Re: [OAUTH-WG] Digest for DPoP

My inclination is to keep digest[1] out of the base DPoP document. I do
believe that including it would add unneeded complexity to regular old DPoP
(there are some subtleties around digest that make it more complicated than
one might expect) and, from a design philosophy perspective, DPoP has
always aspired to be only a proof-of-possession mechanism in and of itself
and not venture into the realm of HTTP message integrity or general
signatures.

[1] Note that the digest header is defined in RFC3230 but there's work
underway to obsolete that RFC with draft-ietf-httpbis-digest-headers-04

On Wed, Feb 17, 2021 at 2:54 PM Justin Richer <jricher@mit.edu> wrote:

> Two different specifications (GNAP and FAPI signatures) have recently
> profiled DPoP to use its signature method to protect a different kind of
> protocol entirely. One thing these methods have in common is that they both
> define an additional field for holding a digest of the HTTP Message Body:
>
>
> https://bitbucket.org/openid/fapi/src/master/Financial_API_Simple_HTTP_Message_Integrity_Protocol.md#markdown-header-521-htd-the-digest-of-the-http-request-or-response-body
>
>
> https://www.ietf.org/archive/id/draft-ietf-gnap-core-protocol-03.html#name-demonstration-of-proof-of-p
>
> Both of these have the same semantics, and we’re changing the name in GNAP
> to align with the FAPI one. This begs the question: do we want to just
> define this field as an optional component in DPoP instead of having these
> profiles do it separately? It would save them from needing to align with
> each other, and anyone else from inventing it again.
>
> Is it worth defining this in DPoP directly, or does that complicate the
> spec too much? I’ve previously raised a similar question on including a
> hash of the access token in the DPoP request to the RS.
>
>  — Justin
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