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

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

But life does get strange at some point.

 

If I have a JSON serialization that has typ being jose, does that mean that
something is wrong in the processing?  Any compact serialization can be
represented as a JSON serialization.  Does that act invalidate anything?

 

Jim

 

 

From: [e-mail] [mailto:[e-mail]] On Behalf Of Mike
Jones
Sent: Sunday, July 14, 2013 2:15 PM
To: Richard Barnes
Cc: Manger, James H; [e-mail]
Subject: Re: [jose] FW: Should we delete the "typ" header field

 

Per the discussion on the call, people made the point that it's useful to
have a generic MIME type for use by applications that choose to use it.  An
analogy was made to the application/json MIME type.  The JSON spec could
have omitted that, leaving it up to applications to always use
application-specific MIME types for JSON objects.  But in practice, it's
proven useful to have application/json, because it will do the job without
an application-specific MIME type in many circumstances.  That was the
reason cited for continuing to have generic JOSE MIME types.  The value
"JOSE" is simply a registered shorthand for "application/jose".

 

                                                            -- Mike

 

From: Richard Barnes [mailto:[e-mail]] 
Sent: Sunday, July 14, 2013 1:33 PM
To: Mike Jones
Cc: Manger, James H; [e-mail]
Subject: Re: [jose] FW: Should we delete the "typ" header field

 

I think James's point is that there's no point to using "typ": "JOSE".  If
you've already parsed the object far enough to read that field, you know
it's a JOSE object.  So we should just remove that value.

 

--Richard

 

On Sun, Jul 14, 2013 at 4:09 PM, Mike Jones <[e-mail]>
wrote:

It can be application-specific if the application chooses to use that value.
I will plan change the sentence in question by adding the words "by
applications" as below to eliminate the potential ambiguity that you
identified:

   The type value "JOSE" MAY be used by applications

   to indicate that this object is a JWS or JWE using the JWS Compact

   Serialization or the JWE Compact Serialization.

 

As for the choice of the names "JOSE" and "JOSE+JSON", those reflect the
decision on the last call to change from the type-specific MIME types
application/jws, application/jwe, application/jws+json, and
application/jwe+json to the more inclusive names application/jose and
application/jose+json.  Both the MIME types exist because the serializations
use different representations, and so their content types need to be
different.

 

                                                            -- Mike

 

From: Manger, James H [mailto:[e-mail]] 
Sent: Saturday, July 13, 2013 6:41 AM
To: Mike Jones; [e-mail]


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

 

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
<http://tools.ietf.org/rfcdiff?difftype=--hwdiff&url2=draft-ietf-jose-json-w
eb-encryption-12.txt> &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:[e-mail]] 
Sent: Saturday, 13 July 2013 1:32 AM
To: Manger, James H; [e-mail]
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'; [e-mail]
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:  <mailto:[e-mail]> [e-mail] [
<mailto:[e-mail]> mailto:[e-mail]] On Behalf Of
Manger, James H
Sent: Thursday, May 30, 2013 12:17 AM
To:  <mailto:[e-mail]> [e-mail]
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:[e-mail]>
mailto:[e-mail]] 
Sent: Thursday, 30 May 2013 4:45 PM
To: Manger, James H;  <mailto:[e-mail]> [e-mail]
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:  <mailto:[e-mail]> [e-mail] [
<mailto:[e-mail]> mailto:[e-mail]] On Behalf Of
Manger, James H
Sent: Wednesday, May 29, 2013 11:08 PM
To:  <mailto:[e-mail]> [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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