  In the current work we discuss the notion of gateways as a means for
interoperability across different blockchain systems. We discuss two key
principles for the design of gateway nodes and scalable gateway protocols,
namely (i) the opaque ledgers principle as the analogue of the autonomous
systems principle in IP datagram routing, and (ii) the externalization of value
principle as the analogue of the end-to-end principle in the Internet
architecture. We illustrate the need for a standard gateway protocol by
describing a unidirectional asset movement protocol between two peer gateways,
under the strict condition of both blockchains being private/permissioned with
their ledgers inaccessible to external entities. Several aspects of gateways
and the gateway protocol is discussed, including gateway identities, gateway
certificates and certificate hierarchies, passive locking transactions by
gateways, and the potential use of delegated hash-locks to expand the
functionality of gateways.

Cryptography and Security