SIP Core                                                  R. Shekh-Yusef
Internet-Draft                                                     Avaya
Updates: 3261 (if approved)                           September 18, 2019
Intended status: Standards Track
Expires: March 21, 2020


   The Session Initiation Protocol (SIP) Digest Authentication Scheme
                  draft-ietf-sipcore-digest-scheme-10

Abstract

   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 broken MD5 algorithm, which might be used
   for backward compatibility reasons only.

Status of This Memo

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
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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  . . . . . . . . . . . . . . . . . . . . . .   5
     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



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   collision resistant, and is now considered a bad choice for a hash
   function [RFC6151].

   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
   future.

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 to [RFC2617] with a reference to [RFC7616]
   in [RFC3261], 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.  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




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   registry named "HTTP Digest Hash Algorithms" specifies the algorithms
   that correspond to 'algorithm' values.

   [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 the ordering of the
   challenge header fields in the response it is processing.  That
   process 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 MD5 algorithm is used to
   calculate the digest, then the digest will be represented as 32
   hexadecimal characters, SHA-256 and SHA-512/256 by 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.





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

   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.

   If the UAC cannot respond to any of the challenges in the response,
   then it SHOULD abandon attempts to send the request, e.g. if the UAC
   does not have credentials or has stale credentials for any of the
   realms, unless a local policy dictates otherwise.


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 from one received response into the single
   response it MUST maintain the order of these header fields.  The
   ordering of values received from proxies relative to values received
   from other 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.




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   SIP clients and servers MUST NOT accept or request Basic
   authentication.

   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
   [RFC5234]:

      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
   SIP.

   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 MAY check that 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 it is not necessarily a failure if the two fields are
   not equivalent.

   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("") =
      "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"




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   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
   and Proxy-Authenticate header fields.  However, for backward
   compatibility reasons, the "qop" parameter is optional for
   RFC3261-based clients and servers to receive.

   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.

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

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


3.  Security Considerations

   This specification adds new secure algorithms to be used with the
   Digest mechanism to authenticate users, but leaves the broken MD5
   algorithm for backward compatibility.

   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, and Adam
   Roach.


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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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
              2014.

   [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, <https://www.rfc-editor.org/info/rfc8174>.




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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,
              <https://www.rfc-editor.org/info/rfc5234>.

   [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,
              <https://www.rfc-editor.org/info/rfc6151>.

Author's Address

   Rifaat Shekh-Yusef
   Avaya
   425 Legget Dr.
   Ottawa, Ontario
   Canada

   Phone: +1-613-595-9106
   EMail: rifaat.ietf@gmail.com
























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