(2026-08) Non-Interactive Translation of Winternitz Signatures to Lamport Signatures via Secret Sharing
2026-08-13
BitVM2 brought arbitrary program execution to Bitcoin, yielding the first light-client-based bridge to its second layers and reducing the trust required at setup to a single honest participant. Its successors, BitVM3 and BABE, move the disputed computation off-chain into Garbled Circuits (GCs), cutting worst-case on-chain dispute costs by roughly three orders of magnitude and so opening participation beyond well-capitalized operators; BABE in turn cuts the off-chain storage and setup costs of BitVM3's garbled circuits by a comparable factor. What still reaches the chain, however, is bulkier than it need be. BitVM2 commits its data with compact Winternitz one-time signatures (WOTS), whereas BitVM3 and BABE must reveal GC input labels on-chain and so fall back on far bulkier Lamport signatures, which play two roles at once: GC input labels and Bitcoin-verifiable commitments. WOTS cannot simply be substituted, because its hash chains are monotonic—an evaluator holding one state's preimage can hash forward and obtain several active labels on a single input wire, destroying the circuit's privacy. Recovering BitVM2's compactness therefore calls for a practical GC whose input labels are WOTS signatures—a construction that has remained out of reach. We close this gap with a non-interactive \emph{WOTS-to-Lamport translation gadget}: the garbler commits on-chain using compact WOTS chains, and the evaluator expands them off-chain into exactly the orthogonal Lamport labels the GC consumes, learning nothing about the mutually exclusive ones. A naive translation table that enumerated messages would be exponentially large. Two symmetries bring it down to quadratic: reconstruction from a Shamir sharing depends only on \emph{how many} shares are held and not on \emph{which}, which collapses the exponentially many messages onto the single checksum weight WOTS already computes; and monotonicity, the very property that made WOTS unusable, orders an evaluator's access by inclusion. We model the gadget as a garbling scheme and prove it adaptively private. Applied to BABE, it restores WOTS-scale commitments to a GC-based protocol, cutting total on-chain script size by more than and bringing dispute transactions within Bitcoin's standardness limit.