Internet Engineering Task Force (IETF) R. Shekh-Yusef
Request for Comments: 8760
Category: Standards Track
The Session Initiation Protocol (SIP) Digest Access Authentication
This document updates RFC 3261
by modifying 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
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(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
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Internet Standards is available in Section 2 of RFC 7841
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Table of Contents 1.
Updates to the SIP Digest Access Authentication Scheme 2.1.
Hash Algorithms 2.2.
Representation of Digest Values 2.3.
UAS Behavior 2.4.
UAC Behavior 2.5.
HTTP Digest Authentication Scheme Modifications 2.7.
ABNF for SIP 3.
Security Considerations 4.
IANA Considerations 5.
Normative References 5.2.
The Session Initiation Protocol [RFC3261
] uses the same mechanism as
the Hypertext Transfer Protocol (HTTP) does for authenticating users.
This mechanism is called "Digest Access Authentication". 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
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 (see [RFC6151
The HTTP Digest Access Authentication document [RFC7616
] and adds stronger algorithms that can be used with the
Digest Access Authentication scheme and establishes a registry for
these algorithms, known as the "Hash Algorithms for HTTP Digest
Authentication" IANA 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" IANA registry defined by [RFC7616
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
2. Updates to the SIP Digest Access Authentication Scheme
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" IANA 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
] 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 the order in which to insert
and process them. It provides guidance regarding forking. Finally,
it updates the SIP ABNF as required by the updates.
2.1. Hash Algorithms
The Digest Access Authentication scheme has an "algorithm" parameter
that specifies the algorithm to be used to compute the digest of the
response. The "Hash Algorithms for HTTP Digest Authentication" IANA
registry specifies the algorithms that correspond to 'algorithm'
] 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" IANA 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 set of four bits is represented by its familiar
hexadecimal notation from the characters 0123456789abcdef; that is,
binary 0000 is represented by the character '0', 0001 is represented
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 the originator using multiple WWW-Authenticate/
Proxy-Authenticate header fields with the same realm, then each of
these header fields MUST
use a different digest algorithm. The UAS MUST
add these header fields to the response in the order in which it
would prefer to see them used, starting with the most preferred
algorithm at the top. The UAS cannot assume that the client will use
the algorithm specified in 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
the topmost header field that it supports unless a local policy
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 of the realms unless a local policy
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.
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
response 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
2.6. HTTP Digest Authentication Scheme Modifications
This section describes the modifications and clarifications required
to apply the HTTP Digest Access 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
]; the only semantic changes are specified in bullets 1, 7,
and 8 below.
SIP clients and servers MUST NOT
accept or request Basic
The rules for Digest Access 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 ABNF
URI = Request-URI ; as defined in RFC 3261
, Section 25 2.
The "uri" parameter of the Authorization header field MUST
enclosed in quotation marks.
3. The ABNF 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
] 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; however, it MAY
still accept it 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 Access Authentication scheme,
implementers should assume that the hash of the entity-body
resolves to the hash of an empty string when the entity-body is
empty (that is, when SIP messages have no body):
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 a "qop" parameter received
in an Authorization/Proxy-Authorization header field, and a UAC MUST
be able to properly handle a "qop" parameter received in
WWW-Authenticate and Proxy-Authenticate header fields. However,
for backward compatibility reasons, the "qop" parameter is
optional for clients and servers based on [RFC3261
] to receive.
If the "qop" parameter is not specified, then the default value
A UAS MUST
always send a "qop" parameter in WWW-Authenticate and
Proxy-Authenticate header field values, and a UAC MUST
"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. ABNF for SIP
This document updates the ABNF [RFC5234
] for SIP as follows.
It extends the request-digest as follows to allow for different
request-digest = LDQUOT *LHEX RDQUOT
The number of hex digits is implied by the length of the value of the
algorithm used, with a minimum size of 32. A parameter with an empty
value (empty string) is allowed when the UAC has not yet received a
It extends the algorithm parameter as follows to allow any algorithm
in the registry to be used:
algorithm = "algorithm" EQUAL ( "MD5" / "MD5-sess" / "SHA-256" /
"SHA-512-256" / "SHA-512-256-sess" / token )3.
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 NOT RECOMMENDED
This opens the system to the potential for 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
See Section 5
] for a detailed security discussion of the
Digest Access Authentication scheme.4.
] 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.
5.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261
, June 2002,
] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234
, DOI 10.17487/RFC7234
, June 2014,
] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616
, September 2015,
] 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
[SHA2] National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", DOI 10.6028/NIST.FIPS.180-4,
FIPS 180-4, August 2015,
5.2. Informative References
] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication", RFC 2617
, DOI 10.17487/RFC2617
, June 1999,
] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234
, January 2008,
] 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,
The author would like to thank the following individuals for their
careful review, 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, Alexey Melnikov, and Maxim Sobolev.
425 Legget Dr.