Hybrid key exchange in TLS 1.3
draft-ietf-tls-hybrid-design-16

[Ballot comment]
Thanks to Rifaat Shekh-Yusef for their secdir review.

References:  The references for IND-CCA2, IND-CPA as well as the Fujisaki-Okamoto transform, and maybe HHK should be normative.  My reservation here is the fact that the reference for the first two is a textbook, which is certainly 'stable', but hardly freely available.  I'm going to bet that Katz has references for both of the first two security properties. [note: the textbook reference has a typo - Introductino/Introduction]

[Ballot comment]
Hi Douglas, Scott, and Shay,

Thank you for the effort put into this well-written document. It is an interesting read.

Thanks also to Tim Chown for the OPSDIR review and the authors for engaging.

Please find below some comments:

# Why is this Informational?

The justification in the writeup seems clear to me. I do think that it would be useful to mirror in the abstract or the Introduction the main message grabbed from the writeup:

“It does not modify TLS directly, but rather using existing mechanisms and code point registries. It does not define any specific hybrid key exchanges.”

This statement may be revisited based on the outcome of the next item.

# Relaxing a MUST in the base TLS spec?

CURRENT:
  [TLS13] requires that ``The key_exchange values for each
  KeyShareEntry MUST be generated independently.'' In the context of
  this document, since the same algorithm may appear in multiple named
  groups, this document relaxes the above requirement to allow the same
  key_exchange value for the same algorithm to be reused in multiple
  KeyShareEntry records sent in within the same ClientHello. 

Isn’t this modifying aspects of the base TLS? How to reconcile this with the claim in the previous point?

# IANA considerations

Unless I missed something the document provides guidance for future assignments. Shouldn’t a note be added to the TLS registry to track this guidance and/or add this document as a reference to the registry?

As I’m there,

CURRENT:
For these entries in
  the TLS Supported Groups registry, the "Recommended" column SHOULD be
  "N" and the "DTLS-OK" column SHOULD be "Y".

I guess this should be the value used in the initial registration. The recommended value may in theory evolve in the future (far future, maybe). If so, can we make that explicit in the document?

# Minor/nits

## Section 1.2

OLD: for example, FIPS compliance.
NEW: for example, US National Institute of Standards and Technology (NIST) FIPS compliance.

## Section 1.5

OLD: as long as as
NEW: as long as

## Section 2

CURRENT:
  The main security property for KEMs is indistinguishability under
  adaptive chosen ciphertext attack (IND-CCA2), ..
                                      ^^^^^^^^ 

  A weaker security notion is indistinguishability under chosen
  plaintext attack (IND-CPA),
                    ^^^^^^^^

Can we cite references for these two?

## Section 3.2

CURRENT:
  Recall that in TLS 1.3 a KEM public key or KEM ciphertext is
  represented as a KeyShareEntry:

Can we have an explicit reference to rfc8446#section-4.2.8 for readers convenience?

Idem for:

CURRENT:
  [TLS13] requires that ``The key_exchange values for each

Cheers,
Med

[Ballot comment]
This is a solid and well-written document. Thank you.

In Section 1, it says "Some authors prefer...." It might be useful to be clear which authors are being referenced here -- authors of this RFC? Authors of referenced research papers? Authors of future hybrid specifications?

I appreciate the appendix of related work. You specifically call out one reference as "an overview of post-quantum cryptography as of 2009," which invites the question of why this is the best reference 16 years later. I would either drop the date (if it's still the best overview around) or add a few words stating that while dated, it remains a valuable resource because of [foo].

[Ballot comment]

# Éric Vyncke, INT AD, comments for draft-ietf-tls-hybrid-design-14
CC @evyncke

Thank you for the work put into this document.

Please find below some non-blocking COMMENT points/nits (replies would be appreciated even if only for my own education).

Special thanks to Joe Salowey for the shepherd's detailed write-up including the WG consensus *and* the justification of the intended status.

I hope that this review helps to improve the document,

Regards,

-éric


## COMMENTS (non-blocking)

### Section 1.2

s/Algorithms which are widely deployed today, but *which* may be deprecated in the future/Algorithms *that* are widely deployed today, but may be deprecated in the future/ ?

Same for 'next-gen' ;-) I sincerely hope that TLSng will be faster to deploy than IPng, aka IPv6... ;-)

Who is the "we" in `we do not limit`? The authors, the WG, the IETF ? Please avoid using ambiguous "we" in an IETF document.

