Just to expand on this, since it wasn't originally clear to me from reading your post, the contact privacy feature is about using SGX enclaves for the purpose of populating your known contacts on Signal. When you log into Signal for the first time your phone locally has all of your known contacts, and the Signal app wants to know which of these contacts already have Signal accounts. The secure enclave is a mechanism where you publish your entire contact list from your phone to the Signal servers, then they can send back the subset of those contacts that actually have Signal accounts. The point of the enclave is that this is all done in a way where Signal can't see what contacts you sent them, nor can they determine which contacts were matched and sent back to you.