The security considerations have been reworked in the manner that you
suggested, using an example supplied by James. Thanks to both of you for your
diligence.
-- Mike
From: Mike Jones
Sent: Thursday, November 12, 2015 5:50 PM
To: 'Jim Schaad'; [email protected]
Subject: RE: [jose] JWS Unencoded Payload Option spec addressing shepherd
comments
OK, I can take a stab at rewriting it that way.
It's not that I think there isn't ever a problem but I do think that there are
lots of real cases in which there's only a problem if the producer is
intentionally creating a JWS designed to confuse - in which case the producer
is an attacker. Do see any other cases in which there is a problem to address
other than when the producer is intentionally emitting "b64":true but signing
as if "b64" was false?
The current security considerations text was intended to do a case analysis of
when there is and isn't a problem, breaking things down by who implements the
extension and who doesn't. In my analysis, if the recipient rejects a
signature that it doesn't understand, I didn't consider that a security
problem. Do you agree with that?
While, yes, in retrospect I wish we'd talked about in person too, in fairness,
I did write to the list in my November 2nd note that "I will add the thoughts
above to the Security Considerations section", and that's what I did, keeping
the factual content, but attempting to change the tone from e-mail
conversational to factual.
But I'll try it your way instead, while also keeping the case analysis where
appropriate. In the meantime, please let me know your thoughts on the two
questions I asked above.
-- Mike
From: Jim Schaad [mailto:[email protected]]
Sent: Thursday, November 12, 2015 2:24 PM
To: Mike Jones; [email protected]
Subject: RE: [jose] JWS Unencoded Payload Option spec addressing shepherd
comments
Mike,
I wish that we had talked about this last week at the F2F meeting.
This does not read like a security consideration, it read like an email that is
sent to the list saying that there is nothing to talk about here and it can be
completely ignored. If that is the way that you feel about this, you need to
get census that there is no problem and kill the text.
How I would approach writing this consideration is as follows:
1. Describe what the problem is. While both James and I provided one
scenario that was easy to understand, it should not be thought of as the only
case that there is a problem. The basic description of the problem is that
there are now two different strings that will generate the same signature
value, without there being a weakness in the hash algorithm.
2. Describe the set of things that can be done to address this problem.
a. Use the "crit" parameter
b. Use a profile of the application that states there is only one possible
value
c. Make sure that all implementations support the b64 parameter
Jim
From: jose [mailto:[email protected]] On Behalf Of Manger, James
Sent: Wednesday, November 11, 2015 4:21 PM
To: Mike Jones
<[email protected]<mailto:[email protected]>>;
[email protected]<mailto:[email protected]>
Subject: Re: [jose] JWS Unencoded Payload Option spec addressing shepherd
comments
Mike, thanks for trying to explain with a page of Security Considerations the
ambiguity arising since existing (OLD) JWS implementations will silently ignore
the "b64":false signal.
It is still wrong. The extra text starts by saying "there is no security
problem... since the signature will fail" when "b64":false is ignored, but then
ends by saying "crit":["b64"] is necessary when "b64":false is ignored. It
cannot be both!
There is no security problem if a JWS correctly created using "b64"
with a "false" value is received by an implementation not supporting
the "b64" Header Parameter, since the signature will fail to verify
and the JWS will therefore be rejected.
...
Only if the application dynamically switches between "false" and
"true" values for "b64" (something NOT RECOMMENDED in Section 6),
would it be necessary for the application to require the use of
"crit" with a value of "["b64"]" in such application contexts.
The signature can, in fact, still verify when "b64":false is ignored - giving
the verifier the wrong content.
We could have chosen a different signing input to avoid all ambiguity, but
decided against that (as it was aesthetically nice to keep a lower-layer
invariant somewhere inside JWS implementations of signing-up-to-the-2nd-dot).
We could have required "crit":["b64"], but would prefer to avoid the extra
handful of bytes (and probably aren't that confident that everyone implements
"crit" properly anyway).
We could leave it to users to avoid the ambiguity at higher layers by having
totally separate contexts that do & don't use "b64":false, trying to help them
get it right with advice about "application profiles", "application context", a
"NOT RECOMMENDED", and 5 extra Security Considerations paragraphs (though what
"application" means here is not that clear).
To me, this is a design failure.
The extra text about trust is dangerous.
If the trust established by
verifying the signer's key does not actually establish that the
creator is a trusted party, then this case in which JWS libraries
supporting and not supporting the extension may respectively
interpret the attacker's payload as being encoded or unencoded is the
least of the application's worries.
CAs issuing certificates to millions of HTTPS web sites (used to verify the
signer's key) absolutely DO NOT establish that the creator (web site) is a
trusted party, merely what the creator's name is. Misinterpreting what a signer
means is bad regardless of trust.
--
James Manger
From: jose [mailto:[email protected]] On Behalf Of Mike Jones
Sent: Thursday, 12 November 2015 2:37 AM
To: [email protected]<mailto:[email protected]>
Subject: [jose] JWS Unencoded Payload Option spec addressing shepherd comments
Draft -04 of the JWS Unencoded Payload Option specification addresses the
shepherd comments. Thanks to Jim Schaad for his careful review. The primary
change was adding additional security considerations text, including describing
when "crit" should be used.
The specification is available at:
*
https://tools.ietf.org/html/draft-ietf-jose-jws-signing-input-options-04
An HTML formatted version is also available at:
*
http://self-issued.info/docs/draft-ietf-jose-jws-signing-input-options-04.html
-- Mike
P.S. This note was also published at http://self-issued.info/?p=1474 and as
@selfissued<https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2ftwitter.com%2fselfissued&data=01%7c01%7cmichael.jones%40microsoft.com%7c3a69db7b8b6c4d47da0f08d2937a3d82%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=ggurSMkRVW%2bR8Nv93Mnbsf16CmVGqfjB9lW8SV5gAKM%3d>.
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