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