## Section8.2Symmetric Key Cryptography

Symmetric key cryptography is an application of functions. An encryption function turns readable plain text into unreadable cipher text, and the corresponding decryption function turns the cipher text back into the original plain text. Figure 8.2.1. Alice sends a message to Bob, and Eve eavesdrops on their conversation.

Descriptions of cryptographic protocols are commonly phrased as interactions between Alice, Bob, and Eve. Alice sends a message to Bob, and the eavesdropper Eve listens in on their conversion and tries to break their encryption (Figure 8.2.1). Figure 8.2.2. In a symmetric key encryption scheme, Alice and Bob share a common secret, namely the key $$K\text{.}$$

In a symmetric key encryption scheme, Alice and Bob first have to agree on a common shared key. Alice uses the key to encrypt a message and sends the encrypted message to Bob. Then, Bob uses the key to decrypt the encrypted message that was sent by Alice in order to obtain the message in its original form (Figure 8.2.2). We introduce Caesar ciphers and other substitution ciphers as examples for symmetric key encryption scheme in the following two sections.

Complete the following.

In a symmetric encryption protocol

• select

• Alice

• Bob

• Eve

• Oscar

and
• select

• Alice

• Bob

• Eve

• Oscar

agree on an encryption method, a decryption method, and a key that is used for encryption and decryption.

• select

• Alice

• Bob

• Eve

• Oscar

encrypts a message using the encryption method and the key. She sends the encrypted message to
• select

• Alice

• Bob

• Eve

• Oscar

.

• select

• Alice

• Bob

• Eve

• Oscar

• select

• Alice

• Bob

• Eve

• Oscar

and decrypts the message using the decryption method and the key.

$$\text{Alice}$$

$$\text{Bob}$$

$$\text{Alice}$$

$$\text{Bob}$$

$$\text{Bob}$$
$$\text{Alice}$$