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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.


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.



[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.