Privacy has been a worry throughout human history. As soon as people learned to write, they wanted to be able to encrypt their messages so not everyone could read them. The earliest ciphers were simple, you would replace a letter with a number or another letter. The receiver of the message would use a key to decipher the meaning. Simon Singh explains the history and the mechanics of ciphers in his new paperback, "The Code Book". While secret messages most often are used in government, the military and commerce, Singh told Minnesota Public Radio's Stephanie Curtis, that regular people have used secret codes, too, usually for very personal reasons.