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From: gnu@hoptoad.UUCP
Newsgroups: net.crypt
Subject: Re: Decryption of corrupted files...
Message-ID: <757@hoptoad.uucp>
Date: Sat, 3-May-86 08:20:42 EDT
Article-I.D.: hoptoad.757
Posted: Sat May  3 08:20:42 1986
Date-Received: Tue, 6-May-86 04:08:03 EDT
References: <60@rtgvax.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 18

The traditional way to improve data reliability when there is no chance
to recover the data is called "forward error correction".  It adds some
redundant data to the message.  This kind of thing will be used in
Stargate for example, since you can't ask the satellite to send it
again please over a receive-only link.  Disk error correcting codes
are like this too (Fire codes).

But:  Any kind of redundancy you put in your cleartext (or enciphering
algorithm) will cause it to be easier to break.  A simple example.
Let's say you put a parity bit in each character before encrypting.  A
code-breaker who tried a key that produced bad parity characters would
know that that was the wrong key.  Knowing this, and the enciphering
algorithm, lets a codebreaker eliminate a lot of possible keys quickly.

There's probably no simple way to make your data both easier to recover
(for you) and harder to recover (for a codebreaker).
-- 
John Gilmore  {sun,ptsfa,lll-crg,ihnp4}!hoptoad!gnu   jgilmore@lll-crg.arpa