Actually, Signal uses bloom filters to do "offline", private contact discovery[1]. That's why their contact discovery is rather bandwidth-intensive, as @praseod

Actually, Signal uses bloom filters to do "offline", private contact discovery[1]. That's why their contact discovery is rather bandwidth-intensive, as @praseodym mentioned. [1]: https://whispersystems.org/blog/contact-discovery/ https://whispersystems.org/blog/contact-discovery/