of the response.  The IANA
   registry named the "Hash Algorithms for HTTP Digest Authentication"
   specifies the algorithms that correspond to 'algorithm' values.

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   [RFC3261] specifies only one algorithm, MD5, which is used by
   default.  This document extends [RFC3261] to allow use of any
   algorithm listed in the "Hash Algorithms for HTTP Digest
   Authentication" registry.

   A UAS prioritizes which algorithm to use based on its policy, which
   is specified in section 2.3 and parallels the process used in HTTP
   specified by [RFC7616].

2.2.  Representation of Digest Values

   The size of the digest depends on the algorithm used.  The bits in
   the digest are converted from the most significant to the least
   significant bit, four bits at a time to the ASCII representation as
   follows.  Each four bits is represented by its familiar hexadecimal
   notation from the characters 0123456789abcdef, that is binary 0000 is
   represented by the character '0', 0001 by '1' and so on up to the
   representation of 1111 as 'f'.  If the SHA-256 or SHA-512-256
   algorithm is used to calculate the digest, then the digest will be
   represented as 64 hexadecimal characters.

2.3.  UAS Behavior

   When a UAS receives a request from a UAC, and an acceptable
   Authorization header field is not received, the UAS can challenge the
   originator to provide credentials by rejecting the request with a
   401/407 status code with the WWW-Authenticate/Proxy-Authenticate
   header field respectively.  The UAS MAY add multiple WWW-
   Authenticate/Proxy-Authenticate header fields to allow the UAS to
   utilize the best available algorithm supported by the client.

   If the UAS challenges with multiple WWW-Authenticate/Proxy-
   Authenticate header fields with the same realm, then each one of
   these header fields MUST use a different digest algorithm.  The UAS
   MUST add these header fields to the response in the order that it
   would prefer to see them used, starting with the most preferred
   algorithm at the top, followed by the less preferred algorithms.  The
   UAS cannot assume that the client will use the algorithm specified at
   the topmost header field.

2.4.  UAC Behavior

   When the UAC receives a response with multiple WWW-Authenticate/
   Proxy-Authenticate header fields with the same realm it SHOULD use
   the topmost header field that it supports, unless a local policy

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   dictates otherwise.  The client MUST ignore any challenge it does not

   When the UAC receives a 401 response with multiple WWW-Authenticate
   header fields with different realms it SHOULD retry and add an
   Authorization header field containing credentials that match the
   topmost header field of any one of the realms, unless a local policy
   dictates otherwise.

   If the UAC cannot respond to any of the challenges in the response,
   then it SHOULD abandon attempts to send the request, unless a local
   policy dictates otherwise, e.g. the policy might indicate the use of
   non-Digest mechanisms.  For example, if the UAC does not have
   credentials or has stale credentials for any of the realms, the UAC
   will abandon the request.

2.5.  Forking

   Section 22.3 of [RFC3261] discusses the operation of the proxy-to-
   user authentication, which describes the operation of the proxy when
   it forks a request.  This section clarifies that operation.

   If a request is forked, various proxy servers and/or UAs may wish to
   challenge the UAC.  In this case, the forking proxy server is
   responsible for aggregating these challenges into a single response.
   Each WWW-Authenticate and Proxy-Authenticate value received in
   responses to the forked request MUST be placed into the single
   response that is sent by the forking proxy to the UAC.

   When the forking proxy places multiple WWW-Authenticate and Proxy-
   Authenticate header fields received from one downstream proxy into a
   single response, it MUST maintain the order of these header fields.
   The ordering of values received from different downstream proxies is
   not significant.

2.6.  HTTP Digest Authentication Scheme Modifications

   This section describes the modifications and clarifications required
   to apply the HTTP Digest authentication scheme to SIP.  The SIP
   scheme usage is similar to that for HTTP.  For completeness, the
   bullets specified below are mostly copied from section 22.4 of
   [RFC3261]; the only semantic changes are specified in bullets 1, 7,
   and 8 below.

