Thinking on this a little more, it seems like it would be simpler to state this just as an "allow unsigned" flag, rather than a more general "acceptable algorithms" set. That way we don't have to worry about all the downgrade scenarios when you have "none" along with something else.
""" Implementations that support unsigned JWS objects (i.e., the "alg" value "none") MUST NOT accept such objects as valid unless the application specifies that it is acceptable for a specific object to be unsigned. For example, the "verify" method of a JWS library might have a boolean "acceptUnsigned" parameter that would indicate that "none" is an acceptable "alg" value. Applications MUST specify this flag on a per-object basis, since otherwise they will be vulnerable to downgrade attacks. For example, suppose an application accepts JWS objects over two channels, (1) HTTP and (2) HTTPS with client authentication. It requires a JWS signature on objects received over HTTP, but accepts unsigned JWS objects over HTTPS. If the application sets a global setting that "none" is acceptable, then an attacker could provide it with an unsigned object over HTTP and still have that object considered valid. Instead, the application needs to set the "acceptUnsigned" flag to "false" for each object received over HTTP, and to "true" for each object received over HTTPS. """ This makes the equivalence between the "separate object" and "constrained verify" cases a lot clearer. On the one hand, you would have "jose.somethingNotVerify(JWP)", and on the other hand, you would have "jose.verify(JWP, acceptUnsigned=True)". --Richard On Tue, Sep 3, 2013 at 2:02 PM, Mike Jones <[email protected]>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*** > * > > ** ** > > _______________________________________________ > jose mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/jose > >
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