On 01/05/18 06:56, Gary Dale wrote: > On 2018-01-03 01:59 PM, David Christensen wrote: >> On 01/03/18 06:45, Gary Dale wrote: >>> ... Encryption is similar in concept to compression. >>> ... It could even reduce the space requirements. >> >> I'm curious -- can you cite a compressing cipher code that cannot be >> reduced to independent compression and cipher functions? > > While the intent is different, the techniques are quite similar. One > once-popular code was to replace all words in a message with 2 numbers > each, referring to a page & word number within an agreed-upon book. If > you used it electronically, you could send a binary file where each > original word was reduced to 2 bytes.
So, a substitution cipher using two levels of lookup tables. If the page number-word number index has fewer than 5.1 digits [1], the cipher compresses the message as a side effect.
For file compression, the lookup table(s) are created per-file, are optimized for encoded message size, and are transmitted with the encoded message.
For file encryption, the lookup table(s) may be generic or per-file, are optimized for cryptanalysis difficulty, and are transmitted by an alternate secure channel.
David [1] https://www.wolframalpha.com/input/?i=average+english+word+length
