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

["Jim Schaad" <[e-mail]>]

I agree that [1] is incorrect.  Given that you could encode the same JSON
object either as a JSON object or as a compact serialization (if formed
correctly) signing the serialization into the type structure seems very
awkward.

 

Jim

 

 

From: [e-mail] [mailto:[e-mail]] On Behalf Of
Richard Barnes
Sent: Thursday, May 30, 2013 7:39 AM
To: Mike Jones
Cc: Manger, James H; [e-mail]
Subject: Re: [jose] Should we delete the "typ" header field

 

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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