The Session Initiation Protocol (SIP) Digest Access Authentication Scheme
draft-ietf-sipcore-digest-scheme-15

[Ballot comment]
I am agreeing with Alissa's DISCUSS.

Also, I have a few comments of my own:

1) Last para of Section 2.1:

2.1.  Hash Algorithms

  A UAS prioritizes which algorithm to use based on the ordering of the
  challenge header fields in the response it is preparing.

This looks either wrong or confusing to me. I think you are just saying here
that the order is decided by the server at this point.

  That
  process is specified in section 2.3 and parallels the process used in
  HTTP specified by [RFC7616].

So based on the above, my suggested replacement for both sentences:

  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) Last para of Section 2.4:

  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.

Is trying other non Digest algorithms covered by "SHOULD abandon"?
If yes, maybe you should make this clearer.

  For example, if the UAC does not have
  credentials or has stale credentials for any of the realms, the UAC
  will abandon the request.

3) In Section 2.7:

      request-digest = LDQUOT *LHEX RDQUOT

This now allows empty value. I suggest you specify a minimum number of hex digits allowed in the ABNF.
Or at least change "*LHEX" to "2*LHEX".

[Ballot comment]
** Section 2.  Recommend a reference for SHA-256 and SHA-512/256

** Recommend consistent notation on “SHA-512/256” (Section 2.0 and 2.2) and “SHA-512-256” (Abstract).  “SHA-512-256” makes sense in the ABNF for Section 2.7.

[Ballot discuss]
I appreciate the new text in Section 2.7, but I'm still a little unclear on the ABNF that is specified. As far as I can tell the relevant line from the original ABNF in RFC 3261 is:

    algorithm        =  "algorithm" EQUAL ( "MD5" / "MD5-sess"
                      / token )

And what it is being replaced with is:

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

  Each one of these algorithms might have a "-sess" variant, e.g.,
  MD5-sess, SHA-256-sess, etc, as defined in [RFC7616]

But the point of ABNF is to formally specify the syntax, so just having the note after the definition that says there might be -sess variants leaves it unclear whether those -sess variants are expected in the "algorithm=" line. And if it is valid for them to appear, they need to be formally included in the ABNF line, I think.

[Ballot comment]
Thanks for this document; it's always good to see ways to move off of md5.

The genart thread suggests that we are expecting a new rev of the doc?

I do have some comments, despite balloting Yes; please note especially
the comment on Section 2.6, which may reflect an error in the document.

Abstract

nit: comma after "e.g." (as well as before).

Section 2.1

  A UAS prioritizes which algorithm to use based on the ordering of the
  challenge header fields in the response it is processing.  That

nit: the UAS is *preparing* the response, not processing it, right?

Section 2.4

Why does the first paragraph get an exemption for local policy but the
second paragraph does not?

Section 2.6

  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.

I think there's a subtle difference between what this says and what we want;
we still want a "MUST check" requirement (right?), but the nature of the
check changes, with default still being exact match but a "MAY" option for
the relaxed target-user check.

  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.

Should we remind people what the default is when it's not specified?

[Ballot comment]
Thank you for authoring this short document and deprecating MD5.

I have only two comments and they are the same as Barry's first two comments (weird abstract phrasing and not clear about MD5)

-éric

[Ballot comment]
Thanks for doing this.  I have some editorial comments, and one substantive suggestion that we state more strongly here what's said in 7616: that MD5 is NOT RECOMMENDED.

— Abstract —

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

I suggest that the “, which…” part isn’t useful in the Abstract, and should be removed.  It's said in the Security Considerations, and we don't need to lead with it.

— Section 2 —

  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.

I had a hard time with this sentence.  Let me suggest this:

NEW
  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.
END

— Section 2.1 —

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

That is not the name of the registry; it’s “Hash Algorithms for HTTP Digest Authentication” (which you correctly cite in other sections).

— Section 2.2 —

  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.

As we’re phasing out MD5, I suggest not using it as an example.  Just:

NEW
  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.
END

— Section 2.4 —

  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.

This is really hard to parse; I think your “e.g.” clause gets in the way.  I suggest this:

NEW
  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.  For example, if the UAC does not have
  credentials or has stale credentials for any of the realms, the UAC
  will abandon the request.
END

— Section 2.5 —

  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.

