CBOR Object Signing and Encryption (COSE): AES-CTR and AES-CBC
RFC 9459

Received changes through RFC Editor sync (created alias RFC 9459, changed abstract to 'The Concise Binary Object Representation (CBOR) data format is designed for small code size and small message size.  CBOR Object Signing and Encryption (COSE) is specified in RFC 9052 to provide basic security services using the CBOR data format.  This document specifies the conventions for using AES-CTR and AES-CBC as content encryption algorithms with COSE.', changed pages to 10, changed standardization level to Proposed Standard, changed state to RFC, added RFC published event at 2023-09-14, changed IESG state to RFC Published)

[Ballot comment]
# GEN AD review of draft-ietf-cose-aes-ctr-and-cbc-05

CC @larseggert

Thanks to Vijay Gurbani for the General Area Review Team (Gen-ART) review
(https://mailarchive.ietf.org/arch/msg/gen-art/QYDfbhRi0CKZyV4Wft6YqoOoAPg).

## Nits

All comments below are about very minor potential issues that you may choose to
address in some way - or ignore - as you see fit. Some were flagged by
automated tools (via https://github.com/larseggert/ietf-reviewtool), so there
will likely be some false positives. There is no need to let me know what you
did with these suggestions.

### Grammar/style

#### Section 7, paragraph 2
```
entations should change the key before before reaching this limit. To avoid c
                                ^^^^^^^^^^^^^
```
Possible typo: you repeated a word.

#### Section 8, paragraph 3
```
block values are ever used for more that one plaintext with the same key, th
                                    ^^^^
```
Did you mean "than"?

#### Section 8, paragraph 19
```
to prevent undetected stripping of the the authentication and integrity mecha
                                  ^^^^^^^
```
Possible typo: you repeated a word.

## Notes

This review is in the ["IETF Comments" Markdown format][ICMF], You can use the
[`ietf-comments` tool][ICT] to automatically convert this review into
individual GitHub issues. Review generated by the [`ietf-reviewtool`][IRT].

[ICMF]: https://github.com/mnot/ietf-comments/blob/main/format.md
[ICT]: https://github.com/mnot/ietf-comments
[IRT]: https://github.com/larseggert/ietf-reviewtool

[Ballot comment]
Thanks to Michael B. Jones for the shepherd write-up, without which I would have been a little lost as to why we need this document. The context was helpful.

I have just two small comments about the spec.

First, in

  If an attacker is able to strip the authentication and integrity
  mechanism, then the attacker can replace it with their one of their
  own creation

s/their one/one/

Second, there are two SHOULDs in Section 8. I am curious why they aren’t MUSTs. If SHOULD is the more appropriate choice, would it be possible to provide some commentary as to when it might be fine for an implementor to deviate?

[Ballot comment]
Thanks to Michael B. Jones for the shepherd write-up, without which I would have been a little lost as to why we need this document. The context was helpful.

I have just two small comments about the spec.

First, in

  If an attacker is able to strip the authentication and integrity
  mechanism, then the attacker can replace it with their one of their
  own creation

s/their//

Second, there are two SHOULDs in Section 8. I am curious why they aren’t MUSTs. If SHOULD is really the more appropriate choice, would it be possible to provide some commentary as to when it might be fine for an implementor to deviate?

[Ballot comment]
Thank you to Daniel Migault for the SECDIR review.

** Duplicate normative guidance.  Please say it once.
(a) Instance #1

-- Section 4
For this reason, it is inappropriate to use AES-CTR with static keys. Extraordinary measures would be needed to prevent reuse of an IV value with the static key across power cycles. To be safe, implementations MUST use fresh keys with AES-CTR.

-- Section 8
Therefore, it is inappropriate to use AES-CTR with static keys. Extraordinary measures would be needed to prevent reuse of a counter block value with the static key across power cycles. To be safe, implementations MUST use fresh keys with AES-CTR.

(b) Instance #2

-- Section 4
Implementations MUST use AES-CTR in conjunction with an authentication and integrity mechanism, such as a digital signature.

