The Cryptopals Crypto Challenges are probably the best way to practically learn cryptography - by breaking it. Good luck.

Key Exchange Problems - Computerphile - YouTube

This video quickly explains how public key cryptography works. This video quickly explains how files are hashed, encrypted and decrypted for better digital signatures.

From Moxie’s talk I learned how PGP worked, along with its downsides.

https://dencode.com/ → encode/decode tools