[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?

All of it, I think.  If you mean "whitened" like a stream cipher, and
then put a permutation per-block on top of it, you get at least as much
security as the stream cipher and then deny the opponent the opportunity
to take advantage of the bit-masking properties of stream ciphers.

If you mean "whitened" like XOR a LFSG, I think it would still be pretty
impossible to take any advantage of the linear properties of the LFSG
if the bits were randomly scrambled across a large block.

To be sure of real resistance to attack, however, at least one of
those things - the whitening stream or the permutation selection -
really does need to be cryptographically secure.

				Bear

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