V12
@signalapp's Contact Discovery Service (CDSI) lets you securely discover which of your contacts are on Signal. It's on by default, and the app sends your full contacts list every 48h. Queries run inside an SGX enclave, so even Signal's servers can't see them. SGX encrypts the enclave's memory, so the host can't read the queries directly, and uses ORAM to hide the access pattern.