Most backend tutorials stop at "hash the password, you're secure now." That's good advice for authentication. It's not much use for the rest of a payment system, where you're dealing with data you actually need back and events that don't politely arrive exactly once.
I work on payment infrastructure. Two problems come up constantly here that I almost never see discussed outside of "we had an incident" postmortems: encrypting sensitive fields correctly and making webhook handlers actually idempotent instead of just idempotent-looking.
Encryption: the part hashing can't help you with
Password hashing works because it's one-way. You never need the plaintext back. You just need to compare a new hash against the stored one.
Bank account numbers don't work like that. Neither do BVNs or NINs. You need the original value later, which means encryption has to be reversible. Once that's true, the security story depends far more on how you manage your encryption keys than on which algorithm you picked.
