Simple Crypto Operations

In a pure transposition cipher, the text “AAAAAA” is transformed to “AAAAAA”.

In a pure, static substitution cipher, the text “AAAAAA” is transformed to (say) “ZZZZZZ”.

In a vignere pad, the string “AAAAAA” input  is transformed to an output which is equal to the keyword (or an offset of all its letters, making it easy to guess).

If the keyword were completely random and not used again, but still shared between two parties, one would arrive at the one-time pad, which has perfect secrecy.  Is this possible ? Wikipedia says information-theoretic secrecy is used in top secret communications.

The Enigma cipher was a dynamic substitution cipher where the substitution maps changes as the rotors rotate before each letter is entered. This machine and its code-breaking machine is the subject of the movie The Imitation Game, discussed in my previous blog post.

7 thoughts on “Simple Crypto Operations

    1. The Enigma refers to a specific machine (link added) which used 3 rotors. Each rotor implemented a specific random map from A-Z to A-Z. These rotations produced a variation of the map each time a letter was entered – producing what’s called a polyalphabetic (dynamic) substitution cipher. An enigma.


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  2. Secure Keypads do exist and payment pin entering system do use them in million of terminal every day. They use the same random key generation that you have mention and create a new representation of each number every time you punch numbers.


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