The Signal messenger and protocol.

when turned on, and only for contacts you know the phone number of (that's a quirk, username-only contacts don't work for this), it can verify that the last key you used is the current one. it asks the signal server for XYZ ID's proof ID (which isn't the phone number but a blinded account number) - and you use that on the tree working backwards until you find that yes, that anonymized user's latest signal key is XYZ. Then you double check that both Cloudflare and Trail of Bits agree with Signal's chain of events (that it didn't present a different tree to you).