Re: [jose] Should we delete the "typ" header field

[Mike Jones <[e-mail]>]

Answering Jim's message that inadvertently went unanswered...

Neither of these would be correct because of the last "cty":"JWT" in both examples.  That's because a JWT Claims Set is not a JWT.  You would need to omit the last "cty":"JWT" in both examples for this to be correct.

No, there is no overloading.  JWT uses the absence of a "cty" value to say that the payload is a JWT Claims Set - which is the default case.

                                                                -- Mike

From: Jim Schaad [mailto:[e-mail]]
Sent: Thursday, May 30, 2013 8:48 AM
To: Mike Jones; 'Richard Barnes'
Cc: [e-mail]; 'Dick Hardt'
Subject: RE: [jose] Should we delete the "typ" header field

Ok - I am having some problems with this.  Looking at some concrete psudo-examples

{typ:JWT, ctyp:JWT,...{type:JWT, ctyp:JWT,...{jwt claims object } }


1.       would be a legal or a non-legal description of a signed and then encrypted set of JWT claims?

2.       Does it make a difference if there are or are not other header fields?

{typ:JWE, ctyp:JWT,...{type:JWS, ctyp:JWT,... {jwt claims object }}


1.        Would be a legal or a non-legal description of a signed and then encrypted set of JWT claims?

If typ and ctyp are using the same space, does that mean that a JWT set of claims and a JWT token are being overloaded on the what the string means?  In one case it means a token and in the other case it means a set of claims.




From: Mike Jones [mailto:[e-mail]]
Sent: Thursday, May 30, 2013 8:06 AM
To: Richard Barnes
Cc: Jim Schaad; [e-mail]<mailto:[e-mail]>; Dick Hardt
Subject: RE: [jose] Should we delete the "typ" header field

No, "cty" is used by the derived class to determine the type of the encapsulated field.  But that's not a complete description of the *entire object* - especially not the additional meaning imbued by the additional parameters the derived type may add to the JOSE header.  "typ" is there to provide the type of the entire object, including what you're calling the wrapper parts.

                                                            -- Mike

From: Richard Barnes [mailto:[e-mail]]
Sent: Thursday, May 30, 2013 7:58 AM
To: Mike Jones
Cc: Jim Schaad; [e-mail]<mailto:[e-mail]>; Dick Hardt
Subject: Re: [jose] Should we delete the "typ" header field

Isn't that requirement met by "cty"?  The only thing JOSE adds is a crypto wrapper around the real application content.  If you're an application, you know a JOSE object is the thing you want because it contains the content you want -- it's a JWT because it contains JWT claims.

Inheritance is the wrong metaphor.  This is encapsulation of application data:
if (jws.valid && jws.cty == "application/jwt_claims") {
    jwtClaims = jws.content;
}

--Richard

On Thu, May 30, 2013 at 10:43 AM, Mike Jones <[e-mail]<mailto:[e-mail]>> wrote:
Thanks for sharing the S/MIME details.  Although I was actually making the analogy to MIME - not S/MIME.  Like many analogies, it's imperfect, but I believe still illustrative.

The reason that the analogy isn't perfect is that the JOSE data structures are used to build application-specific data structures that are legal JOSE data structures but also have additional properties - including additional header fields with specific semantics.  (When we agreed to ignore not-understood header fields we let that horse out of the barn.)  For instance, Dick Hardt uses JWEs with issuer and audience fields in the headers, so they can be used by routing software.

Think of JOSE as the base class and the application types built using it as derived classes.  JWT is a derived class.  Dick's structures are a derived class.  These derived classes sometimes need names.  That's what "typ" is for.

                                                            -- Mike

From: Richard Barnes [mailto:[e-mail]<mailto:[e-mail]>]
Sent: Thursday, May 30, 2013 7:34 AM

To: Mike Jones
Cc: Jim Schaad; [e-mail]<mailto:[e-mail]>; Dick Hardt
Subject: Re: [jose] Should we delete the "typ" header field

You're mixing up "typ" and "cty".  If you want to make the analogy to S/MIME, "cty" is the equivalent to Content-Type inside the protected MIME body; "typ" is the content-type on the outer MIME header.  Pasting in an example:

-----BEGIN-----
Content-Type: application/pkcs7-mime; smime-type=signed-data;
     name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m

567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
6YT64V0GhIGfHfQbnj75
-----END-----
<http://tools.ietf.org/html/rfc3851#section-3.4.2>

The outer Content-Type, which is analogous to "typ", MUST be application/pkcs7-mime, with a parameter indicating the type of CMS object.  This is the same as requiring "typ" to be JWE or JWS.  The inner Content-Type (ASN.1/base64 encoded in the example) can be anything, just like "cty".

--Richard




On Thu, May 30, 2013 at 12:53 AM, Mike Jones <[e-mail]<mailto:[e-mail]>> wrote:
Requiring that the "typ" value be only "JWS" or "JWE" would be analogous to the MIME spec requiring that the Content-Type: field be only "text/plain" or "message/external-body".  It would render it useless.

