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Re: [TLS] [Cfrg] Collision issue in ciphertexts.

Watson Ladd <watsonbladd@gmail.com> Mon, 02 November 2015 10:07 UTC

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Date: Mon, 02 Nov 2015 05:07:45 -0500
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From: Watson Ladd <watsonbladd@gmail.com>
To: "Dang, Quynh" <quynh.dang@nist.gov>
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Cc: cfrg@ietf.org, tls@ietf.org
Subject: Re: [TLS] [Cfrg] Collision issue in ciphertexts.
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On Nov 2, 2015 2:14 AM, "Dang, Quynh" <quynh.dang@nist.gov> wrote:
>
> Hi Eric,
>
>
> As you asked the question about how many ciphertext blocks should be safe
under a single key, I think it is safe to have 2^96 blocks under a given
key if the IV (counter) is 96 bits.

This is wrong for PRP, right for PRF. It's not that hard to find the right
result.

>
>
> When there is a collision between two ciphertext blocks when two
different counter values are used , the chance of the same plaintext was
used twice is 1^128.  Collisions start to happen a lot when the number of
ciphertext blocks are above 2^64. However, each collision just reveals that
the corresponding plaintext blocks are probably different ones.

Which breaks IND-$. Let's not be clever, but stick to ensuring proven
definitions are true.

>
>
>
> Quynh.
>
>
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