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Re: [Cfrg] Request For Comments: OCB Internet-Draft

Simon Josefsson <simon@josefsson.org> Fri, 15 July 2011 15:09 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: Ted Krovetz <ted@krovetz.net>
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Date: Fri, 15 Jul 2011 17:09:01 +0200
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Cc: cfrg@irtf.org
Subject: Re: [Cfrg] Request For Comments: OCB Internet-Draft
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Ted Krovetz <ted@krovetz.net> writes:

>> Are there any implications for the key if a nonce is repeated?  Let's
>> say I use the same nonce all the time, and the attacker can do
>> known-plaintext attacks.  Can the attacker recover the key faster than
>> he would be able to if the nonces were not repeated?
>
> No. The only place the OCB key is used is as the key for AES. So, if
> one were able to recover OCB keys in the way you suggest, one would in
> effect have an AES key-extraction method. Since we don't think AES is
> susceptible to key-extraction, neither is OCB.
>
>> I'm trying to get AEAD cipher modes to say more than just "the security
>> properties are lost" when talking about failure modes.  "security
>> properties are lost" can mean so many things, and it is useful to be
>> able to rule out some unwanted side effects.
>
> In the ID we point out that if a nonce is reused during encryption,
> "partial information about past plaintexts will be revealed and
> subsequent forgeries will be possible". That seems specific enough for
> an RFC, don't you think?

Yes, thank you very much for clarifying!

/Simon

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