I can’t make sense of this; can you try rephrasing it?  There’s one response and a single response.  I don’t follow.  There’s values received from proxies and values received from other proxies.  What’s the difference between “proxies” and “other proxies”?  I don’t follow.

— Section 3 —

  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.

I recommend being stronger about this by using text from 7616:

NEW
  This specification adds new secure algorithms to be used with the
  Digest mechanism to authenticate users. The broken MD5 algorithm
  remains only for backward compatibility with [RFC2617], but its use is
  NOT RECOMMENDED.
END

(Via drafts-lastcall@iana.org): IESG/Authors/WG Chairs:

The IANA Functions Operator has reviewed draft-ietf-sipcore-digest-scheme-08, which is currently in Last Call, and has the following comments:

We understand that this document doesn't require any registry actions.

While it's often helpful for a document's IANA Considerations section to remain in place upon publication even if there are no actions, if the authors strongly prefer to remove it, we do not object.

If this assessment is not accurate, please respond as soon as possible.

Thank you,

Sabrina Tanamal
Senior IANA Services Specialist

The following Last Call announcement was sent out (ends 2019-09-12):

From: The IESG
To: IETF-Announce
CC: Jean Mahoney , adam@nostrum.com, sipcore-chairs@ietf.org, sipcore@ietf.org, mahoney@nostrum.com, draft-ietf-sipcore-digest-scheme@ietf.org
Reply-To: ietf@ietf.org
Sender:
Subject: Last Call:  (The Session Initiation Protocol (SIP) Digest Authentication Scheme) to Proposed Standard


The IESG has received a request from the Session Initiation Protocol Core WG
(sipcore) to consider the following document: - 'The Session Initiation
Protocol (SIP) Digest Authentication Scheme'
  as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits final
comments on this action. Please send substantive comments to the
ietf@ietf.org mailing lists by 2019-09-12. Exceptionally, comments may be
sent to iesg@ietf.org instead. In either case, please retain the beginning of
the Subject line to allow automated sorting.

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.




The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-sipcore-digest-scheme/

IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-sipcore-digest-scheme/ballot/


No IPR declarations have been submitted directly on this I-D.

As required by RFC 4858, this is the current template for the Document
Shepherd Write-Up.

Changes are expected over time. This version is dated 24 February 2012.

(1) What type of RFC is being requested (BCP, Proposed Standard,
Internet Standard, Informational, Experimental, or Historic)?  Why
is this the proper type of RFC?  Is this type of RFC indicated in the
title page header?

Proposed Standard, which is indicated in the title page header. 



(2) The IESG approval announcement includes a Document Announcement
Write-Up. Please provide such a Document Announcement Write-Up. Recent
examples can be found in the "Action" announcements for approved
documents. The approval announcement contains the following sections:

Technical Summary

  Relevant content can frequently be found in the abstract
  and/or introduction of the document. If not, this may be
  an indication that there are deficiencies in the abstract
  or introduction.

The authentication framework for the Session Initiation Protocol (SIP, RFC 3261) closely parallels that of the Hypertext Transfer Protocol (HTTP) Digest Access Authentication (RFC 2617). RFC 2617 was obsoleted by RFC 7616, which introduces more secure digest algorithms (e.g., SHA-256 and SHA-512-256). This document updates the authentication scheme used by SIP to add support for these more secure algorithms that are listed in the "Hash Algorithms for HTTP Digest Authentication" registry created by RFC 7616. Although the MD5 algorithm is considered cryptographically broken, it is still supported for backward compatibility.




Working Group Summary

  Was there anything in WG process that is worth noting? For
  example, was there controversy about particular points or
  were there decisions where the consensus was particularly
  rough?

Work on this topic (initially as draft-yusef-sipcore-digest-scheme) started in January 2014, somewhat in parallel with the HTTP Digest Access Authentication work [RFC7616]. The SIPCORE participants who voiced an opinion thought it was a good idea, and provided careful reviews. The draft went through multiple iterations as feedback was incorporated. There was no pushback against the concept on-list; however, at the London IETF 89 SIPCORE WG session, it was discussed that SIP authentication in general needed an overhaul, not just the digest scheme. While some work went into that effort, both the new work and draft-yusef-sipcore-digest-scheme expired about six months later. In 2017 the author resurrected the draft, and again the draft received support and feedback, but then the draft expired later in the year. It was resurrected again in spring 2019, and adopted as a WG item. 



