This is an interesting exercise to help understand cryptography, but if you think cryptography is subtle or magical, you haven’t started looking at cryptanalysis. There are nearly countless possible side channel attacks. This is why we don’t roll our own practically speaking. It can take advanced degrees in mathematics to merely give ourselves some sense of assurance that our home rolled system is safe.

A friend of mine did roll his own security. A decade ago he invented a new homomorphic encryption algorithm (type of encryption that allows meaningful operations on cypertext without revealing secrets). His first step? Getting advanced degrees in mathematics and at least a year of peer review to test against possible attacks. Only years later did he found a company on it. Unfortunately while the system was believed secure, a zero-trust system took too long and too much battery to generate keys on phones.