I think perhaps what you mean by compress is Hash? A hash reduces any block of text into a fixed length "Message Digest"(Fancy word for the result of a hash). For example, there is a hash called MD5. In the MD5 hash, any data you put in results in a 32bit string of random junk. This can be used to compress large amounts of data, into something you can compare too. But Hashes are one way. Once you encrypt data, you can't decrypt it. So it is useful for things like passwords and personal details, but other than that I don't see how it can be very useful. I don't think you can compress raw string data? Perhaps if you find an encryption that reduces it to only a few letters, that will be useful.
