Starting from first principles, students will learn the fundamentals of cryptography. Prime numbers. Factoring. Random numbers. Simple ciphers. One time pads. Key exchange. Generating an RSA key pair. Signing a message. GPG. Best practices for online security. We'll start with basic maths, factoring, string manipulation, learn how to write and brute force a simple cipher, then work our way up to understanding how modern key-exchange works through short examples and audience participation exercises. After taking this class, participants will gain a better understanding of cryptography and understand how to build simple ciphers.