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

[Mike Jones <[e-mail]>]

Thanks for your review, Richard.  Replies are inline below...

> -----Original Message-----
> From: jose [mailto:[e-mail]] On Behalf Of Richard Barnes
> Sent: Wednesday, October 01, 2014 7:36 PM
> To: The IESG
> Cc: [e-mail]; draft-ietf-jose-json-web-
> [e-mail]; [e-mail]
> Subject: [jose] Richard Barnes' Discuss on draft-ietf-jose-json-web-algorithms-
> 33: (with DISCUSS and COMMENT)
> 
> Richard Barnes has entered the following ballot position for
> draft-ietf-jose-json-web-algorithms-33: Discuss
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> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> Section 3.2.
> "This computed HMAC value is then compared to the result of base64url
> decoding the received encoded JWS Signature value."
> Need to add:
> "In order to avoid timing attacks, the comparison of the computed HMAC value
> to the JWS Signature value MUST be done in a constant-time manner."

OK

> Section 3.6.
> I'm not going to object to "none", even though I think it's a very dangerous
> feature because of the risk of confusion between Secured and Unsecured JWS.
> But there needs to be stronger guidance:
> 1. An implementation SHOULD NOT support "none" unless the implementor
> knows that it will be used in application context s that require it.
> 2. If an implementation does support "none", then it MUST NOT accept it as part
> of generic JWS validation.  Instead, it should require the application to explicitly
> signal that an Unsecured JWS is expected for a given validation operation.

As discussed in the working group, your concern about applications inappropriately allowing the use of "none" actually is an instance of a more general concern that applications not allow *any* algorithms to be used that are not appropriate in their application contexts.  This concern is already addressed in the specification at the end of Section 5.2 as follows:

"Finally, note that it is an application decision which algorithms are acceptable in a given context. Even if a JWS can be successfully validated, unless the algorithm(s) used in the JWS are acceptable to the application, it SHOULD reject the JWS."

Since your specific concern is already handled in a more general way, I would like to request that you withdraw this DISCUSS on that basis.  Also, you were one of the contributing authors to the security considerations on this topic in Section 8.5 of JWA (Unsecured JWS Security Considerations), so it's not clear that there's any cause for you to come back with additional wording change requests on this topic at this point.

> Section 4.2.
> Systems that support RSAES-PKCS1-V1_5 key unwrap are commonly vulnerable
> to oracle attacks based on whether they accept the wrapped key or not.
> See, e.g.,
> https://www.w3.org/Bugs/Public/show_bug.cgi?id=25431
> http://cryptosense.com/choice-of-algorithms-in-the-w3c-crypto-api/
> In light of that, it seems irresponsible to include this algorithm without extensive
> security precautions, and especially irresponsible for it to be REQUIRED.  It's
> been dropped from WebCrypto, and is being dropped from TLS in v1.3.

The reasons for its inclusion and security considerations about it are already covered in Section 8.3 of JWA (RSAES-PKCS1-v1_5 Security Considerations).  If you'd like to beef up the text there, specific additional proposed wording would be welcomed.  It's required because it's the one asymmetric key encryption algorithm that appeared to be ubiquitously deployed across development platforms.  Otherwise, there would be no practical basis for interoperability for this functionality.

> Section 6.3.1.
> The descriptions of these parameters are really vague, especially when it comes
> to the "oth" parameters.  Please cite a reference that provides more detail, e.g.,
> RFC 3447.

I agree that an RFC 3447 reference would be appropriate.

> Section 6.3.2.6.
> This section defines the wrong parameter.

Thanks!

> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> Section 1.1.
> The pointer for BASE64URL should be to JWS.  One level of indirection, please :)

Agreed

> Section 3.1, 4.1, and 5.1.
> As I said in the working group, the implementation requirements in these
> registries should be removed.  They are unnecessary for interoperability, and
> highly likely to be ignored by implementors, both because (1) many
> implementations are for specific applications that do not require all of the
> REQUIRED algorithms, and (2) many implementations use cryptographic libraries
> that do not support some REQUIRED algorithms.  I have personally written more
> than one JWS/JWE implementation that ignored these requirements, for exactly
> these reasons.  (This would be a DISCUSS for me, if not for my having made this
> argument already in the WG.)

For what it's worth, apparently Stephen Farrell disagrees with you (as do many in the working group).

> Section 3.2.
> "A key of the same size as the hash output (for instance, 256 bits for
> "HS256") or larger MUST be used with this algorithm."
> A pointer to Section 3 of RFC 2104 here would be helpful.  I was surprised at this
> requirement, given that FIPS 198 says "The size of the key, K, shall be equal to or
> greater than L/2, where L is the size of the hash function output."

The thread with Jim Schaad on this topic concluded as follows:

[JLS] This does not seem to be the statement in sp800-107 rev1 (section
5.3.4) which updated FIPS 198.   It says that the security = min (length of
K, 2*C) where C is the internal barrel length.

Ah, OK.  Thanks for the updated reference.  I stand corrected.
--Richard

> Section 3.4.
> It might be worth noting that though this format seems ad-hoc, it is the same
> used by WebCrypto.

I'll think about if there's a non-intrusive way to add this.  Are there other places also using this representation?  I'd thought that there were.

> Section 4.7.1.1.
> Shouldn't you require that this field MUST encode a 16-octet / 128-bit value?

Actually, 4.7 already states "Use of an Initialization Vector of size 96 bits is REQUIRED with this algorithm."  (See the GCM spec for why.)  But I could add "96 bit" to the description.
 
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				Thanks again,
				-- Mike