                                                            -- Mike

From: [e-mail]<mailto:[e-mail]> [mailto:[e-mail]<mailto:[e-mail]>] On Behalf Of Richard Barnes
Sent: Wednesday, May 29, 2013 8:03 PM
To: Mike Jones
Cc: Jim Schaad; [e-mail]<mailto:[e-mail]>; Dick Hardt

Subject: Re: [jose] Should we delete the "typ" header field

If this is the level of "type" you're referring to, I think we should drop it from the spec.  It's an application-layer thing that the app can add or not according to its wishes.

I'm with Dick on this.  I think we should either have a mandatory indicator of what type of JOSE object this, or nothing at all.   If the former, the allowable values are "JWE" and "JWS".  The "+JSON" options are non-sensical -- the app needs to know what it's parsing before it gets this header.  While I have a preference for the former (for clarity), the latter approach is also OK with me, since the MIME types are specific to JWE/JWS.

Another approach here would be to address the JSON and compact forms separately.  The JSON form has no need of "typ" at all, since the type of the object is completely clear from what fields are there, e.g., "recipients" vs. "signatures".  For the compact form, we could do something like James's "E."/"S." prefix idea, which you need because the dot-separated components have different meanings and no field names to indicate this.

--Richard

On Wed, May 29, 2013 at 8:30 PM, Mike Jones <[e-mail]<mailto:[e-mail]>> wrote:
A standard library is unlikely to know the meanings of all possible "typ" values - and more to the point, it doesn't have to.  It's the application's job to determine that "this blob is a JOSE object" and then pass it to a standard library, which will then ignore the "typ" value.

A standard JOSE library won't know what "typ": "JWT" means.  It won't know what "typ": "BCGovToken" is, should the BC Government want to declare that it's using a token with particular characteristics.  It won't know what "typ": "XMPP" is, should XMPP want to declare that it's using a JOSE data structure with particular characteristics.  Etc.

All these values can be registered in the registry and used by applications that understand them.  That's the application's job - not the library's job.  The "typ" field is just there so that applications have a standard place to make any such declarations that they may need.

                                                                -- Mike

From: Dick Hardt [mailto:[e-mail]<mailto:[e-mail]>]
Sent: Wednesday, May 29, 2013 5:18 PM
To: Mike Jones
Cc: Jim Schaad; [e-mail]<mailto:[e-mail]>

Subject: Re: [jose] Should we delete the "typ" header field

I'd prefer to be able to use standard libraries for creating and parsing tokens, and not specialized libraries dependent on the use case.

I strongly think we either drop "typ" or make it required.

On Wed, May 29, 2013 at 5:03 PM, Mike Jones <[e-mail]<mailto:[e-mail]>> wrote:
It's fine for your application to specify that it's required for your use case.  Not applications need it, so they shouldn't be forced to pay the space penalty of an unnecessary field.

                                                                -- Mike

From: [e-mail]<mailto:[e-mail]> [mailto:[e-mail]<mailto:[e-mail]>] On Behalf Of Dick Hardt
Sent: Wednesday, May 29, 2013 4:56 PM

To: Jim Schaad
Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [jose] Should we delete the "typ" header field

I use it all the time and my code would barf if it was not there.

I think it should be required rather than be a hint if it is going ot be there.

On Wed, May 29, 2013 at 4:40 PM, Jim Schaad <[e-mail]<mailto:[e-mail]>> wrote:
I think the values just changed

However the way you are using it would be an argument to say that it should be a required field.  Are you just using it as a hint if it exists and then looking at the rest of the fields if it is not present?

Jim


From: Dick Hardt [mailto:[e-mail]<mailto:[e-mail]>]
Sent: Wednesday, May 29, 2013 3:49 PM
To: Jim Schaad
Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [jose] Should we delete the "typ" header field

Well, I have been using, but now realize the spec changed or I was confused.

I had been setting "typ" to be either "JWE" or "JWS" depending on the type of token I was creating or parsing as it was easier than looking at "alg"

As currently defined, I don't see value in "typ".

-- Dick


On Wed, May 29, 2013 at 3:02 PM, Jim Schaad <[e-mail]<mailto:[e-mail]>> wrote:
In reading the documents, I am trying to understand the justification for having the "typ" header parameter in the JOSE documents.

The purpose of the field is to hold the type of the object.  In the past, I believe that values which should now be placed in the cty field (such as "JWT") were placed in this field as well.  However the parameter is optional and an implementation cannot rely on its being present.  This means that for all practical purposes all of the code to determine the value of the type field from the values of the alg and enc fields.  If the field was mandatory then this code would disappear at a fairly small space cost and I can understand why the parameter would be present.

Can anybody justify why this field should be present in the document - or should it just disappear?

Jim


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--
-- Dick



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-- Dick

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