> Signal's goal is to make all the messaging in the world that currently uses phone numbers as identifiers --- which, by a long ways, is most messaging in the world --- cryptographically secure.

That goal is fundamentally broken, though, because phone numbers aren’t cryptographically secure. One can use any exploit which allows one to take over a phone number to take over someone’s Signal identity. Yes, all of the target's Signal contacts will receive a message stating they his keys have changed — but they are used to that.

Signal's got some awesome crypto, but it also has some intriguing holes.