Re: [TLS] Comments on nonce construction and cipher text size restriction.

Quynh Dang <quynh97@gmail.com> Tue, 24 May 2016 20:15 UTC

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Date: Tue, 24 May 2016 16:13:47 -0400
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From: Quynh Dang <quynh97@gmail.com>
To: Eric Rescorla <ekr@rtfm.com>
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Cc: tls@ietf.org
Subject: Re: [TLS] Comments on nonce construction and cipher text size restriction.
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Are you worried about 2^96 precomputation and the risk of 1/2^32 of
cracking your key?

Quynh.
On May 24, 2016 3:05 PM, "Eric Rescorla" <ekr@rtfm.com> wrote:

>
>
> On Tue, May 24, 2016 at 12:00 PM, Dang, Quynh (Fed) <quynh.dang@nist.gov>
> wrote:
>
>>
>>
>> On 5/24/16, 2:42 PM, "Martin Thomson" <martin.thomson@gmail.com> wrote:
>>
>> >On 24 May 2016 at 10:46, Dang, Quynh (Fed) <quynh.dang@nist.gov> wrote:
>> >>>We discussed this at quite some length.  I originally took your
>> >>>position, but the IVs add an extra layer of safety at very little
>> >>>cost.
>> >>
>> >> I don¹t see any extra layer here.
>> >
>> >
>> >The argument here is that there are only 2^128 keys and some protocols
>> >have predictable plaintext.  A predictable nonce would allow an
>> >attacker to do some pre-calculation with a large number of keys to get
>> >a chance of a collision (and a break).  It's a long bow, but not
>> >entirely implausible.
>>
>> Ciphers use nonces are designed/proved to be secure when nonces are
>> predictable: nonces are not random values.
>>
>
> I think you may be misunderstanding. There is a time/space tradeoff here
> when the
> nonces are predictable that does not exist when they are random. This is
> not a
> vulnerability in the cipher and applies even if the keystream generator at
> the core
> of the cipher is PRF_k(nonce).
>
> -Ekr
>
>
>> >
>>
>>
>
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