Math Is Fun Forum

HIDDEN KEYS

There are lots of ways to make a simple substitution cipher. One already introduced is the 'random scramble'; where each leter of the alphabet is replaced with another letter from a randomly scrambled cipher alphabet (also known as the Kama Sutra cipher, since it has so many different combinations).

In practice, the random method can be cumbersome. Throughout history, ciphers have been used not simply for fun, but as a vitally important way of seeding a secret message from a sender to a reciever.

A better cipher system is one that allows the sender to encrypt a message quickly, and the reciever to decrypt it just as quickly. If there is a pattern to the scrambling, that makes things easier to remember, and helps keep the sende and receiver in synch. The reciever does not need to 'puzzle out' the plaintextmessage, but instead can quickly create plaintext to ciphertext table.

CAESAR CIPHER

The simplest way of creating a non-random cipher alphabet is called a 'Caesar cipher' after the method uce by Julius Caesar's army. Each plaintext letter is encrypted by shifting it forward three places in the alphabet. An 'A' becomes a 'D', a 'B' becomes an 'E', and so on. A key element of a Caesar cipher is that the ciphertext alphabet is the same order as the plaintext alphabet - it is just shifted left or right by a certain number of characters. Some variations nowadays may shift by a quantity other than three, buy any cipher that keeps the order of the cipher alphabet intact, and merely changes the number of letters by which it is shifted left or right, can be called a Caesar cipher.

Plaintext ABCDEFGHIJKLMNOPQRSTUVWXYZ = Ciphertext DEFGHIJKLMNOPQRSTUVWXYZABC
For example: I like orange juice = L OLNH RUDQJH MXLFH

One version of a Caesar cipher, called ROT-13, was commonly used in internet newsgroups. It effectively performs a Caesar shift of  +13, 'rotating' an alphabet by 13 characters:

The advantage of the number 13 is that it shifts exactly half of the 26-letter alphabet. This means that there is a one to one correspondence when encrypting or decrypting. An A encrypts to an N, and an N encrypts to an A.