[Cryptography] Security of a permute-only system?

[Ray Dillinger]

On 11/25/2015 03:34 PM, Henry Baker wrote:
> Given a message source that's already "whitened", but otherwise unencrypted, how much security can be achieved strictly through an unknown, but random permutation?
> 
> I.e., if n=171, then a random permutation of size n would appear to require 1026 bits to specify it.
> 
> Suppose we simply applied our random permutation to each block of 171 pre-whitened bits.
> 


I forgot to mention this, because at first glance I thought
it was obvious.  However, on reflection it may not be.

You must not apply the SAME permutation to each block;
if you do, then you win absolutely nothing w/r/t security
because your opponent can then undo your permutation
via parallel anagramming.

Like a stream cipher means XORing with a random string of
bits but you NEVER repeat that random string, any security
gained from permutations only holds as long as you don't
repeat the same permutation.

				Bear

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