### Unit 3: Block Ciphers

In this unit, we will start with an explanation of the substitution-permutation network, which works through the series of linked mathematical operations used in block cipher algorithms. Note that substitution-permutation networks take a block of plain text and the key as inputs and then apply several alternating rounds of substitution and permutation boxes to encipher the data.

This unit also uses the complete mathematical algorithm to describe the data encryption standard before finishing with a description of the advanced encryption standard for a symmetric-key encryption adopted by the U.S. government.

**Completing this unit should take you approximately 14 hours.**

### 3.1: Substitution-Permutation Network

Read this article to learn about substitution-permutation networks.

### 3.2: Data Encryption Standard

Read this article to learn about the encryption standards given by National Institute of Standards and Technology. Study the material under the "Data Encryption Algorithm" section in detail. Spend time on this to make sure you know how the encryption algorithm works.

### 3.3: Advanced Encryption Standard

Read this article about the advanced encryption standard.

### 3.4: Abstraction in Cryptography

Watch this lecture about layers of abstraction in cryptography.