Still trying to think of something witty, I will let you know once I get something...
Cryptography Ground Up
Introduction to CGU Series
A Preface to Cryptography
A Preface to Cryptography Part II
A Not-So-Master Class in Modulo
Caeser Cipher - Monoalphabetic Substitution Ciphers
Affine Cipher - Monoalphabetic Substitution Ciphers