> You can improve privacy by one-way transforming the number in a way your systems can't undo (eg hmac with a key in hsm configured for enc only, with rate limi

> You can improve privacy by one-way transforming the number in a way your systems can't undo (eg hmac with a key in hsm configured for enc only, with rate limiting), but naively you lose agility and some useful capability (eg ban accounts with numbers from this range). So store two hashes, one being the whole number, another being the number excluding the last 2 digits. These aren’t difficult problems to solve. The bigger issue is that they’re not revenue generating solutions, but rather the opposite.