This thread started with Joshua's premise that because both compression
and encryption "reduce redundancy", so compression could also function
as encryption.
A clearer way of saying that they "reduce redundancy" is to say that
they increase the entropy (randomness) of a message. Each does so for
different reasons. Compression does it because orderly components of a
message can be encoded much more efficiently than random components.
Encrytion does so to hide any order in the original message.
As M.G. points out, hiding order in a message helps protect
against some forms of attack, but not others. There are other criteria
a cipher should satisfy in order to be "secure" (for example, to defend
against chosen plaintext attack I believe you need it to be difficult to
determine the key from any plaintext/ciphertext pair encrypted with
that key).
M.G's point on combining compression and encryption is quite interesting.
The reason behind it is because since compression already reduces order
in the message (ie. increases entropy), it makes the cipher's job of
hiding order in it much simpler. The other way works quite poorly tho...
compression algorithms tend to have great trouble with encrypted messages
because the messages have too high an entropy for efficient encoding.
So the lesson is: if you want a message both compressed and encrypted,
ALWAYS DO THE COMPRESSION FIRST!!!!
Cheers,
--------------------------------+-----------------------------------
Dennis Clark | Email: dennis@nospam.ilanet.slnsw.gov.au
Programmer, ILANET | Tel: +61 2 230 1424
State Library of NSW | Fax: +61 2 232 8701
"What a tangled WWW we weave!"