[Openid-specs-fapi] JWT Secured Authorization Response Mode (#155)

Torsten Lodderstedt torsten at lodderstedt.net
Thu Aug 16 12:45:15 UTC 2018


Hi Brian, 

Thanks for your feedback. 

> Am 15.08.2018 um 21:35 schrieb Brian Campbell <bcampbell at pingidentity.com>:
> 
> As I said (or tried to say) on the call and reiterating some of the prior email, using response mode "jwt" here is a completely viable approach but it should probably be defined as a generally applicable response mode in that case. Perhaps saying that all authorization response parameters are put as claims in the JWT as the baseline. And then note exceptions like state/s_hash (and maybe id_token) and define how they specifically are handled.  I think also that some more clarity should be provided on how the response=[jwt] and state (and any other special params) are encoded for the redirection - i.e. as query string parameters, or application/x-www-form-urlencoded in a fragment component, or as form post parameters. The draft currently implies query string but there's definitely some potential ambiguity for response type(s) like the hybrid ones that do different things by default. 
> 
> In FAPI part 2 s_hash is defined as, 
> State 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 state 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 HS512, hash the state value with SHA-512, then take the left-most 256 bits and base64url encode them. The s_hash value is a case sensitive string.
> 
> So I think that this draft needs to clarify that the alg header parameter of the response JWT is used to determine the hash algorithm rather than from the ID Token (and there won't be an ID token many times). I imagine that's what most implementations would do anyway but there's definitely room for different interpretations as it's written now. And that should be tightened up. 

Is it reasonable to assume the hash alg of the JWT and the s_hash could differ?

If so, would it make sense to add another claim to JWT indicating the hash alg used to calculate the s_hash? 

>  
> The s_hash in the example is misleading as "s_hash":"44D41668D199FF3D525FA357A25525D738AADF2A7B1E2C819A39E38500ABAED9", is almost certainly not a base64url encoding of the left-most half of the hash of the octets of the ASCII representation of the state value. It looks like hex. 

You are right. Fixed it (hopefully). 

BTW: why is only the left-most half of the hash used? Doesn’t this reduce the collision resistance? 

> 
> Also should a client metadata parameter be defined to say what alg(s) to use on this response JWT? Something akin to id_token_signed_response_alg and id_token_encrypted_response_alg and id_token_encrypted_response_enc but for this response JWT. 

I would suggest the same logic as in the JWT Introspection Response spec. There is a default and the client can setup algorithms using the pattern you mentioned.

What do you think?

Kind regards,
Torsten. 

> 
> 
> 
> 
> On Wed, Aug 15, 2018 at 12:36 PM Brian Campbell <bcampbell at pingidentity.com> wrote:
> [re-sending to the list as I forgot to reply-all earlier before the call]
> 
> Hi Torsten,
> 
> Yes, more text would needed around other response types because code is the only thing described currently with others being out of scope - it says, 'Note: The response mode "jwt" can be combined with other response types, the respective syntax and behavior is out of scope of this draft.'
> 
> I'm not sure I agree that state needs special treatment. But regardless, as a general response mode, there should be more clear definition of what goes in the JWT and what is a normal authz response parameter and how it's all encoded. 
> 
> Kinder regards,
> Brian
> 
> On Wed, Aug 15, 2018 at 7:29 AM Torsten Lodderstedt <torsten at lodderstedt.net> wrote:
> Hi Brian,
> 
> the mode can easily be combined with the grant type „token" (and the text also sketches how). One would include the response parameter access_token, token_type, expires_in, and scope in the JWT. I can also add more text on that. 
> 
> I think it makes sense to treat the state value special because it binds the response object to the initial transaction and can be evaluated _before_ the JWT is being processed. This is a security precaution. 
> 
> Kind regards,
> Torsten. 
> 
> > Am 15.08.2018 um 14:41 schrieb Brian Campbell via Openid-specs-fapi <openid-specs-fapi at lists.openid.net>:
> > 
> > As a Response Mode, I had envisioned that all the authorization response parameters would be passed as claims of the JWT. And would be applicable to any response types. Something like that would more closely mirror OAuth JAR. And be a more generally applicable response mode. 
> > 
> > What is in this draft is more of a specialized treatment of the code response parameter (also state). If that's the extent of the functionality, it's probably more appropriate to be defined as a new response type (I know I suggested response mode but that was with the thinking that it'd be a generalized mode for encoding all the response params). Or, if response mode is used to signal this functionality, the mode value (and spec title) should probably be more true to what it is actually doing. Like response_mode=code_in_jwt_with_state_as_s_hash_and_other_stuff_undefined or just response_mode=jwt_code.
> > 
> > 
> > 
> > On Thu, Aug 9, 2018 at 10:03 AM Torsten Lodderstedt via Openid-specs-fapi <openid-specs-fapi at lists.openid.net> wrote:
> > Hi all, 
> > 
> > please find attached the first version of the draft on the new signed response mode (https://bitbucket.org/openid/fapi/issues/155/support-authorization-and-identity). As this draft mirrors OAuth JAR (as already pointed out by Nat), I choose the name accordingly. 
> > 
> > Looking forward for your feedback.
> > 
> > kind regards,
> > Torsten.
> > 
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