Network access and presumably some centralized key hierarchy.
Same thing that would prevent some one from writing to the private blockchains banks are setting up for clearinghouse purposes.
If your the security of your cryptographic system is dependent on anything but the secret keys being secret it's time to go back to the drawing board.
Bitcoin works because it uses proof of work as a gating mechanism to prevent abuse, closed blockchains usually rely on other mechanisms as the parties involved in them can negotiate the terms via other means.
Same thing that would prevent some one from writing to the private blockchains banks are setting up for clearinghouse purposes.
If your the security of your cryptographic system is dependent on anything but the secret keys being secret it's time to go back to the drawing board.
Bitcoin works because it uses proof of work as a gating mechanism to prevent abuse, closed blockchains usually rely on other mechanisms as the parties involved in them can negotiate the terms via other means.