With PGP, key revocation is pretty straightforward: a self-authenticating revocation certificate gets published to the shared PGP database. This certificate could be kept in the clear in my safe deposit box, could be encrypted to a friend&#x27;s key, could be M-of-N secret shared to N friends&#x27; keys, could be maintained by a third-party service that I&#x27;ve decided I trust, etc. As soon as that certificate is published, everyone sees it immediately.<p>What does a blockchain add to this?