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encryption and compression similarities -- was GRUB and boot partition

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.


[1] https://www.wolframalpha.com/input/?i=average+english+word+length