Internet DRAFT - draft-ietf-sipcore-digest-scheme
draft-ietf-sipcore-digest-scheme
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
draft-ietf-sipcore-digest-scheme-15
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 obsolete MD5 algorithm.
Status of This Memo
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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 . . . . . . . . . . . . . . . . . . . . . . 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
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 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
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, 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
authentication.
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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
[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 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
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"
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
NOT RECOMMENDED.
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,
<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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[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,
<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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