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Re: [TLS] Data volume limits

Watson Ladd <watsonbladd@gmail.com> Wed, 16 December 2015 12:13 UTC

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Date: Wed, 16 Dec 2015 07:13:21 -0500
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From: Watson Ladd <watsonbladd@gmail.com>
To: Henrick Hellström <henrick@streamsec.se>
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Subject: Re: [TLS] Data volume limits
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On Wed, Dec 16, 2015 at 7:07 AM, Henrick Hellström <henrick@streamsec.se> wrote:
> On 2015-12-16 12:17, Eric Rescorla wrote:
>>
>>     Can we see a brief writeup explaining the 2^36 number?
>>
>>
>> I believe Watson provided one a while back at:
>> https://www.ietf.org/mail-archive/web/tls/current/msg18240.html
>
>
> One rather obvious problem with trying to equate probability of loss of
> confidentiality with the advantage for an IND-KPA adversary, is that the
> IND-models don't account for the length of the plain text.
>
> The real life problem is that you lose a lot more information a lot faster,
> by revealing the amount and frequency of the data transfer, than through the
> KPA distinguisher for CTR mode.
>
> And, furthermore, the IND-KPA distinguisher is a fairly well understood
> abstract artifact of CTR mode. It is not obviously relevant to compare it to
> distinguishers for primitives such as RC4, which typically indicate that
> there might be even worse problems.

Sure it is. An attacker can distinguish RC4 from random with a very
high probability (I didn't work it out, but it's an exercise in Bayes
theorem and your favorite biases). I agree this doesn't cover side
channel information related to plaintext length and timing, but there
is a padding facility intended to help with that.

>
>
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