Encryption is the standard means of rendering a communication private. The sender
enciphers each message before transmitting it to the receiver. The receiver (but no
unauthorized person) knows the appropriate deciphering function to apply to the
received message to obtain the original message. An eavesdropper who hears the
transmitted message hears only “garbage” (the ciphertext) which makes no sense to
him since he does not know how to decrypt it.