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Encoding (encrypting) documents has a long history. There are several reasons to do this. The sense used here is for privacy and compactness.

Privacy is based on shared secrets, shared at least between an individual and an IT system keeping their information private. For private exchange between individuals they need to share secrets, preferably secrets that are just used for that exchange. Any exchanged information should look like random, white, noise to anyone who does not have access to the secrets the encoding is based on, as any pattern gives a basis on which to attempt to understand it.

Large and complex secrets are best avoided, as they tend to need to be recorded somewhere in a form others can access. Experience has shown that privacy that depends on secret algorithms is rarely maintained for long. Open algorithms that others are prepared to test for reliability seem to have produced the best results. Secrets that are memorable, to an individual, are needed.

Information theory shows that if information is made as compact as possible, all redundant information is removed from it, then it shares many of the characteristics of white noise. However, there must still be a pattern to it that an open algorithm uses to expand it into a more conveniently usable form. This implies that compaction combined with encoding with at least one secret, which obscures that pattern, should give quite good privacy. For extra security a second level of encoding, which uses a different secret or secrets, can be applied.

Optimum compaction does not necessarily give the best security. If a known, already optimally compacted, document is introduced into the system then this might become the basis of a 'known text' attack. If an optimally compacted document is desired to be exchanged then it should be de-compacted, rearranged, then re-compacted using a shared secret, but not quite optimally. Preferably there should be a number of different compaction algorithms which a shared secret selects between, including within a (already rearranged) document.

Encryption on Wikipedia

(c) ROMsys Ltd, December 2013, permission given to use for non-profit making purposes

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