draft-ietf-sipcore-digest-scheme-10.txt   draft-ietf-sipcore-digest-scheme-11.txt 
SIP Core R. Shekh-Yusef SIP Core R. Shekh-Yusef
Internet-Draft Avaya Internet-Draft Avaya
Updates: 3261 (if approved) September 18, 2019 Updates: 3261 (if approved) October 28, 2019
Intended status: Standards Track Intended status: Standards Track
Expires: March 21, 2020 Expires: April 30, 2020
The Session Initiation Protocol (SIP) Digest Authentication Scheme The Session Initiation Protocol (SIP) Digest Authentication Scheme
draft-ietf-sipcore-digest-scheme-10 draft-ietf-sipcore-digest-scheme-11
Abstract Abstract
This document updates RFC 3261 by updating the Digest Access This document updates RFC 3261 by updating the Digest Access
Authentication scheme used by the Session Initiation Protocol (SIP) Authentication scheme used by the Session Initiation Protocol (SIP)
to add support for more secure digest algorithms, e.g. SHA-256 and 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 SHA-512-256, to replace the broken MD5 algorithm.
for backward compatibility reasons only.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 21, 2020. This Internet-Draft will expire on April 30, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. SIP Digest Authentication Scheme Updates 2. SIP Digest Authentication Scheme Updates
This section describes the modifications to the operation of the This section describes the modifications to the operation of the
Digest mechanism as specified in [RFC3261] in order to support the Digest mechanism as specified in [RFC3261] in order to support the
algorithms defined in the "Hash Algorithms for HTTP Digest algorithms defined in the "Hash Algorithms for HTTP Digest
Authentication" registry described in [RFC7616]. Authentication" registry described in [RFC7616].
It replaces the reference to [RFC2617] with a reference to [RFC7616] It replaces the reference used in [RFC3261] for Digest Access
in [RFC3261], and describes the modifications to the usage of the Authentication, substituting [RFC7616] for the obsolete [RFC2617],
Digest mechanism in [RFC3261] resulting from that reference update. and describes the modifications to the usage of the Digest mechanism
It adds support for the SHA-256 and SHA-512/256 algorithms. It adds in [RFC3261] resulting from that reference update. It adds support
required support for the "qop" parameter. It provides additional for the SHA-256 and SHA-512/256 algorithms. It adds required support
User Agent Client (UAC) and User Agent Server (UAS) procedures for the "qop" parameter. It provides additional User Agent Client
regarding usage of multiple SIP Authorization, WWW-Authenticate and (UAC) and User Agent Server (UAS) procedures regarding usage of
Proxy-Authenticate header fields, including in which order to insert multiple SIP Authorization, WWW-Authenticate and Proxy-Authenticate
and process them. It provides guidance regarding forking. Finally, header fields, including in which order to insert and process them.
it updates the SIP BNF as required by the updates. It provides guidance regarding forking. Finally, it updates the SIP
BNF as required by the updates.
2.1. Hash Algorithms 2.1. Hash Algorithms
The Digest scheme has an 'algorithm' parameter that specifies the The Digest scheme has an 'algorithm' parameter that specifies the
algorithm to be used to compute the digest of the response. The IANA algorithm to be used to compute the digest of the response. The IANA
registry named "HTTP Digest Hash Algorithms" specifies the algorithms registry named the "Hash Algorithms for HTTP Digest Authentication"
that correspond to 'algorithm' values. specifies the algorithms that correspond to 'algorithm' values.
[RFC3261] specifies only one algorithm, MD5, which is used by [RFC3261] specifies only one algorithm, MD5, which is used by
default. This document extends [RFC3261] to allow use of any default. This document extends [RFC3261] to allow use of any
algorithm listed in the "Hash Algorithms for HTTP Digest algorithm listed in the "Hash Algorithms for HTTP Digest
Authentication" registry. Authentication" registry.
A UAS prioritizes which algorithm to use based on the ordering of the A UAS prioritizes which algorithm to use based on the ordering of the
challenge header fields in the response it is processing. That challenge header fields in the response it is preparing. That
process is specified in section 2.3 and parallels the process used in process is specified in section 2.3 and parallels the process used in
HTTP specified by [RFC7616]. HTTP specified by [RFC7616].
2.2. Representation of Digest Values 2.2. Representation of Digest Values
The size of the digest depends on the algorithm used. The bits in The size of the digest depends on the algorithm used. The bits in
the digest are converted from the most significant to the least the digest are converted from the most significant to the least
significant bit, four bits at a time to the ASCII representation as significant bit, four bits at a time to the ASCII representation as
follows. Each four bits is represented by its familiar hexadecimal follows. Each four bits is represented by its familiar hexadecimal
notation from the characters 0123456789abcdef, that is binary 0000 is notation from the characters 0123456789abcdef, that is binary 0000 is
represented by the character '0', 0001 by '1' and so on up to the 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 representation of 1111 as 'f'. If the SHA-256 or SHA-512/256
calculate the digest, then the digest will be represented as 32 algorithm is used to calculate the digest, then the digest will be
hexadecimal characters, SHA-256 and SHA-512/256 by 64 hexadecimal represented as 64 hexadecimal characters.
characters.
