Jidar: A Jigsaw-like Data Reduction Approach Without Trust Assumptions for Bitcoin System

The pioneer blockchain platform Bitcoin has drawn massive attention from various sectors in society, with its promising decentralized, irreversible, and trust-free natures. Increasing volume of Bitcoin transactions makes it impractical for each user to store a full copy of whole ledger, especially for a normal user who makes few transactions and owns constrained storage space. There are already some works conducted to reduce the storage overhead by redesigning the system protocol, based on different trust assumptions. An example is running light nodes, which trust the full nodes to store its relevant data and reply to its data request timely. However, it introduces the risks of permanent data loss, as the data relevant to a light node may be tailored by all the full nodes later. Another attempt is conducted by organizing the nodes into several cooperative units based on a trust assumption, which is hard to satisfy in practice. In this paper, we propose a novel Jigsaw-like Data Reduction (Jidar) approach. Each node in Jidar only stores transactions of interest and relevant Merkle branches from the complete blocks, just like selecting several pieces from the jigsaw puzzles. A node can maintain and verify all its relevant data locally and safely without any trust assumptions. To verify the validity of a newly proposed transaction, Merkle branches relevant to the inputs are provided along with the transaction by the proposer. In view of the requirement that a user wants to cohere all the fragments stored in different nodes into a complete block, just like stitching all the pieces into a complete jigsaw-puzzle picture, a mechanism to query full data is added to Jidar. Experimental results demonstrate that Jidar can reduce a normal node's storage cost to about 1.03% of the original Bitcoin system at a low communication cost