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Re: [Cfrg] ChaCha20 and Poly1305 for IPsec

CodesInChaos <codesinchaos@gmail.com> Tue, 07 January 2014 14:02 UTC

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Date: Tue, 07 Jan 2014 15:01:52 +0100
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From: CodesInChaos <codesinchaos@gmail.com>
To: Watson Ladd <watsonbladd@gmail.com>
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Subject: Re: [Cfrg] ChaCha20 and Poly1305 for IPsec
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> However, DJB did use XSalsa20 instead of XChaCha20 (a hypothetical longer nonce variant) in NaCl

ChaCha was published in January 2008, NaCl was published in March
2009. So I'd guess Salsa20 was chosen over ChaCha because it had seen
more analysis in the eStream contest whereas ChaCha was pretty new at
the time.

I think that's less of a concern now, most attacks were applied
against both Salsa20 and ChaCha, with ChaCha resisting them slightly
better.

The biggest difference in analysis I'm aware of is that there is a
proof of security against differential cryptoanalysis for Salsa20, but
not for ChaCha. This proof relies on the alternation of xor and
addition in Salsa20, it doesn't seem trivial to port it to ChaCha.
http://eprint.iacr.org/2013/328

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