[schoen]

Try https://www.cryptopals.com/ or https://www.coursera.org/learn/crypto.

There's a lot to know, but you can build up to it!

One thing you'll learn about are oracle attacks, in which a bad protocol or implementation can leak secret data to the attacker bit-by-bit (sometimes literally bit-by-bit) in various ways. There are lots of those and they're sadly easy to create and a lot of progress in cryptographic engineering has been about finding ways to make sure oracle attacks can't occur in various real-world deployments.