Caesar cipher

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In cryptography, a Caesar cipher, also known as a Caesar shift cipher or shift cipher, is a substitution cipher in which the cipher alphabet is the plain alphabet rotated left or right by some number of positions. For instance, here is a Caesar cipher using a right rotation of three places ("3" being the key):

Plain:  ABCDEFGHIJKLMNOPQRSTUVWXYZ

Cipher: XYZABCDEFGHIJKLMNOPQRSTUVW

To encipher a message, simply look up each letter of the message in the "plain" line and write down the corresponding letter in the "cipher" line. To decipher, do the reverse. Multiple encryptions and decryptions provide no additional security. This is because two encryptions of shift A and shift B will be equivalent to an encryption with shift A + B. In mathematical terms, the encryption under various keys forms a group.

Table of contents

1 History
2 Breaking the cipher
3 See also
4 External links

History

The Caesar cipher is named after Julius Caesar, who used it with a key of 3 to protect a message of military significance to Marcus Cicero. It was secure at the time because Caesar's enemies could often not even read plaintext, let alone ciphertext; furthermore, no cryptanalytic method was known which would reliably break such a cypher. Others are known to have used such cyphers before Caesar, so it was certainly not invented by him. Since the discovery of frequency analysis in the Arab world around 1000CE, every such cypher has been easily, even trivially, breakable. None are suitable for secure communication now, and indeed haven't been for the past 1000 years or so. An ancient Roman book on cryptography, now lost, is said to have discussed the use of such cyphers at considerable length. Our knowledge of it is due to references in other writings which have survived, as for instance Suetonius.

Breaking the cipher

By graphing the frequencies of letters in the ciphertext and those in the original language of the plaintext, a human can easily spot the value of the key by looking at the displacement of particular features of the graph. For example in the English language the plaintext frequencies of the letters E, T, (most frequent), and Q, Z (least frequent) are particularly distinctive. Computers can also do this trivially by means of an auto-correlation function.

As such a cypher system only has 26 possible keys (or, rather, as many characters as there are in the alphabet used (eg, Polish has more than 26 letters)) it is trivial even for a human to cycle through the keys trying each until they find one which allows the ciphertext to be converted into plaintext. (See also brute force attack.)

The Caesar cipher is much weaker than the random-alphabetc substitution ciphers used in newspaper cryptogram puzzles. The most commonplace Caesar ciphers found today are in children's toys such as secret decoder rings and in the ROT13 algorithm (which, of course, serving only to obscure, is meant to be trivial to decrypt).

External links

The Caesar Shift as discussed on The Beginner's Guide to Cryptography

Classical cryptography