### Unit 7: Digital Signature and Entity Authentication

This unit begins with a general discussion of key exchange methods, or methods designed to distribute keys securely so that they can be later used in a cryptographic algorithm. This unit also describes the difficult problem of computing the discrete logarithm, which is of greatly interest to cryptologists by virtue of its ElGamal signature scheme.

The unit will then cover five additional schemes (trusted certificates, private certificates, a modified Schnorr algorithm, a modified Guillou-Quisquater algorithm, and a modified Mu-Varadharajan algorithm) before ending with an overview and discussion of public key infrastructure and a lecture by James Massey.

**Completing this unit should take you approximately 12 hours.**

### 7.1: Key Exchange

Read this article to learn how to exchange cryptographic keys between users so a cryptographic algorithm can be used.

### 7.2: Discrete Logarithm

Read about the discrete logarithm problem, which is of great interest in the field of cryptography.

### 7.3: The ElGamal Signature Schemes

Read this article to learn about the system parameters, key and signature generation, verification, correctness, and security of the ElGamal signature scheme.

### 7.4: Autokey Identity Schemes

Read this page to learn about autokey identity schemes.

Read this article for general overview of public key infrastructure.

### 7.5: Cryptography - Science or Magic?

Watch this lecture about cryptography. Professor Massey presents a number of topics, including No-break Cryptography, No-leak Secret Sharing, No-key Cryptography, and No-watch Coin Tossing,