2.3. UAS Behavior 2.3. UAS Behavior
When a UAS receives a request from a UAC, and an acceptable When a UAS receives a request from a UAC, and an acceptable
Authorization header field is not received, the UAS can challenge the Authorization header field is not received, the UAS can challenge the
originator to provide credentials by rejecting the request with a originator to provide credentials by rejecting the request with a
401/407 status code with the WWW-Authenticate/Proxy-Authenticate 401/407 status code with the WWW-Authenticate/Proxy-Authenticate
header field respectively. The UAS MAY add multiple WWW- header field respectively. The UAS MAY add multiple WWW-
Authenticate/Proxy-Authenticate header fields to allow the UAS to Authenticate/Proxy-Authenticate header fields to allow the UAS to
utilize the best available algorithm supported by the client. utilize the best available algorithm supported by the client.
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When the UAC receives a response with multiple WWW-Authenticate/ When the UAC receives a response with multiple WWW-Authenticate/
Proxy-Authenticate header fields with the same realm it SHOULD use Proxy-Authenticate header fields with the same realm it SHOULD use
the topmost header field that it supports, unless a local policy the topmost header field that it supports, unless a local policy
dictates otherwise. The client MUST ignore any challenge it does not dictates otherwise. The client MUST ignore any challenge it does not
understand. understand.
When the UAC receives a 401 response with multiple WWW-Authenticate When the UAC receives a 401 response with multiple WWW-Authenticate
header fields with different realms it SHOULD retry and add an header fields with different realms it SHOULD retry and add an
Authorization header field containing credentials that match the Authorization header field containing credentials that match the
topmost header field of any one of the realms. 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, 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 then it SHOULD abandon attempts to send the request unless a local
does not have credentials or has stale credentials for any of the policy dictates otherwise. For example, if the UAC does not have
realms, unless a local policy dictates otherwise. credentials or has stale credentials for any of the realms, the UAC
will abandon the request.
2.5. Forking 2.5. Forking
Section 22.3 of [RFC3261] discusses the operation of the proxy-to- Section 22.3 of [RFC3261] discusses the operation of the proxy-to-
user authentication, which describes the operation of the proxy when user authentication, which describes the operation of the proxy when
it forks a request. This section clarifies that operation. it forks a request. This section clarifies that operation.
If a request is forked, various proxy servers and/or UAs may wish to 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 challenge the UAC. In this case, the forking proxy server is
responsible for aggregating these challenges into a single response. responsible for aggregating these challenges into a single response.
Each WWW-Authenticate and Proxy-Authenticate value received in Each WWW-Authenticate and Proxy-Authenticate value received in
responses to the forked request MUST be placed into the single responses to the forked request MUST be placed into the single
response that is sent by the forking proxy to the UAC. response that is sent by the forking proxy to the UAC.
When the forking proxy places multiple WWW-Authenticate and Proxy- When the forking proxy places multiple WWW-Authenticate and Proxy-
Authenticate header fields from one received response into the single Authenticate header fields received from one downstream proxy into a
response it MUST maintain the order of these header fields. The single response, it MUST maintain the order of these header fields.
ordering of values received from proxies relative to values received The ordering of values received from different downstream proxies is
from other proxies is not significant. not significant.
2.6. HTTP Digest Authentication Scheme Modifications 2.6. HTTP Digest Authentication Scheme Modifications
This section describes the modifications and clarifications required This section describes the modifications and clarifications required
to apply the HTTP Digest authentication scheme to SIP. The SIP to apply the HTTP Digest authentication scheme to SIP. The SIP
scheme usage is similar to that for HTTP. For completeness, the scheme usage is similar to that for HTTP. For completeness, the
bullets specified below are mostly copied from section 22.4 of bullets specified below are mostly copied from section 22.4 of
[RFC3261]; the only semantic changes are specified in bullets 1, 7, [RFC3261]; the only semantic changes are specified in bullets 1, 7,
and 8 below. and 8 below.
SIP clients and servers MUST NOT accept or request Basic SIP clients and servers MUST NOT accept or request Basic
authentication. authentication.
The rules for Digest authentication follow those defined in HTTP, The rules for Digest authentication follow those defined in HTTP,
with "HTTP/1.1" [RFC7616] replaced by "SIP/2.0" in addition to the with "HTTP/1.1" [RFC7616] replaced by "SIP/2.0" in addition to the
following differences: following differences:
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4. The example procedure for choosing a nonce based on Etag does not 4. The example procedure for choosing a nonce based on Etag does not
work for SIP. work for SIP.
5. The text in [RFC7234] regarding cache operation does not apply to 5. The text in [RFC7234] regarding cache operation does not apply to
SIP. SIP.
