[Course] Journey into cryptography

What is cryptography?: What is Cryptography? A story which takes us from Caesar to Claude Shannon.
https://www.khanacademy.org/computing/computer-science/cryptography/crypt/v/intro-to-cryptography

https://www.khanacademy.org/computing/computer-science/cryptography/modern-crypt/v/diffie-hellman-key-exchange-part-2