You literally cannot prove it either way, you can't prove it's not enciphered data, you can't prove it's not random garbage, that's the point. You can say that 'we could develop a safe backdoored system that will allow only lawful decryption in the event of emergency' in the same way you can say 'we can launch probes made of candy to distant planets that will build cities and plant potatoes for us'. It's a fantasy. The keys will leak, criminals etc will still blend into the crowd.