(Phone number share)
User A wants to share phone number. 
Private key of User A is generated using Npub+ phone number.
User A shares that private key.

User B wants to see if they are friends with anyone.
User B takes friend list Npub + phone number list and hashes it (might be lots of random guessing but possible)
If user B generates the same key as user A shared they know they are friends. 

 I think the idea was not to share but to find the npub of your friends who’s phone number you know. 🐶🐾🫡 

 DM'd you 

 Replied 🐶🐾🫡