Efficient Bitcoin Meta-Protocol Transaction and Data Discovery Through nLockTime Field Repurposing

TL;DR

The paper introduces the Lockchain Protocol, a lightweight Bitcoin meta-protocol that repurposes the nLockTime field for efficient transaction discovery and data verification without additional on-chain storage.

Contribution

It presents a novel method to encode metadata in the nLockTime field, enabling scalable transaction filtering and discovery within Bitcoin's existing protocol constraints.

Findings

Enables filtering of transactions by header without inspecting full payloads

Maintains Bitcoin consensus rules while repurposing nLockTime

Improves transaction discovery efficiency at scale

Abstract

We describe the Lockchain Protocol, a lightweight Bitcoin meta-protocol that enables highly efficient transaction discovery at zero marginal block space cost, and data verification without introducing any new on-chain storage mechanism. The protocol repurposes the mandatory 4-byte nLockTime field of every Bitcoin transaction as a compact metadata header. By constraining values to an unused range of past Unix timestamps greater than or equal to 500,000,000, the field can encode a protocol signal, type, variant, and sequence identifier while remaining fully valid under Bitcoin consensus and policy rules. The primary contribution of the protocol is an efficient discovery layer. Indexers can filter candidate transactions by examining a fixed-size header field, independent of transaction payload size, and only then selectively inspect heavier data such as OP RETURN outputs or witness fields. The Lockchain Protocol applies established protocol design patterns to an under-optimised problem domain, namely transaction discovery at scale, and does not claim new cryptographic primitives or storage methods.