Re: [secdir] [jose] secdir review of draft-ietf-jose-json-web-encryption-31

"Jim Schaad" <ietf@augustcellars.com> Thu, 11 September 2014 20:30 UTC

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From: "Jim Schaad" <ietf@augustcellars.com>
To: "'Mike Jones'" <Michael.Jones@microsoft.com>, "'Scott Kelly'" <scott@hyperthought.com>, <secdir@ietf.org>, <draft-ietf-jose-json-web-encryption.all@tools.ietf.org>, <iesg@ietf.org>
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Date: Thu, 11 Sep 2014 13:27:33 -0700
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Subject: Re: [secdir] [jose] secdir review of draft-ietf-jose-json-web-encryption-31
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From: jose [mailto:jose-bounces@ietf.org] On Behalf Of Mike Jones
Sent: Friday, September 05, 2014 4:13 PM
To: Scott Kelly; secdir@ietf.org;
draft-ietf-jose-json-web-encryption.all@tools.ietf.org; iesg@ietf.org
Cc: jose@ietf.org
Subject: Re: [jose] secdir review of draft-ietf-jose-json-web-encryption-31

 

Thanks for the useful review, Scott.  I've cc'ed the working group in my
reply so that they're aware of the contents of your review.  Jim Schaad -
also please see questions to you below.  Replies are inline below:

 

-----Original Message-----
From: Scott Kelly [mailto:scott@hyperthought.com] 
Sent: Saturday, August 30, 2014 6:13 AM
To: secdir@ietf.org; draft-ietf-jose-json-web-encryption.all@tools.ietf.org;
iesg@ietf.org
Subject: secdir review of draft-ietf-jose-json-web-encryption-31

 

I have reviewed this document as part of the security directorate's ongoing
effort to review all IETF documents being processed by the IESG.  These
comments were written primarily for the benefit of the security area
directors.  Document editors and WG chairs should treat these comments just
like any other last call comments.

 

>From the abstract, JSON Web Encryption (JWE) represents encrypted content
using JavaScript Object Notation (JSON) based data structures. A little like
CMS for web transactions.

 

The security considerations section begins 

 

   "All of the security issues that are pertinent to any cryptographic

   application must be addressed by JWS/JWE/JWK agents.  Among these

   issues are protecting the user's asymmetric private and symmetric

   secret keys, preventing various attacks, and helping avoid mistakes

   such as inadvertently encrypting a message to the wrong recipient.

   The entire list of security considerations is beyond the scope of

   this document, but some significant considerations are listed here."

 

  "All the security considerations in the JWS specification also apply

   to this specification.  Likewise, all the security considerations in

   XML Encryption 1.1 [W3C.REC-xmlenc-core1-20130411] also apply, other

   than those that are XML specific."

 

If you are going to point to the JWS specification, you should use a
normative reference. It's fine to point at other references to avoid
re-stating the obvious, but all security considerations *are* within scope,
and require coverage, either directly or by reference. I haven't reviewed
the referenced W3C spec, so I'm not sure that everything has been covered.
The JWS security considerations section only talks about crypto algs and
server identity verification. So, the ADs will want to pay attention here.

 

We plan to remove the sentence "The entire list of security considerations
is beyond the scope of this document, but some significant considerations
are listed here" since several reviewers have taken exception to it.

 

I'm a bit confused by your comment about normative references, because the
JWS reference already is normative.

 

Jim Schaad, etc., do you agree that the XMLENC reference should become
normative?  I'd though that earlier you'd advised me that security
considerations references should be informative.

 

[JLS] Does this need to be understood in order to implement the content
covered in the core of the document?  I would question that needing to
understand the entire XML Encryption document is necessary.  If the material
is core then it might be a better candidate for copy and paste in any event.
This is esp. true given that the material is qualified as - anything that is
not XML and the exercise of making that determination is left up to the
reader.

 

FYI, as part of addressing Russ Housley's comments on the Security
Considerations section, I do expect to explicitly reference a number of
security considerations called out in XMLENC, such as the text on
chosen-ciphertext attacks, backwards compatibility attacks, etc.