Document Quality

  Are there existing implementations of the protocol? Have a
  significant number of vendors indicated their plan to
  implement the specification? Are there any reviewers that
  merit special mention as having done a thorough review,
  e.g., one that resulted in important changes or a
  conclusion that the document had no substantive issues? If
  there was a MIB Doctor, Media Type or other expert review,
  what was its course (briefly)? In the case of a Media Type
  review, on what date was the request posted?

The content of this document has been implemented and deployed in mobile IMS networks. Several reviewers provided substantial feedback and they have been thanked in the Acknowledgments section. The content of the document does not require expert review.


Personnel

  Who is the Document Shepherd? Who is the Responsible Area
  Director?

Document Shepherd: Jean Mahoney
Responsible Area Director: Adam Roach


(3) Briefly describe the review of this document that was performed by
the Document Shepherd.  If this version of the document is not ready
for publication, please explain why the document is being forwarded to
the IESG.

Document Shepherd went through the mail archives back to 2014 to ensure that feedback was addressed. She considers the draft ready to proceed.



(4) Does the document Shepherd have any concerns about the depth or
breadth of the reviews that have been performed?

The Document Shepherd is satisfied with the breadth and depth of reviews performed by the working group.



(5) Do portions of the document need review from a particular or from
broader perspective, e.g., security, operational complexity, AAA, DNS,
DHCP, XML, or internationalization? If so, describe the review that
took place.

None required.



(6) Describe any specific concerns or issues that the Document Shepherd
has with this document that the Responsible Area Director and/or the
IESG should be aware of? For example, perhaps he or she is uncomfortable
with certain parts of the document, or has concerns whether there really
is a need for it. In any event, if the WG has discussed those issues and
has indicated that it still wishes to advance the document, detail those
concerns here.

No concerns.



(7) Has each author confirmed that any and all appropriate IPR
disclosures required for full conformance with the provisions of BCP 78
and BCP 79 have already been filed. If not, explain why.

The author has confirmed conformance with BCPs 78 and 79.



(8) Has an IPR disclosure been filed that references this document?
If so, summarize any WG discussion and conclusion regarding the IPR
disclosures.

No disclosure has been filed.



(9) How solid is the WG consensus behind this document? Does it
represent the strong concurrence of a few individuals, with others
being silent, or does the WG as a whole understand and agree with it? 

The only pushback this document has received has been at IETF 89 in 2014, when the WG wanted to solve a bigger problem. Otherwise it has received WG support and careful review. 



(10) Has anyone threatened an appeal or otherwise indicated extreme
discontent? If so, please summarise the areas of conflict in separate
email messages to the Responsible Area Director. (It should be in a
separate email because this questionnaire is publicly available.)

No one has indicated any discontent with the draft.



(11) Identify any ID nits the Document Shepherd has found in this
document. (See https://www.ietf.org/tools/idnits/ and the Internet-Drafts
Checklist). Boilerplate checks are not enough; this check needs to be
thorough.

idnits 2.16.02 was run, and no issues were found. The Shepherd checked the draft against https://www.ietf.org/standards/ids/checklist/. No issues were found with the draft.



(12) Describe how the document meets any required formal review
criteria, such as the MIB Doctor, media type, and URI type reviews.

No formal review needed.



(13) Have all references within this document been identified as
either normative or informative?

Yes.



(14) Are there normative references to documents that are not ready for
advancement or are otherwise in an unclear state? If such normative
references exist, what is the plan for their completion?

All normative references are to published RFCs.



(15) Are there downward normative references references (see RFC 3967)?
If so, list these downward references to support the Area Director in
the Last Call procedure.

There are no downward normative references.



(16) Will publication of this document change the status of any
existing RFCs? Are those RFCs listed on the title page header, listed
in the abstract, and discussed in the introduction? If the RFCs are not
listed in the Abstract and Introduction, explain why, and point to the
part of the document where the relationship of this document to the
other RFCs is discussed. If this information is not in the document,
explain why the WG considers it unnecessary.

This document updates RFC 3261. This information is clearly captured in the header, abstract, and introduction.



