Compress before encrypting? (Was Re: NSA Helped Yeltsin...)

[Martin Janzen <janzen@idacom.hp.com>]

Jay Prime Positive writes:
>[...]
>If you suspect that some of the non DOD/NSA cyphers might be broken,
>but you are not ready to employ one-time-pads, then you should
>threshold you mesages into N parts so that all N are needed to recover
>the original.  Then encrypt each part under a different cypher.
>
>Perhaps IDEA, and 3DES would be apropriate.  This will not increase
>the size of your messages very much since you compress before
>encrypting -- don't you?

Most compression programs add a characteristic signature to the beginning
of the compressed output file.  If a cryptanalyst guesses that you may
be compressing before encrypting, wouldn't this make his job easier?
To me, this sounds as though you're adding a known bit of "plaintext" to
the start of each message.

If you're encrypting files that you wish to store securely you could just
clip off the signature, I suppose.  But this would be unsuitable for sending
messages, because your compression program is now incompatible with everyone
else's.

Or am I missing something?

-- 
Martin Janzen           janzen@idacom.hp.com