Re: [jose] JWK Thumbprint in JWS/JWE Header

[Brian Campbell <[e-mail]>]

There was more or less such a thing in an earlier draft (
https://tools.ietf.org/html/draft-ietf-jose-jwk-thumbprint-01#section-4) of
what would become RFC 7638: There was some disagreement about it that I
don't quite remember the details of and it was pulled out. I think what
Justin said did come up as justification for not needing something more
explicit. If a thumbprint is used as a kid, then the parties involved need
to know that and also know the hash alg. I realize that doesn't really
answer the question but is a little background/context.



On Thu, Jul 21, 2016 at 7:00 PM, Nathaniel McCallum <[e-mail]>
wrote:

> Specifically, I'm thinking of the problem of validating a thumbprint.
>
> The RFC does not define a hash function. Nor does the output format
> contain a hash function name.
>
> So if I hand JWK to an entity, how does that entity validate that the
> thumbprint in the kid is actually a valid thumbprint and wasn't
> modified? I supose the entity could try all its supported hash
> functions; but that seems a little heavy handed.
>
> The existing kid could be used with contents like: <hash>.<thumbprint>
>
> Alternatively, RFC 7517 uses the x5t and x5t#S256 variants. This is
> precisely why I wondered about a thumbprint specific attribute.
> Something like thp and thp#S256 would make this explicit.
>
> Thoughts?
>
> On Tue, 2016-07-19 at 12:09 -0400, Nathaniel McCallum wrote:
> > Has there been any talk about using a prefix to specify the hash
> > algo?
> >
> > On Tue, 2016-07-19 at 11:24 -0400, Justin Richer wrote:
> > >
> > > This was discussed on the list a while ago, and the thought was
> > > that
> > > you could easily use the JWK thumbprint *as* the “kid” value
> > > instead
> > > of defining a new field for this use case. The header values are
> > > protected by the signature in the normal (compact) JWS/JWE formats,
> > > and ought to be protected in the JSON representations too for
> > > exactly
> > > the reasons you’re talking about.
> > >
> > >  — Justin
> > >
> > > >
> > > >
> > > > On Jul 19, 2016, at 10:48 AM, Nathaniel McCallum <npmccallum@redh
> > > > at
> > > > .com> wrote:
> > > >
> > > > The JWS and JWE specs defined the "kid" header value that can be
> > > > used
> > > > to identify the key used for signing or encryption. Subsequently,
> > > > the
> > > > JWK thumbprint method was defined.
> > > >
> > > > Has anyone put any thought into registering a header value for
> > > > JWS
> > > > and
> > > > JWE headers that indicates the thumbprint of the key used for
> > > > signing
> > > > or encryption? This would be very helpful for key indexes
> > > > especially
> > > > when using unprotected headers since the value of "kid" might be
> > > > modified.
> > > >
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