You could reverse engineer everything on -one- phone and one version of software and be confident every single CPU instruction involving crypto sufficiently ran

You could reverse engineer everything on -one- phone and one version of software and be confident every single CPU instruction involving crypto sufficiently random, in constant time, and not being exfiltrated. This is a -massive- effort, that would also need to rule out hardware backdoors and use some cutting edge techniques, but say you did it. Now a dissident is targeted. Their phone is sent a special OS hotfix no one in the world has reviewed and that compromises the random number generator at a CPI microcode level that would take months of targeted reverse engineering to detect, then relevant messages are decrypted over the wire with the now weakend keys. It then reverts itself when done making detection that much less likely. Whoever has the signing keys make the rules... or anyone who can give secret orders to those holding the signing keys.