The Session Initiation Protocol (SIP) Digest Authentication Scheme
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SIP Core
Digest Auth
This document updates the Digest Access Authentication scheme used by
the Session Initiation Protocol (SIP) to add support for secure digest
algorithms to replace the broken MD5 algorithm.
The SIP protocol uses the same mechanism used by the HTTP
protocol for authenticating users, which is a simple challenge-
response authentication mechanism that allows a server to challenge a
client request and allows a client to provide authentication
information in response to that challenge.
The SIP protocol uses the Digest Authentication scheme that is used
with the HTTP authentication mechanism, which by default uses MD5 as
the default algorithm.
The HTTP Digest Access Authentication document defines
the Digest Authentication scheme and defines a few algorithms that could be
used with the Digest Authentication scheme, and establishes a
registry for these algorithms to allow for additional algorithms to
be added in the future.
This document updates the Digest Access Authentication scheme used by
SIP to support the list of digest algorithms defined in the "Hash Algorithms
for HTTP Digest Authentication" registry defined by .
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 .
This section describes the modifications to the operation of the
Digest mechanism as specified in in order to support the SHA-
256 and SHA-512/256 algorithms as described in , and
also to require support for the "qop" option."
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 "HTTP Digest Hash Algorithms" specifies the algorithms
that correspond to 'algorithm' values, and specifies a priority for
each algorithm.
specifies only one algorithm, MD5, which is used by default.
This document extends to allow use of any registered algorithm.
The priority of the algorithm defines its usage preference. UAs
SHOULD prefer algorithms with higher priorities.
Note that defines a -sess variant for each algorithm;
the -sess variants are not used with SIP.
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.
When a UAS receives a request from a UAC, and an acceptable
Authorization header field is not sent, 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. The UAS MAY include multiple WWW-Authenticate/Proxy-Authenticate
headers to allow the UAS to utilize the best available
algorithm supported by the client.
If the UAS challenges with multiple WWW-Authenticate/Proxy-
Authenticate headers with the same realm, then each one of these
headers MUST use a different digest algorithm. The UAS MUST add these
headers 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.
When the UAC receives a response with multiple 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 include 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 for any of the realms.
Section 22.3 of discusses the operation of the proxy-to-user
authentication, which describes the operation of the proxy when it
forks a request. This section introduces some clarification to 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 UA.
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 the header field values from the various proxies is not
significant.
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 ; the
only semantic changes are specified in bullets 7 and 8 below.
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" 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 , 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 regarding cache operation does not
apply to SIP.
6. 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 server MAY check that the
Request-URI in the Authorization header field value
corresponds to a user for whom the server 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"
8. Servers MUST be able to properly handle "qop" parameter received
in an authorization header field, and clients MUST be able to
properly handle "qop" parameter received in WWW-Authenticate and
Proxy-Authenticate header fields.
Servers MUST always send a "qop" parameter in WWW-Authenticate
and Proxy-Authenticate header field values, and clients MUST
send the "qop" parameter in any resulting authorization header
field.
The usage of the Authentication-Info header field continue to be
allowed, since it provides integrity checks over the bodies and
provides mutual authentication.
This document updates the Augmented BNF for the SIP Protocol 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 must be specified by the specification of
the algorithm used.
It extends the algorithm parameter as follows to allow for SHA2
algorithms to be used:
algorithm = "algorithm" EQUAL ( "MD5" / "SHA-512-256" / "SHA-256" / token )
This specification adds new secure algorithms to be used to 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 man-in-the-middle.
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.
See section 5 of for a detailed security discussion of
the Digest scheme.
defines an IANA registry named "Hash Algorithms
for HTTP Digest Authentication" to simplify the introduction of new
algorithms in the future. This document will use the algorithms defined in that
registry.
The author would like to thank the following individuals
for their careful reviews, comments, and suggestions: Paul Kyzivat,
Olle Johansson, Dale Worley, Michael Procter, Iņaki Baz Castillo,
Tolga Asveren, Christer Holmberg, and Brian Rosen.
Key words for use in RFCs to Indicate Requirement Levels
SIP: Session Initiation Protocol
Hypertext Transfer Protocol (HTTP/1.1): Caching
HTTP Digest Access Authentication