Re: [jose] Richard Barnes' Discuss on draft-ietf-jose-json-web-signature-33: (with DISCUSS and COMMENT)

Richard Barnes <rlb@ipv.sx> Mon, 20 October 2014 16:17 UTC

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From: Richard Barnes <rlb@ipv.sx>
To: Mike Jones <Michael.Jones@microsoft.com>
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Subject: Re: [jose] Richard Barnes' Discuss on draft-ietf-jose-json-web-signature-33: (with DISCUSS and COMMENT)
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On Sat, Oct 11, 2014 at 5:14 PM, Mike Jones <Michael.Jones@microsoft.com>
wrote:

> > From: Richard Barnes [mailto:rlb@ipv.sx]
> > Sent: Friday, October 10, 2014 3:20 PM
> > To: Mike Jones
> > Cc: The IESG; jose-chairs@tools.ietf.org; jose@ietf.org;
> draft-ietf-jose-json-web-signature@tools.ietf.org
> > Subject: Re: [jose] Richard Barnes' Discuss on
> draft-ietf-jose-json-web-signature-33: (with DISCUSS and COMMENT)
> >
> > On Mon, Oct 6, 2014 at 3:54 AM, Mike Jones <Michael.Jones@microsoft.com>
> wrote:
> > Thanks for your review, Richard.  I'm repeating my previous responses
> from my Thursday reply, but this time using ">" quoting rather than colors,
> for better readability by people not using HTML-enabled mail readers...
> >
> > > -----Original Message-----
> > > From: jose [mailto:jose-bounces@ietf.org] On Behalf Of Richard Barnes
> > > Sent: Wednesday, October 01, 2014 9:22 PM
> > > To: The IESG
> > > Cc: jose-chairs@tools.ietf.org; jose@ietf.org;
> draft-ietf-jose-json-web-
> > > signature@tools.ietf.org
> > > Subject: [jose] Richard Barnes' Discuss on
> draft-ietf-jose-json-web-signature-33:
> > > (with DISCUSS and COMMENT)
> > >
> > > Richard Barnes has entered the following ballot position for
> > > draft-ietf-jose-json-web-signature-33: Discuss
> > >
> > > When responding, please keep the subject line intact and reply to all
> email
> > > addresses included in the To and CC lines. (Feel free to cut this
> introductory
> > > paragraph, however.)
> > >
> > >
> > > Please refer to
> http://www.ietf.org/iesg/statement/discuss-criteria.html
> > > for more information about IESG DISCUSS and COMMENT positions.
> > >
> > >
> > > The document, along with other ballot positions, can be found here:
> > > http://datatracker.ietf.org/doc/draft-ietf-jose-json-web-signature/
> > >
> > >
> > >
> > > ----------------------------------------------------------------------
> > > DISCUSS:
> > > ----------------------------------------------------------------------
> > >
> > > Overall, this document is in much more solid shape than when it began.
> > > Thanks to the WG for a lot of hard work.  I only have two remaining
> concerns,
> > > which should hopefully be easy to address.
> > >
> > > Section 7.2.
> > > I've had several implementers trying to use JWS in the JSON
> serialization ask why
> > > it was necessary to include a "signatures" array in cases where
> there's only one
> > > signer.  It seems like this is going to be a major barrier to
> deployment and re-
> > > use, so I would propose including the following text:
> > >
> > > """
> > > In cases where the JWS has been signed by only a single signer, the
> "signatures"
> > > array will contain a single object.  In such cases, the elements of
> the single
> > > "signatures" object MAY be included at the top level of the JWS
> object.  A JSON-
> > > formatted JWS that contains a "signatures" field MUST NOT contain a
> > > "protected", "header", or "signature" field, and vice versa.
> > > """
> > >
> > > This may also require some other changes where "signatures" is relied
> on, e.g.,
> > > in Section 9 of the JWE spec.
> > This was previously proposed (I believe during the Denver interim
> meeting) and rejected by the working group because it complicates both
> producers and parsers by introducing an unnecessary special case.
> Currently, by design, whether there are single or multiple signatures, the
> same syntax is used.  Your proposal would use a different syntax in the
> single signature case than in the multiple signature case.  This is likely
> to result in implementation bugs and inconsistencies.
> >
> > This assertion of complexity is bogus.  Have you implemented this
> syntax, or can you point me to someone who has, and has had problems?  I
> have implemented it and I'm asking for the simpler syntax.  I've gotten the
> same request from others, for JWE.
>
> I'm not saying that anything is insurmountable.  I'm saying that having
> two equivalent syntaxes in the JSON Serialization for the same thing is
> going to make it more likely that some implementations won't correctly
> support both of them.  For those following along, those two syntaxes would
> be:
>
>   {
>    "payload":"<payload contents>",
>    "signatures":[
>     {"protected":"<integrity-protected header 1 contents>",
>      "header":<non-integrity-protected header 1 contents>,
>      "signature":"<signature 1 contents>"}]
>   }
>
> and
>
>   {
>    "payload":"<payload contents>",
>    "protected":"<integrity-protected header 1 contents>",
>    "header":<non-integrity-protected header 1 contents>,
>    "signature":"<signature 1 contents>"
>   }
>

