Polar Coded Merkle Tree: Improved Detection of Data Availability Attacks in Blockchain Systems

Light nodes in blockchain systems are known to be vulnerable to data availability (DA) attacks where they accept an invalid block with unavailable portions. Previous works have used LDPC and 2-D Reed Solomon (2D-RS) codes with Merkle Trees to mitigate DA attacks. While these codes have demonstrated improved performance across a variety of metrics such as DA detection probability, they are difficult to apply to blockchains with large blocks due to generally intractable code guarantees for large codelengths (LDPC), large decoding complexity (2D-RS), or large coding fraud proof sizes (2D-RS). We address these issues by proposing the novel Polar Coded Merkle Tree (PCMT) which is a Merkle Tree built from the encoding graphs of polar codes and a specialized polar code construction called Sampling-Efficient Freezing (SEF). We demonstrate that the PCMT with SEF polar codes performs well in detecting DA attacks for large block sizes.

I. Introduction

Decentralization and security properties of blockchains have led to their applications in a wide variety of fields [1]– [7]. A blockchain is an immutable ledger of transaction blocks. Full nodes in a blockchain system store the entire ledger and validate transactions. However, for better scalability, blockchains also run light nodes who store the header of each block and cannot validate transactions. Light nodes rely on honest full nodes for fraud proofs [8] in order to reject invalid blocks.