1994-05-30 - Re: Compress before encrypting? (Was Re: NSA Helped Yeltsin…)

Header Data

From: Martin Janzen <janzen@idacom.hp.com>
To: cypherpunks@toad.com
Message Hash: f245caa9353cd2fe789a794276d6a4f70392e2552b31fbc0452608444c0a1fa8
Message ID: <9405301616.AA27727@loki.idacom.hp.com>
Reply To: <m0q87CU-0003paC@jpplap>
UTC Datetime: 1994-05-30 16:16:15 UTC
Raw Date: Mon, 30 May 94 09:16:15 PDT

Raw message

From: Martin Janzen <janzen@idacom.hp.com>
Date: Mon, 30 May 94 09:16:15 PDT
To: cypherpunks@toad.com
Subject: Re: Compress before encrypting?  (Was Re: NSA Helped Yeltsin...)
In-Reply-To: <m0q87CU-0003paC@jpplap>
Message-ID: <9405301616.AA27727@loki.idacom.hp.com>
MIME-Version: 1.0
Content-Type: text/plain

Jay Prime Positive writes:
>   From: Martin Janzen <janzen@idacom.hp.com>
>   Date: Fri, 27 May 94 14:43:02 MDT
>   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.
>  In short, you are right, compression algorithms often _do_ include a
>magic number at the begining.
>  However, compression algorithms intended for cryptographic
>applications don't have to include a magic number.  This is especialy
>true if the crypto system is never used without the compression
>algorithm. [...]

OK; so ideally this is something that would be built in to one's
encryption/decryption program.  I was thinking of UNIX compress,
gzip, and the like.

>  Finaly, the state of the art in cryptanalysis (as far as I know),
>sugests that modern crypto systems aren't as vulnerable to known
>plaintext as past systems.  The best attacks I know of (differential,
>and linear cryptanalysis) require masive (about 2^30 blocks for DES)
>amounts of known, or chosen, plaintext -- though miniscule relative to
>the key size (2^56 again for DES).

That's good to know!  Thanks for the explanation, Jay.

Martin Janzen           janzen@idacom.hp.com
Pegasus Systems Group   c/o Hewlett-Packard, IDACOM Telecom Operation