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

[Richard Barnes <[e-mail]>]

That is completely backwards.  [3] is satisfied by "cty".   JWT should be
setting "typ"=JWE/JWS, "cty"=JWT-claims.  If it's doing otherwise, it's
semantically confused and wrong.

On James's points:
(1) I think we all agree that (1) is not a useful application of "typ".
 The current docs are wrong to include the "+JSON" forms.
(2) My preference would be to keep an explicit indicator.  Relying on
structure / algorithm IDs is brittle.
(3) This is "cty", not a use case for "typ"

--Richard


On Thu, May 30, 2013 at 2:45 AM, Mike Jones <[e-mail]>wrote:

>  The purpose of “typ” is your 3.****
>
> ** **
>
> There can already be no confusion about 1 because they’re syntactically
> completely different.  2 is unnecessary because the “alg” value (or the
> existence of “enc”) already distinguishes between JWS and JWE semantics.**
> **
>
> ** **
>
>                                                             -- Mike****
>
> ** **
>
> *From:* [e-mail] [mailto:[e-mail]] *On Behalf
> Of *Manger, James H
> *Sent:* Wednesday, May 29, 2013 11:08 PM
> *To:* [e-mail]
>
> *Subject:* Re: [jose] Should we delete the "typ" header field****
>
>  ** **
>
> > Can anybody justify why this field should be present in the document –
> or should it just disappear?****
>
> ** **
>
> It seems there are at least 3 different meanings given to "typ" in the
> header of a JOSE message:****
>
> ** **
>
> [1] The "typ" value indicates the serialization of the JOSE message. For
> example, "typ":"JWE" and "typ":"JWE+JSON" distinguish the compact
> (dot-separated-b64-blobs) and JSON serializations.****
>
> ** **
>
> [2] The "typ" value indicates the high-level semantics of the JOSE
> structure. For example, "typ":"JWE" and "typ":"JWS" distinguish the
> semantics defined in the separate JWE and JWS specifications.****
>
> ** **
>
> [3] The "typ" value indicates the application-layer semantics of the
> message. For example, "typ":"JWT" value indicates that the message conveys
> a set of claims (as a JSON object) wrapped as a JOSE message (either
> unprotected, signed, MACed, encrypted, or signed then encrypted) that use
> the compact serialization.****
>
> ** **
>
> Indicating the serialization [1] does not seem helpful as the recipient
> needs to know the serialization before they can extract the header to see
> the "typ" value. Indicating the serialization is actually harmful as it
> tightly couples a message to one serialization, whereas serialization is
> generally thought of as a transport-layer choice that is independent of the
> message security or semantics.****
>
> ** **
>
> Indicating the high-level semantics of the JOSE structure [2] is slightly
> useful so a message can be switched to different code according to its
> structure. It is not that useful, however, as further switching is required
> to distinguish different modes (eg unprotected vs asymmetric signature vs
> MAC). This meaning only helps if the field is made mandatory, and the
> presence/absence of the "enc" field or looking up the class of the "alg"
> value are not specified as alternatives.****
>
> ** **
>
> Being able to indicate application-layer semantics [3] could theoretically
> be useful. Perhaps the "profile" attribute or "rel=’profile’" link relation
> in HTML5 is analogous. In this case JOSE should not define values for the
> field. "JWS", "JWS+JSON", "JWE", and "JWE+JSON" make no sense as
> application-layer semantics — and certainly not inside the JOSE message.**
> **
>
> ** **
>
> Most (all?) of the many specs mentioning the "typ" field make it optional,
> and if they suggest particular "typ" values those are only “MAY”s or
> “SHOULD”s — not “MUST”s. Consequently, apps cannot rely on "typ" regardless
> of its meaning.****
>
> ** **
>
> ** **
>
> My suggestions:****
>
> ** **
>
> * For [1], define two media types to distinguish the two serializations,
> not a header field.****
>
> ** **
>
> *  1st preference for [3], drop it from JOSE specs; let an application
> using JOSE (eg JWT) define a field (and value) for this. If the application
> defines the field in a generic fashion for reuse by other applications that
> is a nice bonus.****
>
> ** **
>
> * 2nd preference for [3], define a field (but no values) that can hold an
> application-layer semantics identifier – but only put this definition in a
> spec that defines JOSE messages as a whole (not specs specific to JWE or
> JWS). Use a different name: "app" or "profile" or "mean"ing or "pur"pose.*
> ***
>
> ** **
>
> * For [2], define a mandatory field that indicates the semantics of the
> JOSE structure at a low enough level that a JOSE implementation built on
> top of a crypto library could (almost) work without needing to recognize
> the "alg" value. "typ" would have been a reasonable name for this field but
> is now too polluted with confusion. How about "t"?****
>
> Consider for instance a JOSE implementation that only supports
> "alg":"HS256". To add support for "alg":"HS3" (HMAC with SHA-3) minimal (if
> any) new code is needed in a JOSE layer: perhaps an extra table entry
> mapping the JOSE label "HS3" to a crypto library label (eg "HmacSHA3").
> "t":"mac" can accompany both these algs. To support "alg":"RS512", in
> contrast, requires calls to different crypto library functions (knowing the
> difference between public & private keys for instance). This deserves a
> separate value, say, "t":"sig".****
>
> ** **
>
> --****
>
> James Manger****
>
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