 

In section 5.1 (Message Encryption), step 16 says "Encrypt M..." without
ever defining M. One might guess it stands for Message, but this should be
stated. 

 

Agreed

 

Section 8 (TLS Requirements) points at JWS, but neither document references
the channel binding problem. If you are depending on TLS to provide
essential and necessary security features (which, presumably, you are since
TLS is a MUST), then you should give clear guidance as to how to effectively
use it. JWS requires combined confidentiality and integrity protection, and
also requires server identity verification per RFC6125, but does not mention
channel binding.

 

Scott, is there text on the channel binding problem in another specification
that you'd recommend that we reference or use?  If not, would you mind
supplying proposed text for us to use?

 

Section 11.1 (Using Matching Algorithm Strengths) says

 

  "Algorithms of matching strengths should be used together whenever

   possible.  For instance, when AES Key Wrap is used with a given key

   size, using the same key size is recommended when AES GCM is also

   used."

 

This doesn't quite scan for me, but editorial nits aside, it might be good
to say greater or equal key sizes should be used for wrapping.

 

The "matching strengths" guidance came from Eric Rescorla and I believe was
supported by then-Security AD Sean Turner.  It's not clear to me that the ?
language is better than what's there now, in part because if the strengths
don't match, it's not clear to me which way the inequality should go.

 

And you might want to point to RFC3766 for BCPs when using public keys.

 

The RFC 3766 reference looks like a good one.  Thanks for providing it.

 

Section 11.2 introduces the term "key tainting". "Strict key
management/usage policy" might be better understood. Also, it might be
valuable to use SHOULD here.

 

Jim Schaad, you suggested using the term "key tainting".  Is there a place
where this term is defined, which we could reference?

 

[JLS] There was a definition in the W3C document, but it appears to have
disappeared.  The text they had was

 

"Such mitigations may include restricting a generic key ("tainting") 

> once it has been used with a specific algorithm or operation, and only 

> permit applications to use that key with that same algorithm or 

> operation in the future."

 

Also, Jim, I believe in our in-person discussions of issue #70 (Review of
2119 Language) you'd suggested that we use 2119 keywords in the Security
Considerations statements.  Am I remembering that right, or would you prefer
that the Security Considerations sections use 2119 language?

 

I am generically opposed to the use of 2119 language in security
considerations and appendixes.  If it is of sufficient importance to warrant
this type of language it should be in the core document not in a
considerations section.  However I am not in the majority on this position.

 

I was surprised not to see any mention of the lack of replay protection. TLS
channel binding could presumably be leveraged for this purpose, but in any
event, the fact that JWEs can be replayed should be mentioned.

 

It's not clear to me that being able to decrypt an encrypted object multiple
times if you hold the correct key constitutes an attack, any more than being
able to check a signature multiple times does.

 

I agree with you that some higher-level objects that may use JWE (or JWS)
may want replay protection.  For instance,
http://tools.ietf.org/html/draft-ietf-oauth-json-web-token-25#section-4.1.7
describes a means of replay protection for JWTs.  At most, if we mention
replay protection, I would propose that we say that some applications using
JWE encryption may choose to incorporate replay protection mechanisms, such
as by including IDs in the protected content that change with each
application-level usage.  Would that work for you Scott, or is there
something else you had in mind?

 

As always, if you can supply specific proposed language to address your
concern, that would probably be the clearest statement of what you'd like to
see.

 

I would suggest that the authors read the security considerations in
rfc5652; most of the same concerns apply here, and you could almost
cut/paste from there to here.

 

Thanks.  I expect to reference some of these as well when addressing Russ
Housley's gen-art review comments of JWS.

 

For the ADs: I'm not sure if one of the companion documents provides a
comprehensive threat model, but you will want to pay attention here. This
doc does not.

 

Each doc tries to list security considerations specific to that document and
where they span documents, they are described in one and referenced in
others.

 

                                                                Thanks
again, Scott,

                                                                -- Mike