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Re: Defaults: Getting concrete (round 2)

From: Richard Barnes <rbarnes@bbn.com>
Date: Sun, 21 Apr 2013 21:49:28 -0400
Cc: Wan-Teh Chang <wtc@google.com>, Web Cryptography Working Group <public-webcrypto@w3.org>
Message-Id: <DECA1E9B-6DAF-43CD-82CC-0A32AE3C81B4@bbn.com>
To: Ryan Sleevi <sleevi@google.com>

On Apr 19, 2013, at 6:47 PM, Ryan Sleevi <sleevi@google.com> wrote:

> On Fri, Apr 19, 2013 at 3:16 PM, Wan-Teh Chang <wtc@google.com> wrote:
>> On Thu, Apr 18, 2013 at 11:14 AM, Richard Barnes <rbarnes@bbn.com> wrote:
>>> 
>>> I agree that there are lots of protocols that have defined ways to shove things into the counter and IV fields for CTR and GCM.  They can always override the default.
>>> 
>>> I'm more concerned about newer protocols that haven't done something similar (and probably don't need to).  Those protocols just need something that meets the security requirements, and it's easy enough for the UA to provide that.
>>> 
>>> We've also seen that application designers can also get counter/IV generation badly wrong, as with the recent nonce reuse issue in JOSE:
>>> <http://www.ietf.org/mail-archive/web/jose/current/msg01967.html>
>>> 
>>> So while you're right that there are protocols that will not make use of the default, I think that newer things can benefit from having a safe default here.
>> 
>> 1. Let's first consider the counter field for the CTR mode.
>> 
>> Unless the UA knows about all the CTR mode encryptions that have been
>> done with the key in question, the UA cannot generate a new counter
>> value that hasn't been used before.
>> 
>> This requires the UA to be the exclusive user of the key in question.
>> But if the API allows the key to be exported, the UA won't be the
>> exclusive user of the key.
> 
> Or a new key imported that has been used previously.
> 
> I definitely don't think implementations should be trying to track
> what the 'used' counters are - that's certainly the realm and
> responsibility of a high-level protocol, and no API does this.

Obviously, in the fully general case, there's no way the UA can guarantee uniqueness.

There is one clear case where the UA knows for sure the entire set of counters that has been used, namely for non-exportable keys generated by the UA.  Likewise for exportable keys that have not been exported (which the UA knows).  

As Wan-Teh points out, an RBG-based CTR approach can offer very low probability of counter reuse.  For a 32-bit counter / 96-bit nonce, the probably of nonce re-use would be 2^-96 as long as no single encryption processed more than 16GB.  It wouldn't be strictly FIPS compliant, but it would be practically there.  And if apps care, they can generate their own IVs to ensure uniqueness.

I would posit that these use cases -- UA-generated keys and encryptions <16GB -- cover a broad enough swathe of likely usages that they are worth addressing.


>> 2. As to the IV field for GCM, I think the UA can use the RBG-based
>> construction of the IV in Section 8.2.2 of NIST SP 800-38D. I believe
>> this is what you proposed.
>> 
>> 3. This makes me wonder if an RBG-based construction of the counter
>> field for the CTR mode would also be acceptable if the probability of
>> reusing a counter value is low enough.
>> 
>> Wan-Teh
>> 
> 
> I think the inconsistency argument should be the one we're looking at here.
> 
> The argument for having the UA generate the IV is not one being made
> on technical requirements, but simply on the basis that "People (may)
> use it incorrectly."

I guess our disagreement is on the risk = likelihood * impact calculation for IV re-use.  I'm claiming that the likelihood of a non-crypto-expert developer re-using an IV is non-negligible, and the impact is likely to be severe.  You are apparently claiming that either or both of these factors is effectively zero.  That doesn't really seem plausible to me.


> The fact that the IV needs to be protected
> (outside of the AEAD case) should be a compelling reason enough for us
> to suggest it's a false argument being presented.

Could you clarify in what sense the IV needs to be protected?  I assume you don't mean confidentiality protection [1].  And in any case, I don't really see how that bears on how the IV is generated.

--Richard



[1] From SP 800-38A: "The IV need not be secret, so the IV, or information sufficient to determine the IV, may be 
transmitted with the ciphertext."
Received on Monday, 22 April 2013 01:50:05 UTC

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