(1) Each client app to submit x randomly generated phone numbers for every real number, making any reconstructed social graph useless and deniable. Where x is the slowdown their very cleverly over-engineered solution introduces...
(2) As long as Signal user base is relatively small — submit n-1 digits of the number, then wait for confirmation there is at least one Signal user matching before submitting the final digit.
(3) If user base is larger - submit n-1 digits of the phone number, and receive all of the up to 10 matching users
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).
(1) Each client app to submit x randomly generated phone numbers for every real number, making any reconstructed social graph useless and deniable. Where x is the slowdown their very cleverly over-engineered solution introduces...
(2) As long as Signal user base is relatively small — submit n-1 digits of the number, then wait for confirmation there is at least one Signal user matching before submitting the final digit.
(3) If user base is larger - submit n-1 digits of the phone number, and receive all of the up to 10 matching users