ABSTRACT
Bob wants to send a message to Alice so that Alice, and only Alice, can read it. For example, Alice may own a business and Bob may need to send her his credit card number so that he may make a purchase. This message is usually called the plaintext. To keep the message secret, he encrypts it to a secret, unreadable form called the ciphertext, which he sends to Alice. Alice then decrypts to obtain the original plaintext.
For a simple example, suppose Bob’s original message is ZEBRA. He decides to shift each letter forward by four positions to obtain the ciphertext DIFVE (note that the Z wraps around to the beginning):
