Re: Basic tools of encryption: Transposition and substitution?

Doug wrote:
> I was involved in a project about 2 years ago where we implemented
> DES from scratch, did some other funky PGP based stuff and as I
> recall from then, as with Singh the basic tools of encryption
> were always Transposition and Substitution.
>
> Now, it strikes me as odd that compression doesn't fall into this
> category as well. Surely compression is a highly effective crypto
> technique: by reducing the size of the cypher text compared to
> the plain text, you make it much more difficult to break the
> code.

   Compression itself doesn't have a key. If you send a compressed
message, what is it that keeps eavesdroppers from being able to read it?
Secrecy of algorithm?

> Furthermore, as you approach perfect compression the compressed data
> approaches pure random data (because in theory any pattern can be
> used to further compress the data).
>
> Why doesn't compression fall in as one of the basic crypt techniques?

   Because well-implemented encryption provides confidentiality and
authentication of any message, regardless of whether it's been
compressed or not. Hence, it's convenient to regard compression and
encryption as independent.

--Mike Amling
Received on Tue Jan 3 03:41:57 2006