-- Section 8
Accordingly, implementations MUST use AES-CTR in conjunction with an authentication and integrity mechanism, such as a digital signature

(c) Instance #3

-- Section 4
Implementations MUST use AES-CBC in conjunction with an authentication and integrity mechanism, such as a digital signature.

-- Section 8
Accordingly, implementations MUST use of AES-CBC in conjunction with an authentication and integrity mechanism, such as a digital signature.

** Section 6.  Consistent guidance to implementers:

-- Section 5
Since AES-CBC cannot provide integrity protection for external additional authenticated data, the decryptor MUST ensure that no external additional authenticated data was supplied.
(aside, similar text in Section 4 for AES-CTR)

-- Section 6
COSE libraries that support either AES-CTR or AES-CBC and accept Additional Authenticated Data (AAD) as input should return an error if one of these non-AEAD content encryption algorithm is selected.

Wouldn’t it be more appropriate to say “must return an error”?  Silent failure seems like a bad idea and inconsistent with the protocol guidance.

** Section 8
Since AES has a 128-bit block size, regardless of the mode employed, the ciphertext generated by AES encryption becomes distinguishable from random values after 2^64 blocks are encrypted with a single key. Implementations should change the key before before reaching this limit.

-- Typo. s/before before/before/

-- Why would it be safe to continue to use the key past 2^64 blocks?

** Section 8.  Editorial.
Since AES-GCM uses AES-CTR for encryption, an attacker can switch the algorithm identifier from AES-GCM to AES-CTR, and then strip the authentication tag to bypass the authentication and integrity, allowing the attacker to manipulate the ciphertext.

This is the first reference to AES-GCM.  All other modes got a definition.  Please explain or cite.

[Ballot comment]
Thanks for working on this specification.

I don't have any transport related comments or issues. However, I was expecting a "MUST NOT" in section 5 regarding same IV usage more than once as it says in section 4.

[Ballot comment]
# Internet AD comments for draft-ietf-cose-aes-ctr-and-cbc-04
CC @ekline

* comment syntax:
  - https://github.com/mnot/ietf-comments/blob/main/format.md

* "Handling Ballot Positions":
  - https://ietf.org/about/groups/iesg/statements/handling-ballot-positions/

## Nits

### S8

* "Accordingly, implementations MUST use of"

  -> "Accordingly, implementations MUST use", or
  -> "Accordingly, implementations MUST ensure use of", or something

* "replace it with their one of their"
  -> "replace it with one of their"

[Ballot comment]
Thank you for yet another well written document - I found it well written, comprehensive and clear.

My only note is that a fair bit of the Security Considerations feels like it might work better in an Appendix, but this is truly just an editorial opinion. I also think that much of the Security Considerations (basically everything after the first paragraph) would make a good standalone document (not necessarily an RFC, but published somewhere)

[Ballot comment]
Hi,

Thanks for this document.  I had a few minor comments for you to consider please:

(1) p 2, sec 4.  AES Counter Mode

  When AES-CTR is used as a COSE Content Encryption algorithm, the
  encryptor generates a unique value that is communicated to the
  decryptor.  This value is called an initialization vector (IV) in
  this document.  The same IV and AES key combination MUST NOT be used
  more than once.  The encryptor can generate the IV in any manner that
  ensures the same IV value is not used more than once with the same
  AES key.

Is there any RFC that could be referenced here on how to generate suitable IVs?


(2) p 4, sec 5.  AES Cipher Block Chaining Mode

  AES-CBC mode requires an 16 octet Initialization Vector (IV).  Use of
  a randomly or pseudo-randomly generated IV ensures that the
  encryption of the same plaintext will yield different ciphertext.

