Re: [jose] Whether implementations must understand all JOSE header fields

"Jim Schaad" <> Sun, 30 December 2012 04:25 UTC

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From: Jim Schaad <>
To: 'Mike Jones' <>,
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Date: Sat, 29 Dec 2012 20:24:57 -0800
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Subject: Re: [jose] Whether implementations must understand all JOSE header fields
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From: [] On Behalf Of Mike
Sent: Wednesday, December 26, 2012 2:32 PM
To: Jim Schaad;
Subject: Re: [jose] Whether implementations must understand all JOSE header


That idea seems like it would (1) add to the overall complexity of
implementations, (2) would be counter the goal of interoperable
implementations, and (3) would likely be counter to goal of security.


(1) The way I see it, if an extension mechanism (other than having both
parties understand the extension, which already works) is going to be
defined, we owe users of the specs a clear definition of how to do
not-understood extensions.  Otherwise implementations of each profile will
potentially have to repeat the work of checking the fields used, rather than
doing it in the JOSE libraries themselves, adding to overall implementation


(2)  Sometimes allowing fields to be not understood and sometimes requiring
them to be, and possibly using different extension mechanisms in different
profiles would definitely hurt interoperability.


(3)  If a profile decided that the "alg" field need not be understood, that
would definitely defeat the security purposes of the JOSE specs.  The same
is true of many other fields.  We owe it to users of the specs to be clear
that security-critical fields must be understood.  "Punting" this to
profiles or just putting it in the security considerations unnecessarily
risks having profile writers get this wrong.


[JLS]  I would be shocked if a JWS implementation would report that the
signature verified in this case.  So I don't think that there is any
security issue here.


Yes, in theory, we could include a consensus call question giving people
this option as well, but unless support for it shows up on the list, it
would just seem (to me) to unnecessarily complicate the questions we're
asking people by including an option that's not actually viable from a
complexity, interoperability, or security point of view.


                                                            -- Mike


From: Jim Schaad [] 
Sent: Wednesday, December 26, 2012 2:03 PM
To: Mike Jones;
Subject: RE: [jose] Whether implementations must understand all JOSE header


This does not include my recommended approach which is to punt the issue of
required headers to the application rather than JOSE.





From: [] On Behalf Of Mike
Sent: Friday, December 14, 2012 10:17 AM
Subject: [jose] Whether implementations must understand all JOSE header


Currently the JOSE specs contain the following language declaring that all
header (and some other) fields must be understood:  "Implementations MUST
understand the entire contents of the header; otherwise, the . MUST be
rejected."  There's currently an open issue about whether all header fields
should be required to be understood by implementations using them or whether
to relax this requirement in some circumstances.  We would like the working
group to consider the following potential resolutions to the issue.


1.  Maintain the current requirement that all header fields must be
understood by implementations using them.


PRO:  This is a simple rule designed to result in secure implementations.
Representations of existing JWS & JWE objects are unchanged.

CON:  Extensibility is limited to cases where all participating parties
understand new fields that are introduced.


2A.  Take an alternative approach where, by default, all fields must be
understood, but where an explicit list of fields can be designated as "may
be ignored if not understood".  For instance, an "ign" (may be ignored)
member could be defined whose value is an array listing the member names
that may be safely ignored if not understood.  An example using this field
in a JWS header is:




     "notes":"This signature need not be checked when the moon is full"



(Obviously, the "ign" field MUST NOT contain the value "ign".)


PRO:  This would enable adding non-critical header fields without breaking
implementations.  It would maintain the current semantics, while allowing
explicit exceptions to be made in cases where the fields are not
security-critical and the structure has a meaningful interpretation without
them.  Representations of existing JWS & JWE objects are unchanged.

CON:  Requires coordination of field names and may-be-ignored list, adding
some implementation complexity.


2B.  Take an alternative approach where instead of one header object, there
would be two header objects.  Implementations would be required to
understand all fields in the first header object.  Implementations would be
allowed to ignore fields in the second header object if not understood.  An
example of this approach, adding the second header object after the first in
a JWS, would be:










The first, third, and fourth fields are from the example JWS in the current
spec. The new second field is the base64url encoded representation of the
string '{"notes":"This signature need not be checked when the moon is


PRO:  May-be-ignored fields are easily distinguishable from
must-be-understood fields.

CON:  Implementations must consult/merge two sets of headers to understand
the meaning/structure of the object, adding some implementation complexity.
Existing JWS & JWE object representations would become invalid.


After a discussion period, we believe that it would be useful for the chairs
to make two formal consensus calls of the following kind:


FIRST POLL:  Should all header fields be critical for implementations to

YES - All header fields must continue to be understood by implementations or
the input must be rejected.

NO - A means of listing that specific header fields may be safely ignored
should be defined.


SECOND POLL:  Should the result of the first poll be "NO", which syntax
would you prefer for designating the header fields that may be ignored if
not understood?

A - Define a header field that explicitly lists the fields that may be
safely ignored if not understood.

B - Introduce a second header, where implementations must understand all
fields in the first but they may ignore not-understood fields in the second.


                                                Thanks for considering these

                                                Richard Barnes, John
Bradley, Mike Jones