I have a list of email subscribers. I want any one of them to be able to prove they are one of my subscribers without revealing their specific email address. What’s the easiest way to do this today?

You can solve this using public key cryptography. Email your subs with a unique private key. Set up a validator site where they can come prove it against your public key. You can even ensure each key is signed only once and the email associated with it

Doesn’t this doxx them if I know which priv key I sent them?

Wait, I think this works. Sha256 hash of email+priv key. This is proof of subscription. When they come to validate, you can’t know know who is validating

Yes it will assuming you keep track of which keys goes where which you needn’t have to for this to work . I actually worked on a similar problem 8 years back in an internship but not able to recollect it now. 99% sure there is a RSA based solve somewhere here