Encryption places a message into a code. For a computer, we want to take a string of binary digits and place them into a code. In private key encryption, we have a private key which allows us to convert a binary string into a single integer number. We store or transfer over network the single integer number, to be decrypted later. Private key encryption is simple but the key must be kept secret or else it does us no good. Private key encryption is fine for storing files locally or using secure login such as ssh, but of no value for e-commerce as we cannot allow customers to know the private key. This example demonstrates how to encrypt a 13-bit binary string using a 13-bit private key. We also see how to decrypt the encrypted number.