# CSE4303 Introduction to Computer Security (Lecture 12)

## Asymmetric Encryption

### Motivation

#### Symmetric key exchange: TTPs

Idea: Trusted Third Party (TTP) is always-available key manager

- Assume secure channel exists between every user and TTP
  - KA = shared key between user A and TTP
- Generates/distributes keys to user pairs on demand
  - KAB = shared key between user A and user B

Symmetric-key encryption: challenges

1. Key storage: one per user pair, $O(n^2)$ total for $n$ users
2. Key exchange: how to do it over non-secure channel?

### Key Exchange

[Diffie-Hellman](https://notenextra.trance-0.com/CSE442T/CSE442T_L16/#diffie-helmann-key-exchange)