Thunderbolt: A Formally Verified Protocol for Off-Chain Bitcoin Transfers

Hongbo Wen; Hanzhi Liu; Jingyu Ke; Yanju Chen; Dahlia Malkhi; Yu Feng

Paper 2025/709

Thunderbolt: A Formally Verified Protocol for Off-Chain Bitcoin Transfers

Hongbo Wen, University of California, Santa Barbara, Nubit

Hanzhi Liu, University of California, Santa Barbara, Nubit

Jingyu Ke, Shanghai Jiao Tong University, Nubit

Yanju Chen, University of California, Santa Barbara, Nubit

Dahlia Malkhi

, University of California, Santa Barbara

Yu Feng, University of California, Santa Barbara, Nubit

Abstract

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.

Metadata

Available format(s): PDF
Category: Cryptographic protocols
Publication info: Preprint.
Keywords: blockchain Bitcoin adaptor signatures formal verification
Contact author(s): hongbowen @ ucsb edu
hanzhi @ ucsb edu
windocotber @ sjtu edu cn
yanju @ cs ucsb edu
dahliamalkhi @ ucsb edu
yufeng @ cs ucsb edu
History: 2025-04-19: approved; 2025-04-19: received; See all versions
Short URL: https://ia.cr/2025/709
License: CC BY

BibTeX

@misc{cryptoeprint:2025/709,
      author = {Hongbo Wen and Hanzhi Liu and Jingyu Ke and Yanju Chen and Dahlia Malkhi and Yu Feng},
      title = {Thunderbolt: A Formally Verified Protocol for Off-Chain Bitcoin Transfers},
      howpublished = {Cryptology {ePrint} Archive, Paper 2025/709},
      year = {2025},
      url = {https://eprint.iacr.org/2025/709}
}