Caesar cipher
A Caesar cipher is a substitution cipher in which the cipher alphabet is merely 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 (three being the key):
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher: XYZABCDEFGHIJKLMNOPQRSTUVWTo 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. Because this cipher is a group, multiple encryptions and decryptions provide NO additional security against any attack, including brute-force.
The Caesar cipher is named for Julius Caesar, who used it with key 3 in a message to Marcus Cicero to protect a message of military significance. It was secure at the time because Caesar's enemies could often not even read plaintext, let alone ciphertext. But since it can be very easily broken even by hand, it has not been adequate for secure communication for at least a thousand years since the Arabs discovered frequency analysis and so made all simple substitution cyphers almost trivially breakable. An ancient book on cryptopgraphy, now lost, is said to have discussed the use of such cyphers at considerable length. Our knowledge is due to side comments by other writers, such as Suetonius.
| Table of contents |
|
2 Frequency analysis 3 Brute force 4 Cipher strength 5 External Links |
Identifying the cipher
Because this system is so trivial broken it is often fast enough to perform a brute force attack to discover if this cipher is in use or not.
One easy way for humans to identify it is to examine the letter frequencies of the cipher text and see if they match those found in the underlying language.
Frequency analysis
By graphing the frequencies of letters in the ciphertext and those in the original language of the plaintext, a human can trivially spot the value of the key but looking at the displacement of particular features of the graph. For example in the English language the frequencies of the letters Q,R,S,T have a particularly distinctive pattern.
Computers can also do this trivially by means of an auto-correlation function.
Brute force
As the system only has 26 keys it is trivial even for a human to cycle through the keys trying each one until they find one which allows the ciphertext to be converted into plaintext.
Cipher strength
The Caesar cypher is much weaker than the (competently done) random substitution ciphers used in newspaper cryptogram puzzles. The most common places Caesar ciphers are found today are in children's toys such as secret decoder rings and in the ROT13 cipher on Usenet (which, of course, is meant to be trivial to decrypt).
External Links
- The Caesar Shift as discussed on The Beginner's Guide to Cryptography
