[bqmjjx0kac]

I'm so confused by this use case (the traffic-saving one, not the malicious URL classifier). Why not store the "is-paying-customer" bit in a cookie?

What are we using as the user identifier? Where does it come from, if not a cookie?

Also, this client-side bloom filter kind of leaks your user database, supposing it's keyed on email addresses and your adversary has a gigantic list of email addresses, or is patient enough to enumerate them.

[pronoiac]

> Why not store the "is-paying-customer" bit in a cookie?

You shouldn't trust the client. You probably don't want people to get access to paid features with a relatively easy tweak of cookies.

The client-side filter is more suitable for listing, say, malware URLs, as mostly the response is "no" (go ahead) instead of "maybe", which would require a bit more work (like, network requests) to check if it's blocked or not.