Verifying a public key over a secure channel works trivially for any public key cryptography system. I was referring to ways to establish such secure secondary

Verifying a public key over a secure channel works trivially for any public key cryptography system. I was referring to ways to establish such secure secondary channels. Either verify a key yourself, eg, in person, or use distributed trust to average out the noise. For example, keybase has an approach: linking various identity information to keys. Signal is secure in a strange narrow interpretation of the security. There are problems if you look more broadly.