Proof-carrying data refers to the process of providing a zk-SNARK as input to an arithmetic circuit in order to produce another zk-SNARK which validates the input zk-SNARK.
Proof-carrying data and recursive proofs, as well as zk-SNARKs more generally, are useful concepts for creating interoperability between different trustless protocols.
One design principle of Orbis is that all the world’s demand for zk-rollups could be met by one rollup. In reality, more than one zk-rollup will exist and an open decentralized financial ecosystem cannot become a reality without protocols for interoperability between decentralized protocols.
The concept similar to sending a UTxO between heterogeneous Orbis rollups can be applied to create bridges between zk-rollup protocols based on different zk-SNARK theories.
Applying the concepts requires only that
A zk-SNARK can be created proving the creation of a UTxO burning transaction in the source protocol.
A zk-SNARK can be validated as part of the conditions for validating a UTxO factory transaction in the destination protocol.
The concept of inter-protocol bridges opens the door to a concept of layer 3 rollups, which are rollups that use an L2 rollup in place of an L1 blockchain as the basis for the settlement and commitment protocol. Layer 3 rollups may be useful for specialized applications. This concept could be further extended to layer 4 rollups and beyond.