[jose] Mxed Signature Algorithm TYpes

A side question from this discussion that I would like to see addressed is
should we state any recommendations about mixing different types of
signatures algorithms in the same message.

I would think that it makes sense to say that type types "none", "mac" and
"asymmetric-Signature" should never be mixed in the same message because
these different types of algorithms have significantly different security
properties.

From: jose-bounces@ietf.org [mailto:jose-bounces@ietf.org] On Behalf Of
Richard Barnes
Sent: Tuesday, September 03, 2013 1:33 PM
To: Mike Jones
Cc: jose@ietf.org
Subject: Re: [jose] Text about applications and "alg":"none"

This text is still far too weak, and does not reflect what I remember EKR
saying (in particular, there is no MUST).  It does not address the attack
where an application may, in general, be willing to accept both signed and
unsigned content, but each in specific contexts.  Proposed text:

"""

JWS implementations MUST provide an interface for applications to specify a
list of "alg" values that are acceptable for the validation of a given JWS
object.  JWS implementations MUST NOT indicate that a JWS object is valid if
the "alg" value for the object is "none", unless the application has
specifically indicated that the value "none" is acceptable for the
particular JWS object being validated.  Applications using "none" MUST
indicate support on a per-object basis, in order to avoid downgrade attacks
that arise if more broadly-applicable preferences are specified.

"""

I continue to believe that this is far too subtle, and that applications are
very likely to get it wrong.  It is far simpler and safer to require that a
JWS implementation MUST reject an object with "alg":"none", and have another
content type for unsigned content.

Also, if "none" is going to remain, then it needs to be OPTIONAL.  Given all
the above limitations, I don't see how you could justify it being mandatory.

--Richard

On Tue, Sep 3, 2013 at 2:02 PM, Mike Jones <Michael.Jones@microsoft.com>
wrote:

I took an action item during the last call to write text along the lines
suggested by ekr about applications and "alg":"none".  I propose that the
following text be included:

It is RECOMMENDED that libraries provide applications a means of specifying
the list of acceptable algorithms used in a JWS object in a way that causes
inputs using algorithms outside the specified set to be rejected.  In
particular, it is intended for applications to use this mechanism to exclude
accepting inputs using "alg":"none" in security contexts where non-integrity
protected inputs are not acceptable.

Feedback/proposed wording refinements welcomed.

                                                                -- Mike

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