A cipher is a way to make a word or message secret by changing
or rearranging the letters in the message. (A code affects the
word, not the individual letters.) One type of cipher, called a
TRANSPOSITION CIPHER, is created by simply rearranging the
letters in the word itself. For example, CHYPRAGTOPRY can be
unscrambled to reveal the word CRYPTOGRAPHY. Another cipher, the
SUBSTITUTION CIPHER, is a bit harder. It involves changing the
letters of your message into something else: other letters,
numbers, or symbols. Using a substitution cipher, the word
CRYPTOGRAPHY might look like this: DOHQMRZOFQYH. In this example
C=D, R=O, Y=H, and so on. For your friends to understand the
message, they must know your system. In our substitution cipher,
your friend would need to know the KEY, or how the system is set
up (a=f, etc.). If only you and your friend have the key, then
it is very difficult for others to read your message.