A universal encryption key is useless. Encryption keys are only useful if their distribution is limited.
What you're looking for is an authentication signature-type key, where there is a public portion that can be distributed without limits, and a private portion known only to the user. The public portion of the key can be used to verify that a user originated a message, but cannot be used to sign a message.
Then you just need to get worldwide agreement on what digital signature algorithm should be used for digital ham radio, upgrade every digital radio format to support digital user authentication, upgrade every digital radio in existence with the digital authentication firmware, and add additional CPU power to each radio to support the additional overhead of calculating a digital signature for every packet of data coming out of the radio. And then you have to build, test, and debug the worldwide user management infrastructure that tracks authorized users and their authentication keys.
And if ANY part of that process fails, then either the entire system goes offline, or affected users' radios are effectively bricked.
Somehow I suspect that the occasional jammer is a preferable alternative.