My visceral response is don’t first try to write crypto algorithms|apps in native Crystal.
There’s a whole suite of tried|true|tested code to do the various crypto algorithms, usually in C|C++, that you can create wrappers for and use.
Writing Secure Crypto is HARD!
Unless you plan to take the years to test every routine to verify its security, on all kinds of hardware, against all known (including Quantum) attacks, wrap what’s considered to be secure, and provide a Crystal API to them.
But if you want to do it just for fun, then go ahead.