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Re: Compress before encrypting? (Was Re: NSA Helped Yeltsin...)
Jay Prime Positive writes:
>
> From: Martin Janzen <[email protected]>
> 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 [email protected]
Pegasus Systems Group c/o Hewlett-Packard, IDACOM Telecom Operation