Lovely clear explanations of Public Key Infrastructure.

https://www.cem.me/pki

How TLS 1.2 works

https://tls.ulfheim.net/

http://www.moserware.com/2009/06/first-few-milliseconds-of-https.html

and updated for TLS 1.3

https://tls13.ulfheim.net/

Applied Crypto Hardening

https://bettercrypto.org/

The CrypTool project's aim is to raise awareness and interest in crypto techniques for everyone.

https://www.cryptool.org/

"Automatically decrypt encryptions without knowing the key or cipher, decode encodings, and crack hashes"

https://github.com/Ciphey/Ciphey

"A fun, free platform for learning modern cryptography"

https://cryptohack.org/