Re: [COSE] Stephen Farrell's Discuss on draft-ietf-cose-msg-20: (with DISCUSS and COMMENT)

[Justin Richer <[e-mail]>]

Hi Stephen,

The consensus of the working group after Yokohama was to make the CDDL non-normative and still use it as a description/example format.

https://www.ietf.org/mail-archive/web/cose/current/msg00802.html <https://www.ietf.org/mail-archive/web/cose/current/msg00802.html>

I believe the current draft reflects that state. Yes, CDDL syntax is peppered throughout the document as illustration, but it’s the English text that’s normative.

One point that was discussed was that if the draft did not use CDDL, then another description language would likely need to be made up to make human-readable examples in the text. The group felt that it was better to use an existing (even if unfinished/unofficial) description format but remove the normative dependency.

 — Justin, as chair.

> On Oct 16, 2016, at 3:41 PM, Stephen Farrell <[e-mail]> wrote:
> 
> 
> Hiya,
> 
> Dear cose-chairs - there is stuff in this discuss for you to deal
> with. (Just putting that there in case you're not delving down
> into the body of the mails:-)
> 
> On 16/10/16 22:50, Jim Schaad wrote:
>> I think we dealt with all of the comments.  I cannot help you with getting chair response.
>> 
>> See below for my one issue.
>> 
>> Jim
>> 
>> 
>>> -----Original Message-----
>>> From: Stephen Farrell [mailto:[e-mail]]
>>> Sent: Sunday, October 16, 2016 1:57 PM
>>> To: The IESG <[e-mail]>
>>> Cc: [e-mail]; Goeran Selander
>>> <[e-mail]>; [e-mail];
>>> [e-mail]; [e-mail]
>>> Subject: Stephen Farrell's Discuss on draft-ietf-cose-msg-20: (with DISCUSS and
>>> COMMENT)
>>> 
>>> Stephen Farrell has entered the following ballot position for
>>> draft-ietf-cose-msg-20: Discuss
>>> 
>>> When responding, please keep the subject line intact and reply to all email
>>> addresses included in the To and CC lines. (Feel free to cut this introductory
>>> paragraph, however.)
>>> 
>>> 
>>> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
>>> for more information about IESG DISCUSS and COMMENT positions.
>>> 
>>> 
>>> The document, along with other ballot positions, can be found here:
>>> https://datatracker.ietf.org/doc/draft-ietf-cose-msg/
>>> 
>>> 
>>> 
>>> ----------------------------------------------------------------------
>>> DISCUSS:
>>> ----------------------------------------------------------------------
>>> 
>>> 
>>> Thanks for the updates in -20. I think we've only the following points left. Note
>>> that 2 of those are questions to the WG chairs and not to Jim.
>>> 
>>>   (1.1) table 2: As-is the value type column seems to me to
>>>   make CDDL normative. I don't see the natural language version
>>>   that you said would be normative.
>>> 
>>> Can you help me see that the changes there are such that CDDL is ok as
>>> informative? I'm not sure but as I read it there is still no natural language
>>> statement that a "counter signature" is one (or more) COSE_Signature values.
>> 
>> In section 1.3 the following text was added:
>> 
>> Two syntaxes from CDDL appear in this document as shorthand. These are:
>> 
>>    FOO / BAR - indicates that either FOO or BAR can appear here
>>    [+ FOO] - indicates that the type FOO appears one or more times in an array
>> 
>> The value from the table is:
>> 
>> COSE_Signature / [+ COSE_Signature ]
>> 
>> Section 4.1 contains the text:
>> 
>> The CBOR object that carries the signature and information about the signature is called the COSE_Signature structure.
>> 
>>> From that I think that all of the needed normative text is present.
> 
> Yeah, I get that. But then I think you've embedded some
> CDDL notation at least as normative. (I mean the constructs
> you describe with FOO/BAR above.)
> 
> While I do start to feel pedantry-panic that I'm saying
> it, if really treating CDDL as informative, I think you
> ought say in text that one or more COSE_Signature fields
> need to be in a counter signature.
> 
> But I'm gonna clear that anyway as the whole CDDL is not
> normative thing is really pretence IMO and there's no point
> in me joining in with that further.
> 
> That means that clearing this discuss is now solely down
> to the chairs saying they're happy that Jim's positions
> do reflect wG consensus.
> 
> Cheers,
> S.
> 
> 
> 
> 
>> 
>>> 
>>>   (2) 3.1, alg: so you're disallowing a setup where the kid
>>>   alone identifies the key and algorithm to the recipient?
>>>   That is used in some IETF protocols (OSPF iirc) so rhat's a
>>>   pity, and will in those (maybe less common) cases consume a
>>>   few bytes that could otherwise be saved.  I think, but am not
>>>   sure, that the WG already discussed this, but if not, maybe
>>>   worth a thought? (Or even a 2nd thought:-) And appendix A.1
>>>   is really puzzling - as it provides instructions for how to
>>>   not follow a MUST in the body of the document.
>>> 
>>> I think we left the mail thread on this with you saying "Best to ask the chairs if
>>> they agree that this is WG consensus," as you're an admitteddly strong partisan
>>> on this topic.
>>> 
>>> So, COSE chairs - what's your take? (If you say this is ok with the WG, I'll clear.)
>>> 
>>>   (6) section 10: why MUST the kty values be present always?
>>>   That seems unnecessary in some contexts and I don't get a
>>>   security reason why it's needed e.g. if there's an alg id
>>>   somewhere - can you explain? I can see folks omitting this
>>>   leading to interop problems for not useful reasons. (Same
