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"Signal uses your existing number, doesn’t require a password, and leverages privacy-preserving contact discovery to immediately display which of your contacts are reachable with Signal."

How does contact discovery work? What happens when run on a device without a (valid) SIM?



It looks like it uses bloom filters for the phone number:

isCntactRegisteredWithWhisper calls isPhoneNumberRegisteredWithWhisper:

https://github.com/WhisperSystems/Signal-iOS/blob/19ff47e278...

The implementation is simply checking a bloom filter:

https://github.com/WhisperSystems/Signal-iOS/blob/19ff47e278...

… and the filter is periodically downloaded from their servers:

https://github.com/WhisperSystems/Signal-iOS/blob/19ff47e278...

https://github.com/WhisperSystems/Signal-iOS/blob/19ff47e278...

which ultimately ends up at:

https://master.whispersystems.org:31337/users/directory


They've blogged a bit about this challenge in the past:

https://whispersystems.org/blog/contact-discovery/


Ah, thanks!




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