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

["Stephen Farrell" <[e-mail]>]

Stephen Farrell has entered the following ballot position for
draft-ietf-cose-msg-20: Discuss

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DISCUSS:
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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.

   (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.)


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COMMENT:
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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