6. [RFC7616] requires that a server check that the URI in the 6. [RFC7616] requires that a server check that the URI in the
request line and the URI included in the Authorization header field request line and the URI included in the Authorization header field
point to the same resource. In a SIP context, these two URIs may point to the same resource. In a SIP context, these two URIs may
refer to different users, due to forwarding at some proxy. refer to different users, due to forwarding at some proxy.
Therefore, in SIP, a UAS MAY check that the Request-URI in the Therefore, in SIP, a UAS MUST check if the Request-URI in the
Authorization/Proxy-Authorization header field value corresponds to a Authorization/Proxy-Authorization header field value corresponds to a
user for whom the UAS is willing to accept forwarded or direct 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 requests, but MAY still accept it if the two fields are not
not equivalent. equivalent.
7. As a clarification to the calculation of the A2 value for message 7. As a clarification to the calculation of the A2 value for message
integrity assurance in the Digest authentication scheme, implementers integrity assurance in the Digest authentication scheme, implementers
should assume, when the entity-body is empty (that is, when SIP 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 messages have no body) that the hash of the entity-body resolves to
the hash of an empty string: the hash of an empty string:
H(entity-body) = <algorithm>("") H(entity-body) = <algorithm>("")
For example, when the chosen algorithm is SHA-256, then: For example, when the chosen algorithm is SHA-256, then:
H(entity-body) = SHA-256("") = H(entity-body) = SHA-256("") =
"e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855" "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
8. A UAS MUST be able to properly handle "qop" parameter received in 8. A UAS MUST be able to properly handle "qop" parameter received in
an Authorization/Proxy-Authorization header field, and a UAC MUST be an Authorization/Proxy-Authorization header field, and a UAC MUST be
able to properly handle "qop" parameter received in WWW-Authenticate able to properly handle "qop" parameter received in WWW-Authenticate
and Proxy-Authenticate header fields. However, for backward and Proxy-Authenticate header fields. However, for backward
compatibility reasons, the "qop" parameter is optional for compatibility reasons, the "qop" parameter is optional for
RFC3261-based clients and servers to receive. 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 A UAS MUST always send a "qop" parameter in WWW-Authenticate and
Proxy-Authenticate header field values, and a UAC MUST send the "qop" Proxy-Authenticate header field values, and a UAC MUST send the "qop"
parameter in any resulting authorization header field. parameter in any resulting authorization header field.
The usage of the Authentication-Info header field continues to be The usage of the Authentication-Info header field continues to be
allowed, since it provides integrity checks over the bodies and allowed, since it provides integrity checks over the bodies and
provides mutual authentication. provides mutual authentication.
2.7. Augmented BNF for SIP 2.7. Augmented BNF for SIP
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The number of hex digits is implied by the length of the value of the The number of hex digits is implied by the length of the value of the
algorithm used. algorithm used.
It extends the algorithm parameter as follows to allow for any It extends the algorithm parameter as follows to allow for any
algorithm in the registry to be used: algorithm in the registry to be used:
algorithm = "algorithm" EQUAL ( "MD5" / "SHA-512-256" / "SHA-256" algorithm = "algorithm" EQUAL ( "MD5" / "SHA-512-256" / "SHA-256"
/ token ) / token )
Each one of these algorithms might have a "-sess" variant, e.g.,
MD5-sess, SHA-256-sess, etc, as defined in [RFC7616]
3. Security Considerations 3. Security Considerations
This specification adds new secure algorithms to be used with the This specification adds new secure algorithms to be used with the
Digest mechanism to authenticate users, but leaves the broken MD5 Digest mechanism to authenticate users. The broken MD5 algorithm
algorithm for backward compatibility. 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 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 on-path attacker. The most effective way of dealing with this type
of attack is to either validate the client and challenge it of attack is to either validate the client and challenge it
accordingly, or remove the support for backward compatibility by not accordingly, or remove the support for backward compatibility by not
supporting MD5. supporting MD5.
See section 5 of [RFC7616] for a detailed security discussion of the See section 5 of [RFC7616] for a detailed security discussion of the
Digest scheme. Digest scheme.
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in the future. This document specifies that algorithms defined in in the future. This document specifies that algorithms defined in
that registry may be used in SIP digest authentication. that registry may be used in SIP digest authentication.
This document has no actions for IANA. This document has no actions for IANA.
5. Acknowledgments 5. Acknowledgments
The author would like to thank the following individuals for their The author would like to thank the following individuals for their
careful reviews, comments, and suggestions: Paul Kyzivat, Olle careful reviews, comments, and suggestions: Paul Kyzivat, Olle
Johansson, Dale Worley, Michael Procter, Inaki Baz Castillo, Tolga Johansson, Dale Worley, Michael Procter, Inaki Baz Castillo, Tolga
Asveren, Christer Holmberg, Brian Rosen, Jean Mahoney, and Adam Asveren, Christer Holmberg, Brian Rosen, Jean Mahoney, Adam Roach,
Roach. Barry Leiba, and Roni Even.
6. References 6. References
6.1. Normative References 6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
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