After introducing "component" and "composite", the text states `It is intended that the composite algorithms` while the preceeding text seems to prefer not using "composite". Nothing dramatic but puzzling.

s/that is not Diffie-Hellman-based nor a group/hat is *neither* Diffie-Hellman-based nor a group/ ?

### Section 1.4

s/session authentication cannot be retroactively broken/breaking retroactively session authentication is pointless/ ?

### Section 1.5

When reading the part about the sheer size of the key exchange, I wonder whether some text (perhaps in another doc) should be authored for 'contrained' networks (being in ressource or network bandwidth). Strongly suggest adding a sentence around "Using hybrid in constrained systems may not be suitable" or something similar to state that the TLS WG have not ignored the "IoT world".

### Section 2

While I trust the I-D authors to know more than me, I am puzzled by reading "secret key" rather than "private key", which seems more popular.

Please add another informational reference to explain what "IND-CCA2" is (and later in the text "IND-CPA").

The last sentence contains 2 "MUST", but aren't these "MUST" applicable to *any* key TLS key exchange ? I.e., not only to "hybrid" ones ?

### Section 3.3

As non-US "NIST" may appear, let's s/NIST/US NIST/ (and possibly expand NIST).

### Section 4

Should there be an informal reference to `Classic McEliece`?

Should the "can" in `Clients can retry if a failure is encountered` be replaced by "MAY" or even a "SHOULD" ?

### Section 5

Please add an informal reference to the IANA registry URI https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-8.

I was not expected an informational RFC to state such a strong binding statement `For these entries in the TLS Supported Groups registry, the "Recommended" column should be "N" and the "DTLS-OK" column should be "Y".` (at least this is a "should"... should this has been a "SHOULD" ?).

Suggest to add guidance to IANA to implement `These assignments should be made in a range that is distinct`


## NITS (non-blocking / cosmetic)

### E.g.

AFAIK, "e.g." should be surrounded by ",".

### Use of SVG graphics

To make a much nicer HTML rendering, suggest using the aasvg too to generate SVG graphics. It is worth a try especially if the I-D uses the Kramdown file format ;-)

IESG/Authors/WG Chairs:

IANA has completed its review of draft-ietf-tls-hybrid-design-13. If any part of this review is inaccurate, please let us know.

IANA has a question about the IANA Considerations section of this document.

IANA understands that, upon approval of this document, there are no IANA Actions that need completion. We request that the IANA Considerations section of the document remain in place upon publication.

However, Section 5 of the current draft says: "IANA will assign identifiers from the TLS Supported Groups registry for the hybrid combinations defined following this document." IANA understands this to mean that future drafts will request IANA to make new registrations in the TLS Supported Groups registry while the current draft makes no such requests.

IANA Question --> Is this understanding correct?

For definitions of IANA review states, please see:

https://datatracker.ietf.org/help/state/draft/iana-review

Thank you,

David Dong
IANA Services Sr. Specialist

The following Last Call announcement was sent out (ends 2025-07-01):

From: The IESG
To: IETF-Announce
CC: caw@heapingbits.net, draft-ietf-tls-hybrid-design@ietf.org, durumcrustulum@gmail.com, joe@salowey.net, paul.wouters@aiven.io, tls-chairs@ietf.org, tls@ietf.org
Reply-To: last-call@ietf.org
Sender:
Subject: Last Call:  (Hybrid key exchange in TLS 1.3) to Informational RFC


The IESG has received a request from the Transport Layer Security WG (tls) to
consider the following document: - 'Hybrid key exchange in TLS 1.3'
  as Informational RFC

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
last-call@ietf.org mailing lists by 2025-07-01. 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


  Hybrid key exchange refers to using multiple key exchange algorithms
  simultaneously and combining the result with the goal of providing
  security even if all but one of the component algorithms is broken.
  It is motivated by transition to post-quantum cryptography.  This
  document provides a construction for hybrid key exchange in the
  Transport Layer Security (TLS) protocol version 1.3.

The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-tls-hybrid-design/



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

# Document Shepherd Write-Up for Group Documents

*This version is dated 4 July 2022.*

Thank you for your service as a document shepherd. Among the responsibilities is
answering the questions in this write-up to give helpful context to Last Call
and Internet Engineering Steering Group ([IESG][1]) reviewers, and your
diligence in completing it is appreciated. The full role of the shepherd is
further described in [RFC 4858][2]. You will need the cooperation of the authors
and editors to complete these checks.

Note that some numbered items contain multiple related questions; please be sure
to answer all of them.

## Document History

1. Does the working group (WG) consensus represent the strong concurrence of a
  few individuals, with others being silent, or did it reach broad agreement?

