1994-05-27 - Compress before encrypting? (Was Re: NSA Helped Yeltsin…)

Header Data

From: Martin Janzen <janzen@idacom.hp.com>
To: cypherpunks@toad.com
Message Hash: 6bf6ab0c5a496003ac42d923e781a57d16b70f2bb4c8b34eca557f0880f46a94
Message ID: <9405272043.AA23269@loki.idacom.hp.com>
Reply To: <m0q76Cl-0003paC@jpplap>
UTC Datetime: 1994-05-27 20:43:17 UTC
Raw Date: Fri, 27 May 94 13:43:17 PDT

Raw message

From: Martin Janzen <janzen@idacom.hp.com>
Date: Fri, 27 May 94 13:43:17 PDT
To: cypherpunks@toad.com
Subject: Compress before encrypting?  (Was Re: NSA Helped Yeltsin...)
In-Reply-To: <m0q76Cl-0003paC@jpplap>
Message-ID: <9405272043.AA23269@loki.idacom.hp.com>
MIME-Version: 1.0
Content-Type: text/plain

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

Or am I missing something?

Martin Janzen           janzen@idacom.hp.com