another feature RNGs could provide

[Travis H.]

On 12/26/05, Ben Laurie <ben at algroup.co.uk> wrote:
> Surely if you do this, then there's a meet-in-the middle attack: for a
> plaintext/ciphertext pair, P, C, I choose random keys to encrypt P and
> decrypt C. If E_A(P)=D_B(C), then your key was A.B, which reduces the
> strength of your cipher from 2^x to 2^(x/2)?

Almost true.  The cardinality of the symmetric group S_(2^x) is
(2^x)!, so it reduces it from (2^x)! to roughly sqrt((2^x)!).  That's
still a lot.

I suspect this is some information-theoretic limit for x-bit block ciphers.
--
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