(17) Describe the Document Shepherd's review of the IANA considerations
section, especially with regard to its consistency with the body of the
document. Confirm that all protocol extensions that the document makes
are associated with the appropriate reservations in IANA registries.
Confirm that any referenced IANA registries have been clearly
identified. Confirm that newly created IANA registries include a
detailed specification of the initial contents for the registry, that
allocations procedures for future registrations are defined, and a
reasonable name for the new registry has been suggested (see RFC 5226).

The IANA Considerations section of this document merely points to the registry that RFC 7616 created and specifies that algorithms defined in that registry may be used in SIP digest authentication. It does not create nor modify any IANA registry, and thus has no actions for IANA.



(18) List any new IANA registries that require Expert Review for future
allocations. Provide any public guidance that the IESG would find
useful in selecting the IANA Experts for these new registries.

This document does not create any new IANA registries.



(19) Describe reviews and automated checks performed by the Document
Shepherd to validate sections of the document written in a formal
language, such as XML code, BNF rules, MIB definitions, etc.

The document makes two small changes to the ABNF of SIP [RFC3261]. The working group provided feedback on the ABNF. The Doc Shepherd inspected the ABNF manually and found no issues.

As required by RFC 4858, this is the current template for the Document
Shepherd Write-Up.

Changes are expected over time. This version is dated 24 February 2012.

(1) What type of RFC is being requested (BCP, Proposed Standard,
Internet Standard, Informational, Experimental, or Historic)?  Why
is this the proper type of RFC?  Is this type of RFC indicated in the
title page header?

Proposed Standard, which is indicated in the title page header. 



(2) The IESG approval announcement includes a Document Announcement
Write-Up. Please provide such a Document Announcement Write-Up. Recent
examples can be found in the "Action" announcements for approved
documents. The approval announcement contains the following sections:

Technical Summary

  Relevant content can frequently be found in the abstract
  and/or introduction of the document. If not, this may be
  an indication that there are deficiencies in the abstract
  or introduction.

The authentication framework for the Session Initiation Protocol (SIP, RFC 3261) closely parallels that of the Hypertext Transfer Protocol (HTTP) Digest Access Authentication (RFC 2617). RFC 2617 was obsoleted by RFC 7616, which introduces more secure digest algorithms (e.g., SHA-256 and SHA-512-256). This document updates the authentication scheme used by SIP to add support for these more secure algorithms that are listed in the "Hash Algorithms for HTTP Digest Authentication" registry created by RFC 7616. Although the MD5 algorithm is considered cryptographically broken, it is still supported for backward compatibility.




Working Group Summary

  Was there anything in WG process that is worth noting? For
  example, was there controversy about particular points or
  were there decisions where the consensus was particularly
  rough?

Work on this topic (initially as draft-yusef-sipcore-digest-scheme) started in January 2014, somewhat in parallel with the HTTP Digest Access Authentication work [RFC7616]. The SIPCORE participants who voiced an opinion thought it was a good idea, and provided careful reviews. The draft went through multiple iterations as feedback was incorporated. There was no pushback against the concept on-list; however, at the London IETF 89 SIPCORE WG session, it was discussed that SIP authentication in general needed an overhaul, not just the digest scheme. While some work went into that effort, both the new work and draft-yusef-sipcore-digest-scheme expired about six months later. In 2017 the author resurrected the draft, and again the draft received support and feedback, but then the draft expired later in the year. It was resurrected again in spring 2019, and adopted as a WG item. 



Document Quality

  Are there existing implementations of the protocol? Have a
  significant number of vendors indicated their plan to
  implement the specification? Are there any reviewers that
  merit special mention as having done a thorough review,
  e.g., one that resulted in important changes or a
  conclusion that the document had no substantive issues? If
  there was a MIB Doctor, Media Type or other expert review,
  what was its course (briefly)? In the case of a Media Type
  review, on what date was the request posted?

The content of this document has been implemented and deployed in mobile IMS networks. Several reviewers provided substantial feedback and they have been thanked in the Acknowledgments section. The content of the document does not require expert review.


Personnel

  Who is the Document Shepherd? Who is the Responsible Area
  Director?

Document Shepherd: Jean Mahoney
Responsible Area Director: Adam Roach


(3) Briefly describe the review of this document that was performed by
the Document Shepherd.  If this version of the document is not ready
for publication, please explain why the document is being forwarded to
the IESG.

