Generalized HTLC for Cross-Chain Swapping of Multiple Assets with Co-Ownerships
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A core enabler for blockchain or DLT interoperability is the ability to atomically exchange assets held by mutually untrusting owners on different ledgers. This atomic swap problem has been well-studied for several years, with the Hash Time Locked Contract (HTLC) emerging as a canonical solution. HTLC ensures atomicity of exchange, albeit with caveats for node failure and timeliness of claims. But a bigger limitation of HTLC is that it only applies to a model consisting of two adversarial parties having sole ownership of an asset each. Realistic extensions of the model in which assets may be jointly owned by multiple parties, all of whose consents are required for exchanges, or where multiple assets must be exchanged for one, cannot be handled by HTLC because these scenarios present distinctly new threats of collusions among subsets of parties. In this paper, we generalize the model of asset exchanges across DLT networks, present the consequent threat model, and propose and analyze an augmented HTLC protocol for atomic multi-party-and-asset exchanges. We also show how our protocol can be implemented easily by augmenting existing HTLC capabilities in the Weaver interoperability framework to enable asset exchanges between networks built on Hyperledger Fabric or Corda.
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