Glock: Garbled Locks for Bitcoin

Liam Eagen

Paper 2025/1485

Glock: Garbled Locks for Bitcoin

Liam Eagen, Alpen Labs

Abstract

Bitcoin is a decentralized, permissionless network for digital payments. Bitcoin also supports a limited set of smart contracts, which restrict how bitcoin can be spent, through bitcoin script. In order to support more expressive scripting functionality, Robin Linus introduced the BitVM family of protocols. These implement a weaker form of ``optimistic" smart contracts, and for the first time allowed bitcoin to verify arbitrary computation. BitVM allows a challenger to publish a ``fraud proof" that the computation was carried out incorrectly which can be verified on chain, even when the entire computation cannot. Jermey Rubin introduced an alternative optimistic smart contract protocol called Delbrag. This protocol uses Garbled Circuits (GC) to replace the BitVM fraud proof with by simply revealing a secret. He also introduced the Grug technique for malicious security. We introduce a new formalization of GC based optimistic techniques called Garbled Locks or Glocks. Much like Delbrag, we use the GC to leak a secret and produce a signature as a fraud proof. We further propose the first concretely practical construction that does not require Grug. Like BitVM2 and Delbrag, Glock25 reduces verification of arbitrary bounded computation to verification of a SNARK. In Glock25, we use a designated verifier version of a modified of the SNARK Pari with smaller proof size. We make Glock25 maliciously secure using a combination of Cut-and-Choose, Verifiable Secret Sharing (VSS), and Adaptor Signatures. These techniques reduce the communication, computational, and on-chain complexity of the protocol compared to other approaches to construct a Glock, e.g. based on Groth16.

Note: This is concurrent, independent work to Delbrag and BitVM3.

Metadata

Available format(s): PDF
Category: Applications
Publication info: Preprint.
Keywords: Garbled Circuits SNARKs Bitcoin
Contact author(s): liameagen @ protonmail com
History: 2025-08-20: approved; 2025-08-16: received; See all versions
Short URL: https://ia.cr/2025/1485
License: CC BY

BibTeX

@misc{cryptoeprint:2025/1485,
      author = {Liam Eagen},
      title = {Glock: Garbled Locks for Bitcoin},
      howpublished = {Cryptology {ePrint} Archive, Paper 2025/1485},
      year = {2025},
      url = {https://eprint.iacr.org/2025/1485}
}