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:[email protected]] Sent: Saturday, August 30, 2014 6:13 AM To: [email protected]; [email protected]; [email protected] 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. 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? 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 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
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