It's worth noting that the multi-signer case reduces to the single-signer
case, e.g.:

<script>
var JWS = get_jws();
if ("signatures" in JWS) {
  // i = index of a signature we'll use
  JWS.protected = JWS.signatures[i].protected
  JWS.header = JWS.signatures[i].header
  JWS.signature = JWS.signatures[i].signature
} else if (! ("signature" in JWS) && (("protected" in JWS) || ("header" in
JWS))) {
  throw "malformed JWS";
}
var result = validate_single_signer_jws(JWS)
</script>

Or copy "payload" up into each signature object.  Or iterate through
signatures and see which ones work.  Either way, you're going to build this
with a "verify single signature" primitive.



> Is your motivating use case a profile of the JSON Serialization in which
> multiple signatures aren't used or supported, and so for which the array
> notation would always be superfluous syntax?  Are you concerned that if
> this profile becomes the predominant use case, that some/many
> implementations wouldn't implement the more general multi-signature syntax
> (or would implement it incorrectly, since it would normally not ever be
> used or tested)?
>

If you look at existing uses for signed objects (such as they are),
single-signer is by far the dominant use case, so requiring the multiple
signer notation is would almost always be superfluous syntax.

I'm not all that concerned about support for multi-sig.  If implementations
care about it, it will get built.  And because of the way this has been
designed, it's easy to build once you've got the single-signer version (see
above).



> I'd love to hear others in the working group chime in on this topic...
>
> > > Section 6.
> > > """
> > > These Header Parameters MUST be integrity protected if the information
> that
> > > they convey is to be utilized in a trust decision.
> > > """
> > > This smells really fishy to me.  What's your attack scenario?  I'm
> pretty certain
> > > that there's no way any of these fields can be altered in such a way
> that (1) the
> > > signature will validate, and (2) the recipient will accept a key it
> shouldn't.  By way
> > > of contrast, CMS doesn't sign the certificate fields, and the
> Certificate payload in
> > > TLS is only signed as a side effect of the protocol's verification
> that the remote
> > > end has been the same through the whole handshake (which doesn't apply
> here).
> >
> > The attack scenario is trivial to describe.  If an attacker can change
> information used in a trust decision, the trust decision has no validity.
> Unless the information is integrity-protected, the attacker could change
> the non-integrity-protected portions of the JWS in an undetectable way.
> >
> > That's hand waving, not an attack scenario.  Allow me to elaborate on
> this:
> >
> > There is no possible attack scenario for the key identifiers that
> identify a *key* (vs. a cert) -- jwk, jku, and kid.  For any given signed
> object, there is exactly one key that can validate the signature (otherwise
> the crypto is broken).  If the attacker changes the validation key, then
> the signature won't validate.  So there is no need to integrity protect
> these headers, since there's no point to the attacker changing them.  RFC
> 2634 actually has text to this effect:
> >
> > """
> >    The first version of this attack is a simple denial of service attack
> >    where an invalid certificate is substituted for the valid
> >    certificate. This renders the message unverifiable, as the public key
> >    in the certificate no longer matches the private key used to sign the
> >    message.
> > """
>
> It's not clear to me that enabling the attacker to substitute keys and get
> the recipient to attempt to validate the signature with a key of the
> attacker's choosing doesn't constitute at least a portion of a viable
> attack scenario.  It may or may not (knowing for sure is beyond my specific
> cryptographic expertise in this area) and it may not now but may be in the
> future (when new attacks are created).  Requiring these parameters to be
> integrity protected when used in a trust decision does no harm and may do
> substantial good.
>

It seems pretty clear to me :)  Everything involved in signature
verification is public, so anything the attacker could do by tricking a
third party into verifying he could just as well do himself.  The only
exception to this would be if the verification were somehow a side-channel
for something else happening on the verifier's machine, which seems pretty
outlandish.



