Thus the additional re-encryption of the master key. If this re-encryption key is secure then you don't need to care who the master key is handed out to as onl

Thus the additional re-encryption of the master key. If this re-encryption key is secure then you don't need to care who the master key is handed out to as only an authorized party can decrypt it. There are other, more complicated, crypto schemes that can make this process easier or more secure, but something like this has KISS appeal. An external channel is required here (to transmit the re-encryption key) but that can be as simple as a phone call or email; anything that does not need to pass through the third-party holding the data files.