There is strong working group consensus for this draft.

2. Was there controversy about particular points, or were there decisions where
  the consensus was particularly rough?

No

3. Has anyone threatened an appeal or otherwise indicated extreme discontent? If
  so, please summarize 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.

4. For protocol documents, are there existing implementations of the contents of
  the document? Have a significant number of potential implementers indicated
  plans to implement? Are any existing implementations reported somewhere,
  either in the document itself (as [RFC 7942][3] recommends) or elsewhere
  (where)?

This draft is has several implementations of hybrid groups that are based on the approach from this document already deployed. There is a decent chance you are using a hybrid group right now. Here is an incomplete list:

Chrome, Mozilla, OpenSSL 3.5(To be released, currently supports when used with OQS), wolfSSL, AWS s2n, Cloudflare, Google, BoringSSL, rustTLS

## Additional Reviews

5. Do the contents of this document closely interact with technologies in other
  IETF working groups or external organizations, and would it therefore benefit
  from their review? Have those reviews occurred? If yes, describe which
  reviews took place.

This is a TLS specific draft.

6. Describe how the document meets any required formal expert review criteria,
  such as the MIB Doctor, YANG Doctor, media type, and URI type reviews.

The cryptographic mechanisms used in this document are based the following:
https://eprint.iacr.org/2018/903 (Section 3.2) which has been reviewed and published.

7. If the document contains a YANG module, has the final version of the module
  been checked with any of the [recommended validation tools][4] for syntax and
  formatting validation? If there are any resulting errors or warnings, what is
  the justification for not fixing them at this time? Does the YANG module
  comply with the Network Management Datastore Architecture (NMDA) as specified
  in [RFC 8342][5]?

NA

8. Describe reviews and automated checks performed to validate sections of the
  final version of the document written in a formal language, such as XML code,
  BNF rules, MIB definitions, CBOR's CDDL, etc.

NA

## Document Shepherd Checks

9. Based on the shepherd's review of the document, is it their opinion that this
  document is needed, clearly written, complete, correctly designed, and ready
  to be handed off to the responsible Area Director?

The document shepherd believes this document is ready to hand off to the IESG

10. Several IETF Areas have assembled [lists of common issues that their
    reviewers encounter][6]. For which areas have such issues been identified
    and addressed? For which does this still need to happen in subsequent
    reviews?

No Issues have been identified

11. What type of RFC publication is being requested on the IETF stream ([Best
    Current Practice][12], [Proposed Standard, Internet Standard][13],
    [Informational, Experimental or Historic][14])? Why is this the proper type
    of RFC? Do all Datatracker state attributes correctly reflect this intent?

The draft is currently in the informational track. his document is information because it is just presenting a set of guidelines for the designers of hybrid key exchanges in TLS to follow.  It does not modify TLS directly, but rather using existing mechanisms and code point registries. It does not define any specific hybrid key exchanges.

12. Have reasonable efforts been made to remind all authors of the intellectual
    property rights (IPR) disclosure obligations described in [BCP 79][7]? To
    the best of your knowledge, have all required disclosures been filed? If
    not, explain why. If yes, summarize any relevant discussion, including links
    to publicly-available messages when applicable.

To the best of my knowledge there is no IPR that applies to this draft. The authors
have been reminded of their obligations and to the best of my knowledge IPR declarations
have been filed


13. Has each author, editor, and contributor shown their willingness to be
    listed as such? If the total number of authors and editors on the front page
    is greater than five, please provide a justification.

Yes

14. Document any remaining I-D nits in this document. Simply running the [idnits
    tool][8] is not enough; please review the ["Content Guidelines" on
    authors.ietf.org][15]. (Also note that the current idnits tool generates
    some incorrect warnings; a rewrite is underway.)

There is one formula that is too long that can be adjusted during subsequent revisions
or in the RFC editing process

15. Should any informative references be normative or vice-versa? See the [IESG
    Statement on Normative and Informative References][16].

No

16. List any normative references that are not freely available to anyone. Did
    the community have sufficient access to review any such normative
    references?

Normative references are freely available

17. Are there any normative downward references (see [RFC 3967][9] and [BCP
    97][10]) that are not already listed in the [DOWNREF registry][17]? If so,
    list them.

No

18. Are there normative references to documents that are not ready to be
    submitted to the IESG for publication or are otherwise in an unclear state?
    If so, what is the plan for their completion?

No

19. Will publication of this document change the status of any existing RFCs? If
    so, does the Datatracker metadata correctly reflect this and are those RFCs
    listed on the title page, in the abstract, and discussed in the
    introduction? If not, explain why and point to the part of the document
    where the relationship of this document to these other RFCs is discussed.