>>>   comment applies in other cases where kty is a MUST, e.g.
>>>   12.1.2, 12.2.1.)
>>> 
>>> I think this is the similar to discuss point (2) above.
>>> 
>>> So again, COSE chairs, can you confirm that this design does reflect WG
>>> consensus and isn't just a thorough and good editor getting his way? (If you say
>>> this is ok with the WG, I'll clear.)
>>> 
>>> 
>>> ----------------------------------------------------------------------
>>> COMMENT:
>>> ----------------------------------------------------------------------
>>> 
>>> 
>>> OLD COMMENTS BELOW, I DIDN'T CHECK THEM. Happy to chat more about 'em
>>> if that's useful.
>>> 
>>> - 1.4: the 2nd last paragraph is unclear to me. Probably just needs re-phrasing.
>>> 
>>> - 1.5: I'd add a reference to RFC5116.
>>> 
>>> - 3.1, crit: The statement that security libraries or application code can handle
>>> this is odd - isn't that an API requirement? (I'm not objecting, but it's odd.)
>>> 
>>> - 3.1, "content type" is the space there intended?  If so, maybe add quotes or a
>>> comma or something to disambiguate the name and descriptive text? Same for
>>> other multi-word names here.
>>> 
>>> - 3.1, "all the keys may need to be checked" - really?  Or do you mean all the
>>> keys associated with this kid?
>>> 
>>> - 3.1, IV/Partial IV - I think it's an error to define this here. What if some
>>> algorithm can't use that kind of (0|partial)^IV but needs something else instead?
>>> Shouldn't all mechanism for handling IVs be defined by the algorithm/mode?
>>> (This isn't a discuss because I can't think of a good counter example and there'd
>>> be other ways around the problem too probably.)
>>> 
>>> - 4.1: signingTime is often needed with signatures. Isn't that common enough to
>>> want to define a way to do it, as an option?
>>> 
>>> - 4.1: If I sign with a private key corresponding to a 2047 or 2049 bit RSA public
>>> key modulus, then is it clear what to put where in the signature bstr? (Yes, that'd
>>> be dumb, but I wonder is what to do well enough defined, as I don't think you
>>> can rule it out in all cases.) Since you don't include RSA here I guess it's ok to skip
>>> this, but maybe you need to say that such issues need to be handled in the
>>> definition of signature algs.
>>> 
>>> - 4.3: "cannot bleed" isn't clear enough maybe, give an example perhaps where
>>> the decoder can fail to disambiguate a boundary?
>>> 
>>> 4.4, last para: I disagree that one must (even lowercase
>>> must) check the signing identity.  That's application behaviour and should be
>>> stated here in such concrete terms.
>>> At least s/must also/may also want to/
>>> 
>>> (Note - the above were comments on -18, but also seem to work based on -19.
>>> Subsequent comments are on -19.)
>>> 
>>> - 7.1: "starting at the same base IV" - are you missing "and incrementing" or
>>> something? Otherwise I think this seems unclear.
>>> 
>>> - 8.2.1: is the phrasing of the 1st para right? would it be better to say that the
>>> value of a key for EdDSA MUST NOT be used for ECDH and vice-versa. (Or maybe
>>> points instead of
>>> keys?)
>>> 
>>> - 8.2.1: you need a reference for batch signing. (Or could it be omitted?)
>>> 
>>> - section 9: I think it'd be good to be clearer about the strength of truncated
>>> MAC values. (And I can't recall the right thing to say off the top of my head:-)
>>> 
>>> - 11: RFC2898 is about to be obsoleted by [1]. I suspect it'd be better to refer to
>>> the draft as that should be published soon. (Same for RFC3447 btw.)
>>> 
>>>   [1] https://datatracker.ietf.org/doc/draft-moriarty-pkcs5-v2dot1/
>>> 
>>> - 12.4: Why "OKP"? And saying there's no "simple way" to do point validation
>>> seems fairly opaque, a reference there or explanatory text would be good. (Ah,
>>> it's in section 13, maybe shuffle the text or include a pointer.) Octet key pair
>>> doesn't seem like that good a name to me btw.
>>> 
>>> - 12.5: The 1st para seems wrong. (Or at least is unclear to
>>> me.) "Encrypted with <foo> and <bar>" seems ambiguous anyway, does it mean
>>> double encryption or two parallel ciphertexts?
>>> (I assume the former.) What's the algebraic thing you're trying to explain?  It'd
>>> be good to provide that for such relatively complex operations I think. Is this
>>> what you mean?
>>> 
>>> 		KW(KDF(DH-shared),CEK)
>>> 
>>> - Table 22: The EC2 or OKP value is fixed per curve and the cryptographic
>>> function being performed so seems unnecessary.
>>> Do you really need it so? Why? (I'm not buying that some future form of ECC
>>> might mean this is needed btw - and codepoints aren't expensive here, right? So
>>> other forms of ECC can burn codepoints when that's needed and in the
>>> meantime we'd save bytes and complexity.)
>>> 
>>> - Section 15: Do we have any examples of such a profile?  I think it'd be great if
>>> we did and could add an informative reference here (even if that's to an early I-
>>> D).
>>> 
>>> - section 19: I don't get how ECDSA is normative and the cfrg curves are not.
>>> Same for RFC6979. Maybe these all could do with checking? (No big deal IMO
>>> but maybe worth it.)
>>> 
>>> - Appendices A.1 (as already noted) and A.2 are a puzzle.
>>> Why say in the body of the document to do <foo> and then an appendix that
>>> says how to do <not-foo>?
>>> 
>>> - Appendix C and the implementation status section: Many thanks - great to see
>>> that! (I didn't check 'em though:-)
>>> 
>>> - Thanks also for speedily handling the extensive secdir review.
>>> 
>>>   [2] https://www.ietf.org/mail-archive/web/secdir/current/msg06801.html