International Association
for Cryptologic Research

We present Bitcoin Thunderbolt, a novel off-chain protocol for asynchronous, secure transfer of Bitcoin UTXOs between uncoordinated users. Unlike prior solutions such as payment channels or the Lightning Network, Bitcoin Thunderbolt requires no prior trust, direct interaction, or continuous connectivity between sender and receiver. At its core, Bitcoin Thunderbolt employs a Byzantine fault-tolerant committee to manage threshold Schnorr signatures, enabling secure ownership delegation and on-chain finalization.

Our design supports recursive, off-chain UTXO transfers using tweakable, verifiable signature components. The protocol tolerates up to $f$ malicious nodes in a $3f+1$ committee and ensures correctness, consistency, and one-time spendability under asynchronous network conditions.

We formally verify Bitcoin Thunderbolt’s key security properties, namely, unforgeability, ownership soundness, and liveness—using the Tamarin prover. Our results demonstrate that Thunderbolt provides robust, scalable, and non-interactive off-chain Bitcoin transfers, significantly expanding the practical utility of Bitcoin for decentralized applications.