The document does not change the status of any RFC

20. 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 aspects of the document requiring IANA assignments are
    associated with the appropriate reservations in IANA registries. Confirm
    that any referenced IANA registries have been clearly identified. Confirm
    that each newly created IANA registry specifies its initial contents,
    allocations procedures, and a reasonable name (see [RFC 8126][11]).

This document does not specify any new registries, it uses the TLS Supported Groups Registry

21. List any new IANA registries that require Designated Expert Review for
    future allocations. Are the instructions to the Designated Expert clear?
    Please include suggestions of designated experts, if appropriate.

NA

[1]: https://www.ietf.org/about/groups/iesg/
[2]: https://www.rfc-editor.org/rfc/rfc4858.html
[3]: https://www.rfc-editor.org/rfc/rfc7942.html
[4]: https://wiki.ietf.org/group/ops/yang-review-tools
[5]: https://www.rfc-editor.org/rfc/rfc8342.html
[6]: https://wiki.ietf.org/group/iesg/ExpertTopics
[7]: https://www.rfc-editor.org/info/bcp79
[8]: https://www.ietf.org/tools/idnits/
[9]: https://www.rfc-editor.org/rfc/rfc3967.html
[10]: https://www.rfc-editor.org/info/bcp97
[11]: https://www.rfc-editor.org/rfc/rfc8126.html
[12]: https://www.rfc-editor.org/rfc/rfc2026.html#section-5
[13]: https://www.rfc-editor.org/rfc/rfc2026.html#section-4.1
[14]: https://www.rfc-editor.org/rfc/rfc2026.html#section-4.2
[15]: https://authors.ietf.org/en/content-guidelines-overview
[16]: https://www.ietf.org/about/groups/iesg/statements/normative-informative-references/
[17]: https://datatracker.ietf.org/doc/downref/

# Document Shepherd Write-Up for Group Documents

*This version is dated 4 July 2022.*

Thank you for your service as a document shepherd. Among the responsibilities is
answering the questions in this write-up to give helpful context to Last Call
and Internet Engineering Steering Group ([IESG][1]) reviewers, and your
diligence in completing it is appreciated. The full role of the shepherd is
further described in [RFC 4858][2]. You will need the cooperation of the authors
and editors to complete these checks.

Note that some numbered items contain multiple related questions; please be sure
to answer all of them.

## Document History

1. Does the working group (WG) consensus represent the strong concurrence of a
  few individuals, with others being silent, or did it reach broad agreement?

There is strong working group consensus for this draft.

2. Was there controversy about particular points, or were there decisions where
  the consensus was particularly rough?

No

3. Has anyone threatened an appeal or otherwise indicated extreme discontent? If
  so, please summarize 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.

4. For protocol documents, are there existing implementations of the contents of
  the document? Have a significant number of potential implementers indicated
  plans to implement? Are any existing implementations reported somewhere,
  either in the document itself (as [RFC 7942][3] recommends) or elsewhere
  (where)?

This draft is has several implementations of hybrid groups that are based on the approach from this document already deployed. There is a decent chance you are using a hybrid group right now. Here is an incomplete list:

Chrome, Mozilla, OpenSSL 3.5(To be released, currently supports when used with OQS), wolfSSL, AWS s2n, Cloudflare, Google, BoringSSL, rustTLS

## Additional Reviews

5. Do the contents of this document closely interact with technologies in other
  IETF working groups or external organizations, and would it therefore benefit
  from their review? Have those reviews occurred? If yes, describe which
  reviews took place.

This is a TLS specific draft.

6. Describe how the document meets any required formal expert review criteria,
  such as the MIB Doctor, YANG Doctor, media type, and URI type reviews.

The cryptographic mechanisms used in this document are based the following:
https://eprint.iacr.org/2018/903 (Section 3.2) which has been reviewed and published.

7. If the document contains a YANG module, has the final version of the module
  been checked with any of the [recommended validation tools][4] for syntax and
  formatting validation? If there are any resulting errors or warnings, what is
  the justification for not fixing them at this time? Does the YANG module
  comply with the Network Management Datastore Architecture (NMDA) as specified
  in [RFC 8342][5]?

NA

8. Describe reviews and automated checks performed to validate sections of the
  final version of the document written in a formal language, such as XML code,
  BNF rules, MIB definitions, CBOR's CDDL, etc.

