Re: Basic tools of encryption: Transposition and substitution?

Doug wrote:
> Is the difference between "compression" and "encryption" just one
> of those random things where a line is drawn in the sand arbitrarily,
> or is compression actually not much use for encryption?

Lossless compression using a fixed algorithm can readily be
undone by the attacker. Compression has value in increasing
the entropy density (reducing the redundancy), which makes
statistical methods of attack less successful.

The division of cryptographic primitives into transposition
versus substitution arose a long time ago, when those were
almost the only methods in use. (There was also some
steganography.) Since they're both just mappings they fit
into a more general notion of transformation, these days
usually referred to as simply "coding". Similarly for the
distinction between codes and ciphers, the distinction being
merely in the basic units that are handled atomically in the
system, which these days go down to the bit level or even
lower (e.g. "chips" used in modern wireless modulation).
Received on Tue Jan 17 16:48:37 2006