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Re: GRUB and boot partition




On 2018-01-03 01:59 PM, David Christensen wrote:
On 01/03/18 06:45, Gary Dale wrote:
On 2018-01-02 02:35 PM, Pascal Hambourg wrote:

Encryption does not require extra space (except for header and block padding). Encrypted data have the same size of cleartext data.

+1


Encryption is similar in concept to compression.

AFAIK common use on Debian systems:

1.  Encryption functions [1] are reversible functions that transform data 1:1 in size, and are lossless.

2.  Compression functions [2] are reversible functions that strive for size ratios of N:1, where N > 1 (uncompressed:compressed), and can be lossless or lossly.


A related topic is hashing functions [3] (N:fixed, can be difficult to reverse).


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?


Encryption and compression both work by replacing strings of letters with something else.

I agree that encryption and compression are both forms of coding [4].


Encryption and compression both work by replacing strings of letters with something else. What we call clear text, for example, is just a  > Caesar cipher where each letter is replaced by a number (its ASCII code or unicode).

ASCII [5] is a transliteration code -- replace a token from one alphabet (Roman letters, Arabic numerals, common English symbols, etc.) with a token from another alphabet (7-bit binary number). Spelling, grammar, and meaning are unchanged.


The goal of a substitution cipher is to conceal meaning, and can be done without changing alphabets (e.g. rot13 [6]).  Spelling and grammar are unchanged (which facilitates cryptanalysis).


Morse code, on the other hand, replaces common letters with shorter sequences of dots and dashes than less common letters.

I agree that Morse Code [7] uses frequency-based techniques.


David


References:

[1] https://en.wikipedia.org/wiki/Encryption

[2] https://en.wikipedia.org/wiki/Data_compression

[3] https://en.wikipedia.org/wiki/Hash_function

[4] https://en.wikipedia.org/wiki/Code

[5] https://en.wikipedia.org/wiki/ASCII

[6] https://en.wikipedia.org/wiki/ROT13

[7] https://en.wikipedia.org/wiki/Morse_code


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.