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
.

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