MysteryTwisterC3 is an excellent way to learn RSA, ECC, and so on through the process of cryptanalysis. https://mysterytwister.org/home/welcome/ To learn how to implement stuff correctly, I recommend trying to work through an algorithm (say SHA-1 for example), and try to find people that explain it. Once you're ready to implement it yourself, go through the list of flaws in common implementations (side-channel, power analysis, diff, linear, etc) and try to prove whether your implementation is vulnerable or not vulnerable. Once you've done this more than once, you can do it for a lot of things.

It turns out one of the best books on cryptography is in a bundle if you like to read, Applied Cryptography: https://www.humblebundle.com/books/cybersecurity-2022-wiley-...

Schneier has answered this question also: https://www.schneier.com/crypto-gram/archives/1999/1015.html...

Out of curiosity— can you think of any high-risk ways to learn to write crypto code? Do it while driving? Start throwing untested ideas into production in a large database of personal information? Do it for an international drug cartel?

I was curious about how prevalent the use of crypto is as referring to cryptocurrency.

I had to get to the fifth page of Google results for "crypto" to get to this non-cryptocurrency gem: https://www.rottentomatoes.com/m/crypto

cryptopals is a good way to learn about practical crypto exploits

+1 for https://cryptopals.com/

You might want to check out: https://github.com/cryptosubtlety/intuitive-advanced-cryptog...

I feel obligated to say the lowest risk way is to not write it at all. Don’t roll your own crypto.