Unit 7
Cryptography.
The mathematics of secrets: modular arithmetic, primes, key length, and agreeing on a key over a channel everyone hears.
Before this unit
- Bits and data
- Numbers Mathematics
The unit, lesson by lesson
Lessons
4 lessons · 24 practice questions · about 2 hours
Clock arithmetic
Modular arithmetic: a remainder as a place on a clock, addition that wraps back to zero, and powers that leap around the dial and in the end return to 1.
Prime numbers
What makes a number prime, how the sieve of Eratosthenes finds them all, why they thin out, and why primes are easy to multiply and hard to find again.
Keys and guessing
A message locked with a key, the trouble of passing the key, and why every extra bit in the key doubles the time a codebreaker needs to try them all.
The Diffie–Hellman key exchange
How two strangers agree on a secret key over a channel everyone can hear: mixing paint, the same idea in modular arithmetic, and why Eve cannot work out the key herself.