Substitution (cryptography)

As substitution (from Latin : substituere = "replace") is called in cryptography , ie in the branch of science of cryptology , which deals with the secret writing is concerned, one of the two basic encryption classes .

method

Substitution is characterized by the fact that letters or characters or groups of letters or groups of characters in a plain text to be encrypted are replaced (substituted) by other characters, called ciphertext characters .

Because plain text characters are substituted by ciphertext characters, they lose their "character", that is, their appearance, and are therefore no longer recognizable to unauthorized eyes, at least at first glance. Claude Shannon referred to this with the word "confusion".

A simple example of a substitution would be to replace each letter in this text with the one following it in the alphabet , i.e. replacing A with B, B with C, and so on. This particularly simple encryption method, but with a shift of three instead of just one alphabet position, was already used by Gaius Iulius Caesar and is called Caesar encryption in his honor .

In terms of the classification of the different substitution methods, the Caesar encryption belongs to the simple ( i.e. monographic ) monoalphabetic substitutions . Monographic because single characters are replaced and monoalphabetic because only a single fixed alphabet is used as the key . An example of a bigraphic substitution, in which not single characters but pairs of characters are substituted, is the Playfair method .

In contrast to the monoalphabetic substitutions are the polyalphabetic substitutions , in which several (many) different alphabets are used for encryption. Examples of this are Vigenère encryption and the ENIGMA key machine .

The second basic encryption class besides substitution is transposition , in which the characters do not change their "character", but rather their place, that is, their position in the text. Substitution and transposition are classic encryption methods, which, however, still form the basis of modern encryption methods in a variety of ways, such as the Advanced Encryption Standard (also called Rijndael by and after its developers ).

See also

• Cryptography terminology
• Homophonic encryption - The plaintext characters can be substituted by different ciphertext characters.
• One-Time-Pad - Perfect form of substitution encryption
• Polygram substitution (also: polygraphic substitution) - Instead of individual plaintext characters, character N-grams (e.g. groups of letters) are substituted.

literature

• Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, ISBN 3-540-67931-6 .
• Alfred J. Menezes, Paul C. van Oorschot, Scott A. Vanstone: Handbook of Applied Cryptography. CRC Press, Boca Raton FL et al. 1996, ISBN 0-8493-8523-7 , p. 17.
• Fred B. Wrixon: Codes, Ciphers & Other Secret Languages. From the Egyptian hieroglyphs to computer cryptology. Könemann, Cologne 2000, ISBN 3-8290-3888-7 , p. 168ff.

Web links

• CrypTool.de Free cryptology tutorial CrypTool
• CrypTool-Online : Online encryption of Caesar and other classic methods
• Cryptography playground : online encryption by Caesar and a number of other classic methods

Individual evidence

1. ↑ Friedrich L. Bauer: Deciphered secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, p. 46 ff.
2. ^ Claude Shannon : Communication Theory of Secrecy Systems. In: Bell System Technical Journal. Vol 28, October 1949, ISSN  0096-8692 , p. 708. Retrieved January 7, 2012. PDF; 0.6 MB