| HN Mirror

The database isn’t secret here. The server indeed has its own copy - it would have to otherwise what is the client query resolving against. What’s secret is which phone numbers are contacting the client. So instead of sending the phone number to the server, you send an encrypted version of the phone numbers. This encrypted version is then checked against the encrypted database. This prevents the evil server from discovering the phone number the client is checking.

If you read the docs, a perfectly valid implementation is an HTTP request that sends the unencrypted database to the client which then checks the numbers locally - it achieves equivalent security priorities. The advantage here is that the database can be large enough to make distribution less practical than just doing a lookup per number and that’s where the HE comes in.

Remember: evil in a security context means someone trying to actively circumvent your protection guarantees, but you’re making an assumption that the database needs to be secret when it may not as the privacy and security guarantees are about the client’s information. Apple isn’t necessarily saying the database is secret since it’s just “this phone number is likely spam”. Of course, it’s possible that the server itself can’t even generate a valid query. It’s possible Apple designed it such that the query has to be generated on a valid Apple device to begin with (since it has a chain of trust to each device manufactured).