[Openid-specs-ab] Question about at_hash with EdDSA

Brian Campbell bcampbell at pingidentity.com
Tue Jun 2 21:24:44 UTC 2020

Not sure it's well-trodden exactly but
has some treatment of the subject.

On Tue, Jun 2, 2020 at 3:09 PM Justin Richer via Openid-specs-ab <
openid-specs-ab at lists.openid.net> wrote:

> I haven’t been able to find a clean answer for this, but apologies if this
> is well-trodden already.
> The ODIC spec defines at_hash (and c_hash and others) as using "the hash
> algorithm used is the hash algorithm used in the alg Header Parameter of
> the ID Token's JOSE Header”. This is clear enough for things like RS256 and
> the like.
> However, the definition of the “EdDSA” JOSE algorithm in RFC8037 (
> https://tools.ietf.org/html/rfc8037) does not define a hash algorithm in
> the same way. Edwards signatures as defined in RFC8032 (
> https://tools.ietf.org/html/rfc8032) seem to internally use SHA-512, but
> I’m not positive that’s every time or just in the case where you do the
> pre-hashing calculation. Regardless, the JOSE spec is silent on the matter.
> So the question is: which hash algorithm do we use for an “EdDSA” signed
> token when calculating at_hash and its ilk?
>  — Justin
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