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

Watson Ladd <watsonbladd@gmail.com> Tue, 24 December 2013 14:45 UTC

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Date: Tue, 24 Dec 2013 09:45:52 -0500
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From: Watson Ladd <watsonbladd@gmail.com>
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Subject: [Cfrg] Comments on draft-mcgrew-standby-cipher-00/draft-irtf-cfrg-cipher-catalog-01
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Dear all,
David McGrew invited us to comment on these, and since they cover
similar ground I decided to take them both on at once.

The basic problem I see is one is a bibliography and the other is a
wishlist. I don't see them
as being useful to someone simply asking the questions "what should I
use?" and "how do I
ensure algorithm agility is useful?"

The cipher catalog needs to be kept up to date, and I don't really see
a reasonable mechanism for doing it in the form of an internet draft.
Something along the lines of the X lounge, where X is hash function,
pairing, block cipher, stream cipher, or ala EFD/Safecurves is
probably a better format. It also needs a summary of the best attacks:
I should see something like "chosen ciphertext 2^36 texts, 2^192.5
time" for all ciphers, not just some. It's also missing Salsa20 and
ChaCha20.

The standby cipher document is a wishlist. It lists desirable
features, but doesn't propose any block cipher that can meet those
requirements. Instead it hopes that someone, somewhere will propose a
cipher that works.

That said these are the sorts of documents we need to see more of. I
personally think we should make a document "How to use a short shared
secret to protect a stream of messages", so as not to expose WGs to
the complexity of modes of operation.
It's a very solved problem, lots of protocols try to solve it, and
many get it wrong. Before I write a draft I'ld like to see if anyone
has strong objections to AES-GCM with counters for
nonces/XSalsa20+Poly1305 with counters for nonces. This is what is
deployed in Chrome, and both are fast on commodity chips and have
explicit security guarantees.

If possible both should be options: AES-GCM for speed on new Intel
chips, XSalsa20+Poly1305 on all other 32-bit chips with side channel
security.

Sincerely,
Watson