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

"Manger, James H" <James.H.Manger@team.telstra.com> Sat, 13 July 2013 13:41 UTC

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From: "Manger, James H" <James.H.Manger@team.telstra.com>
To: Mike Jones <Michael.Jones@microsoft.com>, "jose@ietf.org" <jose@ietf.org>
Date: Sat, 13 Jul 2013 23:41:24 +1000
Thread-Topic: [jose] FW: Should we delete the "typ" header field
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Subject: Re: [jose] FW: Should we delete the "typ" header field
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Mike,

The “typ” changes (below) make no sense.
The first sentence is marginally clearer that “typ” is some sort of label for a higher-layer (above JOSE processing), eg values are application-specific. But the very next sentence suggests “JOSE” as a value! Argh!! How can that be “application-specific”? It is the definition of not application-specific. Defining “JOSE+JSON” in the next sentence is even worse. That makes It seem that “typ” is indicating the serialization, which everyone (including yourself) have agreed is wrong.


-- from http://tools.ietf.org/rfcdiff?difftype=--hwdiff&url2=draft-ietf-jose-json-web-encryption-12.txt

4.1.10.  "typ" (Type) Header Parameter

   The "typ" (type) header parameter is MAY be used to declare the type of
   this
   object. complete JWE object in an application-specific manner in
   contexts where this is useful to the application.  This parameter has
   no effect upon the JWE processing.  The type value "JWE" is "JOSE" MAY be used
   to indicate that this object is a JWS or JWE using the JWS Compact
   Serialization or the JWE Compact Serialization.  The type value "JWE+JSON"
   is
   "JOSE+JSON" MAY be used to indicate that this object is a JWS or JWE
   using the JWS JSON Serialization or the JWE JSON Serialization.
   Other type values MAY be used, and if not understood, SHOULD be
   ignored.  The "typ" value is a case sensitive string.  Use of this
   header parameter is OPTIONAL.

   MIME Media Type [RFC2046] values MAY be used as "typ" values.

   "typ" values SHOULD either be registered in the IANA JSON Web
   Signature and Encryption Type Values registry [JWS] or be a value
   that contains a Collision Resistant Namespace.


--
James Manger

From: Mike Jones [mailto:Michael.Jones@microsoft.com]
Sent: Saturday, 13 July 2013 1:32 AM
To: Manger, James H; jose@ietf.org
Subject: RE: [jose] FW: Should we delete the "typ" header field

I’ve applied the proposed changes in the -12 drafts.

                                                            -- Mike

From: Mike Jones
Sent: Thursday, May 30, 2013 12:22 AM
To: 'Manger, James H'; jose@ietf.org<mailto:jose@ietf.org>
Subject: RE: [jose] FW: Should we delete the "typ" header field

I agree, given this thread, that clarified wording is in order along the lines of that which you suggest.  Thanks for writing it.  I’ll put doing so on my to-do list.

The purpose of the “JWS” and “JWE” types is to provide standard identifiers for applications that may want to use them, either in the “typ” field or the “cty” field (the values of which are in the same namespace).  I’ll plan to similarly clarify that the semantics of “cty” are application-specific and the contents of “cty” do not affect the JOSE processing.

                                                            Best wishes,
                                                            -- Mike

From: jose-bounces@ietf.org<mailto:jose-bounces@ietf.org> [mailto:jose-bounces@ietf.org] On Behalf Of Manger, James H
Sent: Thursday, May 30, 2013 12:17 AM
To: jose@ietf.org<mailto:jose@ietf.org>
Subject: [jose] FW: Should we delete the "typ" header field

> The purpose of “typ” is your 3.

Then why define "typ":"JWS", "typ":"JWS+JSON", "typ":"JWE", and "typ":"JWE+JSON"?
These values cause confusion as they only make sense for purposes [1] and [2].

The text describing "typ" says it declares the “type of this object”.
This clearly also causes massive confusion. The object is obviously a JOSE message; it is also obviously a JWS or JWE; it might also be a JWT, or a missile launch instruction, or a meeting invitation, or anything. “type” and “object” are too generic for readers to get the same understanding.

At a minimum it should be radically rephrased. Perhaps:
“The "xxx" header parameter can be used to hold an application-specific string identifying the meaning of a JOSE message to an application. This parameter has no affect on JOSE processing.”

--
James Manger

From: Mike Jones [mailto:Michael.Jones@microsoft.com]
Sent: Thursday, 30 May 2013 4:45 PM
To: Manger, James H; jose@ietf.org<mailto:jose@ietf.org>
Subject: RE: [jose] Should we delete the "typ" header field

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: jose-bounces@ietf.org<mailto:jose-bounces@ietf.org> [mailto:jose-bounces@ietf.org] On Behalf Of Manger, James H
Sent: Wednesday, May 29, 2013 11:08 PM
To: jose@ietf.org<mailto:jose@ietf.org>
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