https://toc.cryptobook.us/ https://toc.cryptobook.us/ http://swarm.cs.pub.ro/~mbarbulescu/cripto/Understanding%20Cryptography%20by%20Christof%20Paar%20.pdf htt

https://toc.cryptobook.us/ https://toc.cryptobook.us/ http://swarm.cs.pub.ro/~mbarbulescu/cripto/Understanding%20Cryptography%20by%20Christof%20Paar%20.pdf http://swarm.cs.pub.ro/~mbarbulescu/cripto/Understanding%20C... and "Applied cryptography" by Bruce Schneier. the credential is "I understand cryptography" which you can print on a piece of paper if you like