[Openid-specs-fapi] A simpler signing solution

[Dave Tonge]

Dear WG

After looking in more detail at the proposed OBE signing spec, I'm really
quite concerned and think that this WG should work on something else
simpler as soon as possible.

My suggestion is:
 - abandon "detached" jwts
 - build on dPoP by defining one additional claim - `htd` - the http body
digest
 - recommend that any info in headers that needs to be integrity protected
is put in the body

So you would end up with a JWT with a standard header (no need for any
`crit` claims), and a body that would be something like this:

{
     "jti":"-BwC3ESc6acc2lTc",
     "htm":"POST",
     "htu":"https://server.example.com/payment",

     "htd":"SHA-256=+xeh7JAayYPh8K13UnQCBBcniZzsyat+KDiuy8aZYdI=",
     "iat":1562262616
}
The `htd` value would be created according to the instructions in RFC3230

Verification rules would be the same as dPoP, but with the addition of the
verification of the `htd` value.

The advantage of this approach:
 - should be supported by all standard JWT libraries
 - should be much easier to get interoperability as there aren't the same
serialisation problems as draft-cavage or the OBE profile.
 - only one additional claim needs to be registered in the IANA registry

Any feedback?

@anders - I know you will suggest rather using rfc8785, and I think that as
a WG we should definitely keep monitoring support for that, but the reality
is that at the moment there isn't widespread adoption.

Thanks

Dave




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Moneyhub Enterprise is a trading style of Moneyhub Financial Technology 
Limited which is authorised and regulated by the Financial Conduct 
Authority ("FCA"). Moneyhub Financial Technology is entered on the 
Financial Services Register (FRN 809360) at https://register.fca.org.uk/ 
<https://register.fca.org.uk/>. Moneyhub Financial Technology is registered 
in England & Wales, company registration number 06909772. Moneyhub 
Financial Technology Limited 2020 © Moneyhub Enterprise, Regus Building, 
Temple Quay, 1 Friary, Bristol, BS1 6EA. 

DISCLAIMER: This email 
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