https://signal.miraheze.org/wiki/Secure_Value_Recovery

https://github.com/signalapp/SecureValueRecovery2

If you want to do an empirical test, create a signal account set a pin. Send a message to someone. Then delete signal. Recreate the account using the same phone number, recover using the pin and send a message. The receiver of that message will not get a warning that the signing key has changed.

The only way that's possible is if the key, or a derived key, is recoverable from the network. That is de facto proof that the keys or a key generation mechanism is in the cloud. Which is probably fine for personal communication.

But if I'm a nation state, this represents a significant security risk, especially when you're worried about another nation-state peaking at your communication. I.e France is buddy buddy with the US, but they probably don't want the US to read all of their internal communication.

SGX https://en.m.wikipedia.org/wiki/Software_Guard_Extensions

https://sslab-gatech.github.io/sgx101/pages/attestation.html

SGX is a inside chip secure enclave created by Intel, a company headquartered in the United States, that uses key management, and signing keys from Intel. Intel could be compelled by domestic intelligence to provide their keys, or to sign certain enclave software. I'm not saying it's happened, but I am saying this is part of the risk assessment a nation state would use to evaluate a messaging platform

So a nation state attack might look something like this: Intel is compelled to release their signing keys, the signal user enclave is copied, using the combination of both of these a special SGX environment will be set up to brute Force the passwords, with no limit. The limit will be removed, and it will operate in the SGX environment, and brute forcing a six-digit pin is trivial if you're not rate limited. This is just one possibility, SGX has at least nine known side channel attacks at the moment, I'm sure there's more that haven't been published.