I assume you can still build a social graph with number 1 with enough people using the service if you just check for 2 way connections between contacts (i.e. if

I assume you can still build a social graph with number 1 with enough people using the service if you just check for 2 way connections between contacts (i.e. if Jane submits a contact list including Simon, Pete and Paul, and Paul submits a contact list including Sarah, Francis and Jane, I can imply that Jane & Paul are real contacts). 2 & 3 seem to match the 'bloom filter' concept described here: https://signal.org/blog/contact-discovery/ https://signal.org/blog/contact-discovery/