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# Unit 2: Cryptography

## About this unit

How have humans protected their secret messages through history? What has changed today?

Explore how we have hidden secret messages through history.

### Learn

Assess your understanding of the code breaking presented in the ancient cryptography lesson. This series of articles and exercises will prepare you for the upcoming challenge!

Ready to try your hand at real-world code breaking? This adventure contains a beginner, intermediate and super-advanced level. See how far you can go!

### Learn

### Practice

A new problem emerges in the 20th century. What happens if Alice and Bob can never meet to share a key in the first place?

### Learn

This is a system of arithmetic for integers. These lessons provide a foundation for the mathematics presented in the Modern Cryptography tutorial.

### Learn

### Practice

Why do primes make some problems fundamentally hard? To find out we need to explore primality tests in more detail.

### Learn

Would access to coin flips speed up a primality test? How would this work?