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

Ted Krovetz <ted@krovetz.net> Fri, 15 July 2011 15:04 UTC

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From: Ted Krovetz <ted@krovetz.net>
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Subject: Re: [Cfrg] Request For Comments: OCB Internet-Draft
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> 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?