Document Shepherd went through the mail archives back to 2014 to ensure that feedback was addressed. She considers the draft ready to proceed.



(4) Does the document Shepherd have any concerns about the depth or
breadth of the reviews that have been performed?

The Document Shepherd is satisfied with the breadth and depth of reviews performed by the working group.



(5) Do portions of the document need review from a particular or from
broader perspective, e.g., security, operational complexity, AAA, DNS,
DHCP, XML, or internationalization? If so, describe the review that
took place.

None required.



(6) Describe any specific concerns or issues that the Document Shepherd
has with this document that the Responsible Area Director and/or the
IESG should be aware of? For example, perhaps he or she is uncomfortable
with certain parts of the document, or has concerns whether there really
is a need for it. In any event, if the WG has discussed those issues and
has indicated that it still wishes to advance the document, detail those
concerns here.

No concerns.



(7) Has each author confirmed that any and all appropriate IPR
disclosures required for full conformance with the provisions of BCP 78
and BCP 79 have already been filed. If not, explain why.

The author has confirmed conformance with BCPs 78 and 79.



(8) Has an IPR disclosure been filed that references this document?
If so, summarize any WG discussion and conclusion regarding the IPR
disclosures.

No disclosure has been filed.



(9) How solid is the WG consensus behind this document? Does it
represent the strong concurrence of a few individuals, with others
being silent, or does the WG as a whole understand and agree with it? 

The only pushback this document has received has been at IETF 89 in 2014, when the WG wanted to solve a bigger problem. Otherwise it has received WG support and careful review. 



(10) Has anyone threatened an appeal or otherwise indicated extreme
discontent? If so, please summarise the areas of conflict in separate
email messages to the Responsible Area Director. (It should be in a
separate email because this questionnaire is publicly available.)

No one has indicated any discontent with the draft.



(11) Identify any ID nits the Document Shepherd has found in this
document. (See https://www.ietf.org/tools/idnits/ and the Internet-Drafts
Checklist). Boilerplate checks are not enough; this check needs to be
thorough.

idnits 2.16.02 was run, and no issues were found. The Shepherd checked the draft against https://www.ietf.org/standards/ids/checklist/. No issues were found with the draft.



(12) Describe how the document meets any required formal review
criteria, such as the MIB Doctor, media type, and URI type reviews.

No formal review needed.



(13) Have all references within this document been identified as
either normative or informative?

Yes.



(14) Are there normative references to documents that are not ready for
advancement or are otherwise in an unclear state? If such normative
references exist, what is the plan for their completion?

All normative references are to published RFCs.



(15) Are there downward normative references references (see RFC 3967)?
If so, list these downward references to support the Area Director in
the Last Call procedure.

There are no downward normative references.



(16) Will publication of this document change the status of any
existing RFCs? Are those RFCs listed on the title page header, listed
in the abstract, and discussed in the introduction? If the RFCs are not
listed in the Abstract and Introduction, explain why, and point to the
part of the document where the relationship of this document to the
other RFCs is discussed. If this information is not in the document,
explain why the WG considers it unnecessary.

This document updates RFC 3261. This information is clearly captured in the header, abstract, and introduction.



(17) Describe the Document Shepherd's review of the IANA considerations
section, especially with regard to its consistency with the body of the
document. Confirm that all protocol extensions that the document makes
are associated with the appropriate reservations in IANA registries.
Confirm that any referenced IANA registries have been clearly
identified. Confirm that newly created IANA registries include a
detailed specification of the initial contents for the registry, that
allocations procedures for future registrations are defined, and a
reasonable name for the new registry has been suggested (see RFC 5226).

The IANA Considerations section of this document merely points to the registry that RFC 7616 created and specifies that algorithms defined in that registry may be used in SIP digest authentication. It does not create nor modify any IANA registry, and thus has no actions for IANA.



(18) List any new IANA registries that require Expert Review for future
allocations. Provide any public guidance that the IESG would find
useful in selecting the IANA Experts for these new registries.

This document does not create any new IANA registries.



(19) Describe reviews and automated checks performed by the Document
Shepherd to validate sections of the document written in a formal
language, such as XML code, BNF rules, MIB definitions, etc.

The document makes two small changes to the ABNF of SIP [RFC3261]. The working group provided feedback on the ABNF. The Doc Shepherd inspected the ABNF manually and found no issues.