Caesar Ciphers

One of the earliest substitution cipher described by Julius Caesar in the Gallic Wars. qIn this cipher each of the letters A to W is encrypted by being represented by the letter that occurs three places after it in the alphabet. qAlthough Caesar used a ‘shift’ of 3, a similar effect could have been achieve using any number from 1 to 25. ¤In fact any shift is now commonly regarded as defining a Caesar Cipher.

The encryption key and decryption key are both determined by a shift but the encryption and decryption rules are different. qWe could have changed the formulation slightly to make the two rules coincide and have different encryption and decryption keys. ¤A shift of 26 has the same effect as a shift of 0 and, for any shift from 0 to 25, encryption with that shift is the same as decryption with the new shift obtained by subtracting the original shift from 26. ¤E.g: encryption with shift 8 is the same as decryption with shift 26 - 8 =18.

¤This enable us to use the same rule for encryption and decryption with the decryption key 18 corresponding to the encryption key 8. Caesar ciphers are vulnerable to exhaustive key search attack. ¤To work through all the 26 keys. Furthermore the key can be determined from knowledge of a single pair of corresponding plaintext and ciphertext characters.

single key search may not identify the key uniquely. ¤It is much more likely merely to limit the number of possibilities by eliminating some obviously wrong ones. ¤An exhaustive search for the encryption key for cryptogram HSPPW yields two possibilities that lead to complete English words for the assumed message. ¤These shifts are 4, that gives DOLLS, and 11, that gives WHEEL. ¤When this happens we need more information, possibly the context of the message, or some extra ciphertext, before we can determine the key uniquely.