[10373] in cryptography@c2.net mail archive
Re: Losing the Code War by Stephen Budiansky
daemon@ATHENA.MIT.EDU (Jim Gillogly)
Mon Feb 4 14:00:21 2002
Message-ID: <3C5ECCD2.95E81B8D@acm.org>
Date: Mon, 04 Feb 2002 10:02:58 -0800
From: Jim Gillogly <jim@acm.org>
MIME-Version: 1.0
To: Joshua Hill <josh@untruth.org>
Cc: "Trei, Peter" <ptrei@rsasecurity.com>,
marius <marius.corbu@analog.com>, "'Ben Laurie'" <ben@algroup.co.uk>,
cryptography@wasabisystems.com
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Joshua Hill wrote:
>
> marius wrote:
> > Not quite true. Encrypting each message twice would not increase the
> > "effective" key size to 112 bits.
> > There is an attack named "meet in the middle" which will make the
> > effective key size to be just 63 bits.
>
> Peter Trei wrote:
> > Don't forget that the MITM attack (which Schneier claims
> > takes 2^(2n) = 2^112 time), also requires 2^56 blocks
> > of storage.
> [...]
> > I don't lose sleep over MITM attacks on 3DES.
>
> Unless I'm mistaken, the 2^63 operation MITM attack referenced in the
> original message referred to Double-DES, not Triple-DES. The original
> cited value of 2^63 is incorrect; the Double-DES MITM attack (as proposed
> by Merkle and Hellman) is a known plaintext attack that takes 2^57
> operations, with 2^56 blocks of storage.
>
> Your provided values are correct for attacking Triple-DES, but I don't
> think that's what the original author was referring to.
Since 2^56 blocks of storage is borderline ridiculous, van Oorschot and
Wiener did a nice paper in Journal of Cryptology in 1999 that shows how
to do the time-memory tradeoff. For 2^40 blocks of storage it takes about
2^72 operations for 2DES. The equivalent 3DES attack costs closer to 2^175
with reasonable storage requirements.
--
Jim Gillogly
14 Solmath S.R. 2002, 17:59
12.19.8.17.5, 7 Chicchan 3 Pax, Third Lord of Night
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