The IV initialization text differs compared to section 4, I assume that this is intentional and these modes have fundamentally different IV initialization requirements?                                                                                   
                                                                                                                                                                                                                                                           
                                                                                                                                                                                                                                                           
(3) p 6, sec 5.2.  AES-CBC COSE Algoritm Identifiers                                                                                                                                                                                                       
                                                                                                                                                                                                                                                           
  The following table defines the COSE AES-CBC algorithm values.  Note                                                                                                                                                                                     
  that these algorithms are being registered as "Deprecated" to avoid                                                                                                                                                                                     
  accidental use without a companion integrity protection mechanism.                                                                                                                                                                                       
  +=========+=======+==========+========================+=============+                                                                                                                                                                                   
  | Name    | Value | Key Size |      Description      | Recommended |                                                                                                                                                                                   
  +=========+=======+==========+========================+=============+                                                                                                                                                                                   
  | A128CBC |  TBD4 |  128    |      AES-CBC w/      |  Deprecated |                                                                                                                                                                                   
  |        |      |          |      128-bit key      |            |                                                                                                                                                                                   
  +---------+-------+----------+------------------------+-------------+                                                                                                                                                                                   
  | A192CBC |  TBD5 |  192    |      AES-CBC w/      |  Deprecated |                                                                                                                                                                                   
  |        |      |          |      192-bit key      |            |                                                                                                                                                                                   
  +---------+-------+----------+------------------------+-------------+                                                                                                                                                                                   
  | A256CBC |  TBD6 |  256    |      AES-CBC w/      |  Deprecated |                                                                                                                                                                                   
  |        |      |          |      256-bit key      |            |                                                                                                                                                                                   
  +---------+-------+----------+------------------------+-------------+                                                                                                                                                                                   

I wanted to check that "Deprecated" is really the best choice for "Recommended" for both AES-CTR and AES-CBC.  I read 'deprecated' as meaning that other COSE algorithms should be used in preference to these, but it wasn't clear that is the intent here.  I note that this column contains some entries with a value such as "Filter Only", hence wondering it these should be labelled as "Confidentiality only", perhaps with the description indicating that integrity must be handled separately?

Thanks,
Rob

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

The IANA Functions Operator has completed its review of draft-ietf-cose-aes-ctr-and-cbc-03. If any part of this review is inaccurate, please let us know.

The IANA Functions Operator understands that, upon approval of this document, there is a single action which we must complete.

In the COSE Algorithms registry on the CBOR Object Signing and Encryption (COSE) registry page located at:

https://www.iana.org/assignments/cose/

six new registrations are to be made from the -65534 to -261 range as follows:

Name: A128CTR
Value: [ TBD-at-Registration ]
Description: AES-CTR w/ 128-bit key
Capabilities: [kty]
Change Controller: IESG
Reference: [ RFC-to-be ]
Recommended: Deprecated

Name: A192CTR
Value: [ TBD-at-Registration ]
Description: AES-CTR w/ 192-bit key
Capabilities: [kty]
Change Controller: IESG
Reference: [ RFC-to-be ]
Recommended: Deprecated

Name: A256CTR
Value: [ TBD-at-Registration ]
Description: AES-CTR w/ 256-bit key
Capabilities: [kty]
Change Controller: IESG
Reference: [ RFC-to-be ]
Recommended: Deprecated

Name: A128CBC
Value: [ TBD-at-Registration ]
Description: AES-CBC w/ 128-bit key
Capabilities: [kty]
Change Controller: IESG
Reference: [ RFC-to-be ]
Recommended: Deprecated

Name: A192CBC
Value: [ TBD-at-Registration ]
Description: AES-CBC w/ 192-bit key
Capabilities: [kty]
Change Controller: IESG
Reference: [ RFC-to-be ]
Recommended: Deprecated

Name: A256CBC
Value: [ TBD-at-Registration ]
Description: AES-CBC w/ 256-bit key
Capabilities: [kty]
Change Controller: IESG
Reference: [ RFC-to-be ]
Recommended: Deprecated

Would it be acceptable to list the IETF as the change controller for the COSE Algorithms registrations instead of the IESG? There has been a preference for doing so, as described in the expired document at https://datatracker.ietf.org/doc/html/draft-leiba-ietf-iana-registrations-00, but it hasn’t been recorded in a permanent document yet.

As this document requests registrations in an Expert Review or Specification Required (see RFC 8126) registry, we will initiate the required Expert Review via a separate request. This review must be completed before the document's IANA state can be changed to "IANA OK."

The IANA Functions Operator understands that this is the only action required to be completed upon approval of this document.

Note:  The actions requested in this document will not be completed until the document has been approved for publication as an RFC. This message is meant only to confirm the list of actions that will be performed.