   SIP clients and servers MUST NOT accept or request Basic

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   The rules for Digest authentication follow those defined in HTTP,
   with "HTTP/1.1" [RFC7616] replaced by "SIP/2.0" in addition to the
   following differences:

   1.  The URI included in the challenge has the following BNF

      URI = Request-URI ; as defined in [RFC3261], Section 25

   2.  The 'uri' parameter of the Authorization header field MUST be
   enclosed in quotation marks.

   3.  The BNF for digest-uri-value is:

      digest-uri-value = Request-URI

   4.  The example procedure for choosing a nonce based on Etag does not
   work for SIP.

   5.  The text in [RFC7234] regarding cache operation does not apply to

   6.  [RFC7616] requires that a server check that the URI in the
   request line and the URI included in the Authorization header field
   point to the same resource.  In a SIP context, these two URIs may
   refer to different users, due to forwarding at some proxy.
   Therefore, in SIP, a UAS MUST check if the Request-URI in the
   Authorization/Proxy-Authorization header field value corresponds to a
   user for whom the UAS is willing to accept forwarded or direct
   requests, but MAY still accept it if the two fields are not

   7.  As a clarification to the calculation of the A2 value for message
   integrity assurance in the Digest authentication scheme, implementers
   should assume, when the entity-body is empty (that is, when SIP
   messages have no body) that the hash of the entity-body resolves to
   the hash of an empty string:

      H(entity-body) = <algorithm>("")

   For example, when the chosen algorithm is SHA-256, then:

      H(entity-body) = SHA-256("") =

   8.  A UAS MUST be able to properly handle "qop" parameter received in
   an Authorization/Proxy-Authorization header field, and a UAC MUST be
   able to properly handle "qop" parameter received in WWW-Authenticate

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   and Proxy-Authenticate header fields.  However, for backward
   compatibility reasons, the "qop" parameter is optional for
   RFC3261-based clients and servers to receive.  If the "qop" parameter
   is not specified, then the default value is "auth".

   A UAS MUST always send a "qop" parameter in WWW-Authenticate and
   Proxy-Authenticate header field values, and a UAC MUST send the "qop"
   parameter in any resulting authorization header field.

   The usage of the Authentication-Info header field continues to be
   allowed, since it provides integrity checks over the bodies and
   provides mutual authentication.

2.7.  Augmented BNF for SIP

   This document updates the Augmented BNF [RFC5234] for SIP as follows.

   It extends the request-digest as follows to allow for different
   digest sizes:

      request-digest = LDQUOT *LHEX RDQUOT

   The number of hex digits is implied by the length of the value of the
   algorithm used, with the minimum size of 32.  A parameter with an
   empty value (empty string) is allowed when the UAC has not yet
   received a challenge.

   It extends the algorithm parameter as follows to allow for any
   algorithm in the registry to be used:

      algorithm = "algorithm" EQUAL ( "MD5" / "MD5-sess" / "SHA-256" /
      "SHA-256-sess" / "SHA-512-256" / "SHA-512-256-sess" / token )

3.  Security Considerations

   This specification adds new secure algorithms to be used with the
   Digest mechanism to authenticate users.  The obsolete MD5 algorithm
   remains only for backward compatibility with [RFC2617] but its use is

   This opens the system to the potential of a downgrade attack by an
   on-path attacker.  The most effective way of dealing with this type
   of attack is to either validate the client and challenge it
   accordingly, or remove the support for backward compatibility by not
   supporting MD5.

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   See section 5 of [RFC7616] for a detailed security discussion of the
   Digest scheme.

4.  IANA Considerations

   [RFC7616] defines an IANA registry named "Hash Algorithms for HTTP
   Digest Authentication" to simplify the introduction of new algorithms
   in the future.  This document specifies that algorithms defined in
   that registry may be used in SIP digest authentication.

   This document has no actions for IANA.

5.  Acknowledgments

   The author would like to thank the following individuals for their
   careful reviews, comments, and suggestions: Paul Kyzivat, Olle
   Johansson, Dale Worley, Michael Procter, Inaki Baz Castillo, Tolga
   Asveren, Christer Holmberg, Brian Rosen, Jean Mahoney, Adam Roach,
   Barry Leiba, Roni Even, Eric Vyncke, Benjamin Kaduk, Alissa Cooper,
   Roman Danyliw, and Alexey Melnikov, and Maxim Sobolev.  .

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, H., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC7234]  Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June

   [RFC7616]  Shekh-Yusef, R., Ahrens, D., and S. Bremer, "HTTP Digest
              Access Authentication", RFC 7616, September 2015.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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   [SHA2]     "SHA: SECURE HASH STANDARD, FIPS 180-2", August 2002.

6.2.  Informative References

   [RFC2617]  Franks, J., M. Hallam-Baker, P., L. Hostetler, J., D.
              Lawrence, S., J. Leach, P., Luotonen, A., and L. C.
              Stewart, "HTTP Authentication: Basic and Digest Access
              Authentication", RFC 2617, June 1999.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,

Author's Address

   Rifaat Shekh-Yusef
   425 Legget Dr.
   Ottawa, Ontario

   Phone: +1-613-595-9106

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