But that sounds like un-necessary round trips. I feel like the point in an API or backend service doing this, is because it knows better and has more computing power. I'd rather have my service take an incoming list and return with whether it's a member or not. In fact returning full hashes seems even worse, since now the server has not only replied with the correct match, but potentially more strangers. You could milk an API of it's users much quicker that way.

Yes, but the server doesn't know which of the responses is actually your friend, thus preserving privacy.

Right, but I'm thinking the server still needs to be able to say "This person who ever they are is definitely the number you have". Like, I wouldn't check my password hashes with only the first 10 bytes, I guess that's why I'm still stuck on the truncated idea.

No, the server just says "I know these email addresses starting with 'foo': [email protected], [email protected], [email protected]". If your friend's email address is in there, you know they have Signal.

The hashing is so people can't easily get a list of all the phone numbers, which is easy to work around, but, then again, they could just hammer the endpoint querying for all the various numbers anyway.