Suppose you and your friends wanted to set up a system of sending secret messages to each other You might say, “Instead of using letters, let’s use numbers.” Each number will stand for a certain letter of the alphabet.

You would then have a code. Cryptography is writing using a secret code. Sometimes the word “cipher” is used instead of code. Did you know that Julius Caesar used a cipher to keep his message secret from enemy eyes? In modern times, ciphers and codes are used by both government and business for important and secret messages.

In general, there are two kinds of cipher. One kind is the substitution of a number, letter, or other symbol for each letter in a message. The other kind is the transposition or rearrangement of the order of the letters in a message.

There are endless ways in which these two types can be used. The first type is the simpler system and is the one boys and girls usually use in a homemade cipher. The word recode is usually used for a message which can be translated by use of a code-book held by both the sender and the receiver of the message.

Codes and ciphers can be “broken,” or solved, by direct methods of deciphering and decoding. To do this, the key to a cipher or code book is necessary. These are sometimes hard to find.

A scientific method of reading cryptograms (secret messages) has been developed and is called crypto-analysis. A person reading cryptograms usually must determine what language the secret message uses. He must decide whether the message is in cipher or code. Tables of the frequency of the use of letters in a language, and many other things, are necessary in breaking ciphers and codes.