[Openid-specs-ab] Key Binding - hash of code

[Filip Skokan]

Hello Dick,

The scheme could still work (i.e. use the same hash that the signature
algorithm of the DPoP Proof uses), but I would just suggest to just do
fixed SHA-256 like DPoP implementations already do with the ath claim.

c_hash and code_hash seems redundant so you'll have some explaining to do
when registering these claims in the IANA JWT Claims Registry.

And the reason not to include the entire code is not to save a couple bytes
here and there, it's to prevent issues with OPs that might use JWTs as
their authorization codes. You'd be duplicating a JWT that's already part
of the request payload inside another JWT that goes into a header
potentially running into server HTTP Header limits.

S pozdravem,
*Filip Skokan*


On Thu, 21 Aug 2025 at 17:43, Dick Hardt <dick.hardt at gmail.com> wrote:

> Hey Filip
>
> You suggested we include a hash of the code rather than the code in the
> DPoP JWT, similar to `at_hash` -> let's call if `code_hash`
>
> In at_hash, the hash algorithm is based on the hash algorithm used in the
> ID Token's JOSE Header. In the code_hash case, the RP does not yet have the
> ID Token, so it does not know which hash algorithm to use. Given this, the
> code path for generating the `at_hash` is not going to be the same for
> `code_hash`. (see snippet from spec below)
>
> While there are some OPs that generate long codes, I think we are adding
> another step for implementers to stumble so that we save some bytes
> sometimes. Was there another motivation for keeping it short? It is a one
> time call in the flow.
>
> /Dick
>
> > at_hash
> OPTIONAL. Access Token hash value. Its value is the base64url encoding of
> the left-most half of the hash of the octets of the ASCII representation of
> the access_token value, where the hash algorithm used is the hash
> algorithm used in the alg Header Parameter of the ID Token's JOSE Header.
> For instance, if the alg is RS256, hash the access_token value with
> SHA-256, then take the left-most 128 bits and base64url-encode them. The
> at_hash value is a case-sensitive string.
>
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