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

[John Bradley <[e-mail]>]

Once upon a time there were multiple security token formats all being dot separated base64url encoded segments with the first one containing a JSON object that told the recipient what it was.

Telling the difference between a JWT, JSS, Signed Request or other token type is important in the overall processing.  If you don't know that the sender intended it to be a JWT then you could possibly do things with optional user controlled content to generate a token that might be mis interpreted at the other end.  

JWT may also place additional constraints on JOSE headers beyond what is now a quite liberal ignoring of unknown headers in JOSE

Given that we may have multiply wrapped payloads knowing what is at the soft chewy centre of the token is important so the processing can be consistent through all the layers.

"typ"is optional because in the simple single layer signature case it is typically not needed as the body is available.

John B.
On 2013-05-30, at 11:05 AM, Mike Jones <[e-mail]> wrote:

> 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]; 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]> 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]] 
> Sent: Thursday, May 30, 2013 7:34 AM
> 
> To: Mike Jones
> Cc: Jim Schaad; [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]> 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]] On Behalf Of Richard Barnes
> Sent: Wednesday, May 29, 2013 8:03 PM
> To: Mike Jones
> Cc: Jim Schaad; [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]> 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]] 
> Sent: Wednesday, May 29, 2013 5:18 PM
> To: Mike Jones
> Cc: Jim Schaad; [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]> 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]] On Behalf Of Dick Hardt
> Sent: Wednesday, May 29, 2013 4:56 PM
> 
> To: Jim Schaad
> Cc: [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]> 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]] 
> Sent: Wednesday, May 29, 2013 3:49 PM
> To: Jim Schaad
> Cc: [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]> 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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