For definitions of IANA review states, please see:

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

Thank you,

David Dong
IANA Services Specialist

The following Last Call announcement was sent out (ends 2023-05-05):

From: The IESG
To: IETF-Announce
CC: cose-chairs@ietf.org, cose@ietf.org, draft-ietf-cose-aes-ctr-and-cbc@ietf.org, michael.jones@microsoft.com, paul.wouters@aiven.io
Reply-To: last-call@ietf.org
Sender:
Subject: Last Call:  (CBOR Object Signing and Encryption (COSE): AES-CTR and AES-CBC) to Proposed Standard


The IESG has received a request from the CBOR Object Signing and Encryption
WG (cose) to consider the following document: - 'CBOR Object Signing and
Encryption (COSE): AES-CTR and AES-CBC'
  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
last-call@ietf.org mailing lists by 2023-05-05. 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


  The Concise Binary Object Representation (CBOR) data format is
  designed for small code size and small message size.  CBOR Object
  Signing and Encryption (COSE) is specified in RFC 9052 to provide
  basic security services using the CBOR data format.  This document
  specifies the conventions for using AES-CTR and AES-CBC as Content
  Encryption algorithms with COSE.




The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-cose-aes-ctr-and-cbc/



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

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?

It reached broad agreement.

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

The reasons for the use of unauthenticated encryption algorithms took explanation, but once explained, the working group was comfortable with it.

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 recommends) or elsewhere
(where)?

This is not a protocol document.  These algorithms are being used to encrypt firmware.

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.

No.  That said, a review by the authors of draft-ietf-suit-manifest might be useful.

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.

IANA reviews have not yet taken place.

7. If the document contains a YANG module, has the final version of the module
been checked with any of the recommended validation tools 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?

The document contains no YANG module.

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.

The document does not utilize formal language.

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?

Yes

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

I'm not aware of any such issues in this document.

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

Proposed Standard is being requested.  The specification normative describes how to use cryptographic algorithms, therefore this is an appropriate document type.

12. Have reasonable efforts been made to remind all authors of the intellectual
property rights (IPR) disclosure obligations described in BCP 79? 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.

The authors are aware of no intellectual property pertaining to this specification.

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 is not enough; please review the "Content Guidelines" on
authors.ietf.org. (Also note that the current idnits tool generates
some incorrect warnings; a rewrite is underway.)

The only possible nits identified are references to NIST specifications.  These are not downrefs in this context.

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

The references are appropriately categorized.

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

None

17. Are there any normative downward references (see RFC 3967 and BCP
97) that are not already listed in the DOWNREF registry? 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.

No

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).

The IANA actions are clearly described and appropriate.

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.

None

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?

It reached broad agreement.

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

The reasons for the use of unauthenticated encryption algorithms took explanation, but once explained, the working group was comfortable with it.

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 recommends) or elsewhere
(where)?

This is not a protocol document.  These algorithms are being used to encrypt firmware.

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.

No.  That said, a review by the authors of draft-ietf-suit-manifest might be useful.

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.

IANA reviews have not yet taken place.

7. If the document contains a YANG module, has the final version of the module
been checked with any of the recommended validation tools 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?

The document contains no YANG module.

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.

The document does not utilize formal language.

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?

Yes

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

I'm not aware of any such issues in this document.

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

Proposed Standard is being requested.  The specification normative describes how to use cryptographic algorithms, therefore this is an appropriate document type.

12. Have reasonable efforts been made to remind all authors of the intellectual
property rights (IPR) disclosure obligations described in BCP 79? 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.

The authors are aware of no intellectual property pertaining to this specification.

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 is not enough; please review the "Content Guidelines" on
authors.ietf.org. (Also note that the current idnits tool generates
some incorrect warnings; a rewrite is underway.)

The only possible nits identified are references to NIST specifications.  These are not downrefs in this context.

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

The references are appropriately categorized.

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

None

17. Are there any normative downward references (see RFC 3967 and BCP
97) that are not already listed in the DOWNREF registry? 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.

No

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).

The IANA actions are clearly described and appropriate.

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.

None