W3C home > Mailing lists > Public > public-webcrypto@w3.org > April 2013

Re: Defaults: Getting concrete (round 2)

From: Richard Barnes <rbarnes@bbn.com>
Date: Tue, 23 Apr 2013 07:16:43 -0700
Cc: Ryan Sleevi <sleevi@google.com>, Wan-Teh Chang <wtc@google.com>, Web Cryptography Working Group <public-webcrypto@w3.org>
Message-Id: <A8215264-8966-4DF8-AD4B-B88A794A1D5F@bbn.com>
To: "Acar, Tolga" <tolga.acar@intel.com>

On Apr 22, 2013, at 1:42 PM, "Acar, Tolga" <tolga.acar@intel.com> wrote:

> 
> 
>> -----Original Message-----
>> From: Ryan Sleevi [mailto:sleevi@google.com]
>> Sent: Monday, April 22, 2013 11:09 AM
>> To: Richard Barnes
>> Cc: Wan-Teh Chang; Web Cryptography Working Group
>> Subject: Re: Defaults: Getting concrete (round 2)
>> 
>> On Sun, Apr 21, 2013 at 6:49 PM, Richard Barnes <rbarnes@bbn.com> wrote:
>>> 
>>> 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).
>> 
>> No, this is not at all a realistic statement.
>> 
>> No crypto library tracks IVs. When you think of the space of possible IVs
>> (2^128), it's entirely unreasonable to suggest that UAs should.
>> Otherwise, you've implemented a trivial DoS. Heck, you've implemented
>> arbitrary storage, simply by allowing a web application to flip bits in IV
>> consumption.
>> 
>> Given how much push back there was simply to track *algorithm* usage, I'm
>> truly surprised that a discussion about tracking *IVs* is being entertained.
>> 
>> And to what end? The security assurances provided by such a solution are
>> dwarfed by the complexity of implementation and the security risks therein.
>> 
>> I again propose that we drop this topic.
> +1
> 
> IV: No crypto layer tracks IVs, and should not track IVs. In practice, IVs are random numbers.

This is my point.  We in this group know this, but the average developer does not.  So the API should just provide a random number.


> CTR: Unless there is a monotonic counter, no one tracks counters (protocols may for session keys, and that amounts to counters). Besides, monotonic counters in distributed systems is a pain in the back side where the same key may be used.

Random message nonces solve this problem with a very high degree of assurance.  There is only a need to track nonces if you wan to be strictly FIPS-compliant.

--Richard




> 
>> 
>>> 
>>> 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.
>> 
>> We should NOT be baking cryptographic protocols into the low-level API
>> - which is exactly what this is.
>> 
>>> 
>>> 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.
>> 
>> Strongly disagree here, for the reasons above.
>> 
>>> 
>>> 
>>>>> 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.
>> 
>> I suggest your logic is fundamentally flawed by attempting to design for the
>> non-crypto-expert here. We've repeatedly had this conversation
>> - particularly whenever defaults are brought up. We've repeatedly assessed
>> that "no API can serve two masters" - attempting to split the API like that
>> only serves to make a worse API (which I think the discussion on this thread
>> clearly shows how wildly inconsistent and unpredictable it becomes).
>> 
>> Again, I'm extremely sympathetic to the non-crypto-expert here - but I don't
>> think it's at all reasonable or well-thought-out to try to shoe-horn it in as a
>> last minute design consideration. These are, again, the exact same points
>> that were raised the last time we discussed defaults - and from a number of
>> people, not just 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.
>> 
>> Integrity, not confidentiality. The fundamental issue of the "Cryptographic
>> Doom Principle".
>> 
>> You're arguing that IV generation prevents footguns by some measurable
>> sense. I'm repeatedly asserting that this is demonstrably not the case
>> - and that whatever incremental value derived from trying to do so is vastly
>> eclipsed by both the implementation and cognitive complexity from the
>> wildly inconsistent API needed to service this.
>> 
>>> 
>>> --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 Tuesday, 23 April 2013 14:17:15 UTC

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