Tuesday, December 2, 2008

Substitution Ciphers

A substitution cipher is one in which parts of the plaintext are substituted for something else according to the rules of the cipher. The secret message is deciphered using the inverse substitution.

Monoalphabetic Ciphers

A monoalphabetic cipher uses the same substitution across the entire message. For example, if you know that the letter A is enciphered as the letter K, this will hold true for the entire message. These types of messages can be cracked by using frequency analysis, educated guesses and trial and error.

* Caesar Cipher
* Atbash Cipher
* Keyword Cipher
* Pigpen / Masonic Cipher
* Polybius Square

Polyalphabetic Ciphers

In a polyalphabetic cipher, the substitution may change throughout the message. In other words, the letter A may be encoded as the letter K for part of the message, but latter on it might be encoded as the letter W.

* Vigenère Cipher
* Beaufort Cipher
* Autokey Cipher
* Running Key Cipher

Polygraphic Ciphers

Instead of substituting one letter for another letter, a polygraphic cipher performs substitutions with two or more groups of letters. This has the advantage of masking the frequency distribution of letters, which makes frequency analysis attackes much more difficult.

* Playfair Cipher
* Bifid Cipher
* Trifid Cipher
* Four-square cipher

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