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encryption and compression similarities -- was GRUB and boot partition

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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

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encryption and compression similarities -- was GRUB and boot  partition David Christensen <dpchrist@holgerdanske.com> - 2018-01-05 22:40 +0100

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