> > With regard to the certificate identifiers ("x5u", "x5c", "x5t", and
> "x5t#S256"), the risks that Jim points out [RFC2634, Section 5] are real,
> but only apply in certain narrow circumstances.  Namely, the only time a
> risk arises is when two certificates have been issued for the same public
> key, with different attributes.  This is exceedingly rare in practice, and
> all current secure messaging systems get along fine without protection
> against this attack.  And it might not even be an attack -- you could
> envision cases with "x5u" where the signer purposely presents different
> certificates to different relying parties!
> > So the blanket requirement that these fields MUST be integrity protected
> is not appropriate.  It is only required for certain special situations
> using certificates.  Proposed revision:
> > Delete: "These Header Parameters MUST be integrity protected if the
> information that they convey is to be utilized in a trust decision."
>
> Researching the history a bit more, this text was added to address issue
> #104 http://trac.tools.ietf.org/wg/jose/trac/ticket/104 raised by Jim
> Schaad.  Unless Jim agrees that it is now somehow unnecessary, I don't
> believe that it's reasonable for us to now remove it.  Also, see Jim's
> related comments about trust statements and trust decisions in issue #74.
>

I would note that issue #104 was closed without any discussion on the list
(AFAICT).  And that issue only related to "jku", so the extension to all
the other fields is unnecessary.  See my response to Jim for why it's not
necessary for "jku" either.

As far as issue #74, that appears to be about not using the URI as an
element of the trust decision unless the identity in the URI is
authenticated.  I read that as to mean that the subject of the URI
authenticated itself, e.g., using HTTPS server authentication.




> > Add new paragraph: "In situations where multiple certificates with
> different attributes may be issued over the same public key, there is a
> risk that one of these certificates may be substituted for another. In such
> situations, the creator of a JWS object MUST integrity protect the "x5u",
> "x5c", "x5t", and "x5t#S256" attributes, if present."
> >
> > For what it's worth, Sean had us add language in a number of places that
> basically said that information is only as trustworthy as its source and
> the means by which it is obtained.  If I remember correctly, this was one
> of those places.
> >
> > > ----------------------------------------------------------------------
> > > COMMENT:
> > > ----------------------------------------------------------------------
> > >
> > > Section 2.
> > > In the definition of "Unsecured JWS", it would be good to note that
> this requires
> > > "alg" == "none".
> >
> > OK
> >
> > > Section 3.3.
> > > Why doesn't this section have a JSON-encoded form as well?
> >
> > Because it's meant to be a simple introductory example to help people
> get their head around the concept - not a complete tutorial.  Other
> examples of JSON-encoded objects are found elsewhere in the document and
> lots of them are found in draft-ietf-jose-cookbook.
> >
> > > Appendix A.5.
> > > I would prefer if this example could be removed.  JWT is the only use
> case for
> > > Unsecured JWS right now, and there's already an example in that
> document.
> >
> > Mike> Given that it's important that implementers using them understand
> Unsecured JWSs, there is motivation to retain the example.  I'd be
> interested in what others in the working group think, given that there was
> substantial support for retaining this functionality when its removal was
> proposed.
> >
> > > Thanks for Appendix C.  FWIW, it has been implemented:
> > >
> http://dxr.mozilla.org/mozilla-central/source/dom/crypto/CryptoBuffer.cpp#85
> >
> > You're welcome.
> >
> > > _______________________________________________
> > > jose mailing list
> > > jose@ietf.org
> > > https://www.ietf.org/mailman/listinfo/jose
> >
> >                                 Thanks again!
> >                                 -- Mike
>
>                                         -- Mike
>
>