Chapter 6 introduced the concept of symmetric cryptography. As the name suggests, a symmetric encryption scheme uses the same key for encryption and decryption; the key is known to the sender and recipient of the message but must remain unknown to anyone else. To communicate secretly, a sender uses an invertible cryptographic function to encrypt a plaintext m with the key k and then sends the resulting ciphertext c to the recipient. The recipient obtains the ciphertext c and uses the key k with the inverse function, thereby recovering the plaintext m. The safest way for the sender and the recipient to exchange the key so it remains unknown to outsiders is via an in-person meeting. This feature is a significant weakness of symmetric encryption methods. The Caesar Cipher introduced in Sect. 6.2 exemplifies such a symmetric encryption method.