An early user of cryptographic techniques was the famous Roman commander-inchief and statesman Julius Caesar (100–44 BC). In the second century AD Suetonius  (The Twelve Caesars I,56) describes:

his private letters to friends, the more confidential passages of which he wrote in cipher:

to understand their apparently incomprehensible meaning one must number the letters of the alphabet from 1 to 22, and then replace each of the letters that Caesar has used with the one which occurs three numbers lower—for instance, D stands for A.

The numbers 1 to 22 are used above because the Romans used only 22 letters in their alphabet. But, for convenience, let’s pretend Caesar used the English alphabet and used the same alphabetical order as we do. We obtain Caesar’s cipher if we write the ciphertext alphabet beneath the cleartext alphabet, shifted three positions to the left. Notice that the alphabet wraps around at the end.

                  Cleartext: abc ... wxy z

                  Ciphertext: DEF ... Z ABC

We encipher a cleartext letter by replacing it by the letter beneath it. For instance,  the message “the cleartext” becomes WKH FOHDUWHAW. (Caesar didn’t use the trick of omitting blanks.) Deciphering is equally easy; we replace a ciphertext letter by the cleartext letter above it. So OHWWHU is deciphered to the message  “letter.”

Caesar chose a three-letter skip for no particular reason; one could of course get the ciphertext alphabet by shifting any number of positions. The key is the number of places you shift; Caesar’s key was 3. Since the English alphabet consists of 26 letters, we obtain exactly 26 such ciphers; they are called additive ciphers. The one with key 0 (or 26) is no good—the ciphertext is exactly the same as the plaintext—so there are 25 useful additive ciphers in English.

In the movie 2001: A Space Odyssey, the computer is called HAL; this can be obtained by moving the letters IBM one step to the left (or 25 steps to the right).