Re: [Cfrg] Comments on draft-mcgrew-standby-cipher-00/draft-irtf-cfrg-cipher-catalog-01

Robert Ransom <rransom.8774@gmail.com> Sat, 28 December 2013 02:15 UTC

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Date: Fri, 27 Dec 2013 18:15:46 -0800
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From: Robert Ransom <rransom.8774@gmail.com>
To: David McGrew <mcgrew@cisco.com>
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Subject: Re: [Cfrg] Comments on draft-mcgrew-standby-cipher-00/draft-irtf-cfrg-cipher-catalog-01
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On 12/27/13, David McGrew <mcgrew@cisco.com> wrote:

> (I'd like
> to see an AEAD algorithm based on salsa20 with poly1305, assuming that
> the consensus is that salsa20 is OK, and does not need to be swapped
> with chacha.)

ChaCha is faster than Salsa20, and also provides security with fewer
rounds.  There is no technical reason to prefer Salsa20 over ChaCha.


> My opinion: whatever application is making use of the cryptography
> should *not* have to deal with nonces.   Even if the algorithm requires
> distinct nonces, if we have a single encrypter per key, then it is
> trivial to have nonces generated inside of the crypto implementation,
> and have all of that complexity hidden from the calling application.

Tarsnap had a single process encrypting messages with each symmetric
key.  Its author accidentally removed the code to increment the nonce
between messages, and because its protocol sent the nonce explicitly
with each message, no one noticed the flaw for over a year (see
<http://www.daemonology.net/blog/2011-01-18-tarsnap-critical-security-bug.html>).

Never reuse a symmetric encryption *key* for more than one
non-interactive message or one interactive connection, and never send
an explicit nonce with any message -- use a fixed nonce for
non-interactive protocols, and use the message number as the nonce in
an interactive connection protocol.


Robert Ransom