Since we're specifically talking about Signal, I think that it's worth mentioning that Signal is uniquely predictable here. They published their entire cryptosystem, it's been extensively inspected by the cryptography community, there are multiple open-source implementations that agree with the published mathematics, and I strongly suspect that more than a few people have sat down to verify that the bytes coming out of the app are actually produced by the published protocols. Claiming that that's not "working the way it does" is reaching out into territory along the lines of Trusting Trust, the unproven existence of trapdoor functions, and the Problem of Induction.