Subject: [OAUTH-WG] DPoP - access token hash format

From: Dmitry Telegin <[email-masked]>
Date: Wed, 27 Oct 2021 16:24:59 +0300

As of -03, the "ath" DPoP proof claim has been introduced:

ath: hash of the access token (REQUIRED). The value MUST be the result of a
> base64url encoding (with no padding) the SHA-256 hash of the ASCII encoding
> of the associated access token's value.
>

OpenID Connect has a similar concept used to bind ID token to access token:

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.
>
>
OIDC derives the hashing algorithm from the token header, while DPoP uses
SHA-256 unconditionally. OIDC uses the left-most half of the hash, while
DPoP uses the whole hash. Would it make sense to be aligned with OIDC on
this?

Regards,
Dmitry
Backbase

---

From: Brian Campbell <[email-masked]>
Date: Wed, 27 Oct 2021 09:34:32 -0600

There's discussions around this in the mail and meeting archives, if you
want to dig into it. But generally the "at_hash" approach has proven to be
complicated while not really achieving the algorithm agility it aims for.
We opted for something more straightforward with "ath" in DPoP.

On Wed, Oct 27, 2021 at 7:25 AM Dmitry Telegin <dmitryt=
[email-masked]> wrote:

> ath: hash of the access token (REQUIRED). The value MUST be the result of
>> a base64url encoding (with no padding) the SHA-256 hash of the ASCII
>> encoding of the associated access token's value.
>>
>
> _______________________________________________
> OAuth mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/oauth
>

-- 
_CONFIDENTIALITY NOTICE: This email may contain confidential and privileged 
material for the sole use of the intended recipient(s). Any review, use, 
distribution or disclosure by others is strictly prohibited.  If you have 
received this communication in error, please notify the sender immediately 
by e-mail and delete the message and any file attachments from your 
computer. Thank you._