Losing the Code War by Stephen Budiansky

[Trei, Peter]

> Joshua Hill[SMTP:josh at untruth.org] 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.
> 
> 				Josh
> 
Either way, my point stands: any attack which requires 2^56 blocks
of storage is probably intractable for the time being, imho. 10 years
from now, I'm not so sure.

Peter Trei


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