Network access and presumably some centralized key hierarchy.<p>Same thing that would prevent some one from writing to the private blockchains banks are setting up for clearinghouse purposes.<p>If your the security of your cryptographic system is dependent on anything but the secret keys being secret it&#x27;s time to go back to the drawing board.<p>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.