NA

## Document Shepherd Checks

9. Based on the shepherd's review of the document, is it their opinion that this
  document is needed, clearly written, complete, correctly designed, and ready
  to be handed off to the responsible Area Director?

The document shepherd believes this document is ready to hand off to the IESG

10. Several IETF Areas have assembled [lists of common issues that their
    reviewers encounter][6]. For which areas have such issues been identified
    and addressed? For which does this still need to happen in subsequent
    reviews?

No Issues have been identified

11. What type of RFC publication is being requested on the IETF stream ([Best
    Current Practice][12], [Proposed Standard, Internet Standard][13],
    [Informational, Experimental or Historic][14])? Why is this the proper type
    of RFC? Do all Datatracker state attributes correctly reflect this intent?

The draft is currently in the informational track. his document is information because it is just presenting a set of guidelines for the designers of hybrid key exchanges in TLS to follow.  It does not modify TLS directly, but rather using existing mechanisms and code point registries. It does not define any specific hybrid key exchanges.

12. Have reasonable efforts been made to remind all authors of the intellectual
    property rights (IPR) disclosure obligations described in [BCP 79][7]? To
    the best of your knowledge, have all required disclosures been filed? If
    not, explain why. If yes, summarize any relevant discussion, including links
    to publicly-available messages when applicable.

To the best of my knowledge there is no IPR that applies to this draft. The authors
have been reminded of their obligations and to the best of my knowledge IPR declarations
have been filed


13. Has each author, editor, and contributor shown their willingness to be
    listed as such? If the total number of authors and editors on the front page
    is greater than five, please provide a justification.

Yes

14. Document any remaining I-D nits in this document. Simply running the [idnits
    tool][8] is not enough; please review the ["Content Guidelines" on
    authors.ietf.org][15]. (Also note that the current idnits tool generates
    some incorrect warnings; a rewrite is underway.)

There is one formula that is too long that can be adjusted during subsequent revisions
or in the RFC editing process

15. Should any informative references be normative or vice-versa? See the [IESG
    Statement on Normative and Informative References][16].

No

16. List any normative references that are not freely available to anyone. Did
    the community have sufficient access to review any such normative
    references?

Normative references are freely available

17. Are there any normative downward references (see [RFC 3967][9] and [BCP
    97][10]) that are not already listed in the [DOWNREF registry][17]? If so,
    list them.

No

18. Are there normative references to documents that are not ready to be
    submitted to the IESG for publication or are otherwise in an unclear state?
    If so, what is the plan for their completion?

No

19. Will publication of this document change the status of any existing RFCs? If
    so, does the Datatracker metadata correctly reflect this and are those RFCs
    listed on the title page, in the abstract, and discussed in the
    introduction? If not, explain why and point to the part of the document
    where the relationship of this document to these other RFCs is discussed.

The document does not change the status of any RFC

20. 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 aspects of the document requiring IANA assignments are
    associated with the appropriate reservations in IANA registries. Confirm
    that any referenced IANA registries have been clearly identified. Confirm
    that each newly created IANA registry specifies its initial contents,
    allocations procedures, and a reasonable name (see [RFC 8126][11]).

This document does not specify any new registries, it uses the TLS Supported Groups Registry

21. List any new IANA registries that require Designated Expert Review for
    future allocations. Are the instructions to the Designated Expert clear?
    Please include suggestions of designated experts, if appropriate.

NA

[1]: https://www.ietf.org/about/groups/iesg/
[2]: https://www.rfc-editor.org/rfc/rfc4858.html
[3]: https://www.rfc-editor.org/rfc/rfc7942.html
[4]: https://wiki.ietf.org/group/ops/yang-review-tools
[5]: https://www.rfc-editor.org/rfc/rfc8342.html
[6]: https://wiki.ietf.org/group/iesg/ExpertTopics
[7]: https://www.rfc-editor.org/info/bcp79
[8]: https://www.ietf.org/tools/idnits/
[9]: https://www.rfc-editor.org/rfc/rfc3967.html
[10]: https://www.rfc-editor.org/info/bcp97
[11]: https://www.rfc-editor.org/rfc/rfc8126.html
[12]: https://www.rfc-editor.org/rfc/rfc2026.html#section-5
[13]: https://www.rfc-editor.org/rfc/rfc2026.html#section-4.1
[14]: https://www.rfc-editor.org/rfc/rfc2026.html#section-4.2
[15]: https://authors.ietf.org/en/content-guidelines-overview
[16]: https://www.ietf.org/about/groups/iesg/statements/normative-informative-references/
[17]: https://datatracker.ietf.org/doc/downref/