Subject: [COSE] Feedback on draft-ietf-cose-webauthn-algorithms-00

From: Matt Palmer <[email-masked]>
Date: Fri, 24 May 2019 18:09:47 +1000

I'm looking to implement support for sec256k1 in a JWS library.  The
specifications for the NIST curves in RFC7518 are all of the form "ECDSA
using <curve> and <hash alg>", which makes it very obvious what's going on. 
However, the definitions in this draft don't make mention of the hash
algorithm to be used.

My vague understanding is that the hash algorithm used for ECDSA should be
equal in size with the curve used, so my assumption is that these signatures
are using SHA-256, however I think it would be extremely useful if that were
made explicit in the specification, to prevent any possible confusion.

Thanks,
- Matt

---

From: Jim Schaad <[email-masked]>
Date: Fri, 24 May 2019 12:16:50 -0700

This is a difference between how COSE and JOSE operate.  

For JOSE, the hash and curve are specified and fixed.
For COSE, the hash is fixed and the following text exists:

The ECDSA signature algorithm is parameterized with a hash function
   (h).  In the event that the length of the hash function output is
   greater than the group of the key, the leftmost bytes of the hash
   output are used.

I would agree that the document should be updated to reflect the difference
in behavior between the two standards.

Jim

> -----Original Message-----
> From: COSE <[email-masked]> On Behalf Of Matt Palmer
> Sent: Friday, May 24, 2019 1:10 AM
> To: [email-masked]
> Subject: [COSE] Feedback on draft-ietf-cose-webauthn-algorithms-00
> 
using
> <curve> and <hash alg>", which makes it very obvious what's going on.
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