• Table of Contents

Symmetric Cipher Model

Symmetric encryption, also known as single-key encryption was one of most widely used of the two types of encryption we’ve briefly covered in the first lecture.

Figure 2.1 Simplified model of symmetric encryption.

A symmetric encryption scheme has five ingredients:

1. Plaintext: Original, intelligible input message or data.
2. Encryption Algorithm: Performs substitutions and transformations on the plaintext.
3. Secret Key (Private Key): Input into the algorithm.
4. Ciphertext: Scrambled output dependent on plaintext and private key.
5. Decryption Algorithm: Reverses encryption process. Transforms ciphertext back into original plaintext.

Classical encryption techniques enables us to illustrate the basic approaches to symmetric encryption.

Substitution Techniques

The two basic building blocks of all encryption techniques are substitution and transposition. Substitution involve the substituting a ciphertext symbol for a plaintext symbol.

Caesar Cipher

The simplest, use of a substitution cipher was by Julius Caesar. The Caesar cipher involves replacing each letter of the alphabet with the letter positioned three places down the alphabet.

Figure 2.2 Caesar cipher.

To understand this substitution better, we can assign numerical equivalents to each letter, as shown below.

Figure 2.3 Numerical equivalent of the alphabet.

Then, the algorithm can be expressed as: