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

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

I am going to go into more detail than my last message

 

The issue is a question of which part of the code is responsible for what
set of processing.

 

If a library says that an item is either encrypted or signed and it wasn't,
or it was unable to apply those transformations on request, then you have a
security problem.  If you hand the library a piece of garbage and it says
that it was not signed (or encrypted) then you do not have a security
problem, you have a different type of problem.

 

The same thing is true with the ZIP example you gave earlier.  Is the
application or the JW* library responsible for dealing with the question of
zipping the content?  If you say the application is responsible, then it
does not matter of the JW* code does not recognize the ZIP parameter.  If
you say the JW* library is responsible, then it does matter if the JW* code
does not recognize the ZIP parameter.  You then also have the question
should the JW* code have the "right" to apply a ZIP operation even if it was
not asked for by the application.  It is after all part of the library and
thus could be applied at any time.

 

The position of mandatory understanding of fields is much more justifiable
if one states that the library is responsible for enforcing everything about
the header.  This would mean that the concept of a signing time field in the
header would not make any sense.  The JW* library has no way of knowing what
the semantics of the field is going to mean to the application and thus has
no method of enforcing it.  (Does the object expire a specific amount of
time after the signing time?  Does there need to be some type of allowance
for clock skews?  Is there an interaction between a certificate if given and
the signing time as a header field?  Are there fields in the key container
that say when a key can be used and thus need to be enforced?)  

 

If on the other hand one believes that there is a possibility that there is
auxiliary data which is not part of the content but is part of the context
in which the signature (or encryption) was applied, it is impossible for a
JW* library to be able to enforce any of this context and it must be
deferred to the application for enforcement.

 

To directly address your points below:

 

1.        We owe developers a clear description of what to do with the items
that we defined.  If an application defines another field, then it will
defined what to do with that item and not the JW* code and the application
would be expected to enforce the semantics and not the JW* code.  It does
mean that each application will have its own list of what items can appear,
however it does not mean that the JW* code cannot supply a library routine
for doing the field name checking.

2.       I am completely unable to understand why allowing a field for one
application and not for another application hurts interoperability or
enforcement of the fields.   But as I am doing this in the application, I
only have one standard that I am working with (until such a time as the
application is updated).  

3.       If a profile decides that the alg field need not be present, which
is different that it's value not being understood, then that is up to the
application.  As long as the application does not make a security claim
about the content that as transmitted that is a perfectly fine thing for it
to do.  It might just be transmitted as plain unsigned, unencrypted data in
the same format as the current JW* formats.  There is a different issue
involved with the value of the alg field not being understood.  That is
something that the JW* libraries are perfectly competent to deal with and is
not the same type of extension mechanism as we are discussing here.

 

I completely disagree that the option is either  not viable, or is a
security problem.  I think that this is your narrow view of the world
affecting how you are evaluating the proposal that I have provided.  However
as long as you are going to stay in the camp that says there is no auxiliary
data which is not part of the content of the message, then your view is a
consistent one.  I happen to strongly disagree with that view of the world.

 

Jim

 

 

 

 

From: [e-mail] [mailto:[e-mail]] On Behalf Of Jim
Schaad
Sent: Saturday, December 29, 2012 8:25 PM
To: 'Mike Jones'; [e-mail]
Subject: Re: [jose] Whether implementations must understand all JOSE header
fields

 

 

 

From: [e-mail] [mailto:[e-mail]] On Behalf Of Mike
Jones
Sent: Wednesday, December 26, 2012 2:32 PM
To: Jim Schaad; [e-mail]
Subject: Re: [jose] Whether implementations must understand all JOSE header
fields

 

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
complexity.

 

(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 [mailto:[e-mail]] 
Sent: Wednesday, December 26, 2012 2:03 PM
To: Mike Jones; [e-mail]
Subject: RE: [jose] Whether implementations must understand all JOSE header
fields

 

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

 

Jim

 

 

From: [e-mail] [mailto:[e-mail]] On Behalf Of Mike
Jones
Sent: Friday, December 14, 2012 10:17 AM
To: [e-mail]
Subject: [jose] Whether implementations must understand all JOSE header
fields

 

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:

 

    {"alg":"ES256",

     "ign":["notes"],

     "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:

 

    eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9

    .

 
eyJub3RlcyI6IlRoaXMgc2lnbmF0dXJlIG5lZWQgbm90IGJlIGNoZWNrZWQgd2hlbiB0aGUgbW9v
biBpcyBmdWxsIn0

    .

 
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9p
c19yb290Ijp0cnVlfQ

    .

    dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

 

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
full"}'.

 

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
understand?

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
questions,

                                                Richard Barnes, John
Bradley, Mike Jones