Paper 2019/792

TICK: Tiny Client for Blockchains

Wei Zhang and Jiangshan Yu and Qingqiang He and Nan Guan

Abstract

Currently, a lightweight blockchain client cannot verify a transaction directly. Rather, it generally considers a transaction to be valid and finalized if there are six confirmations on this transaction. This is not desired for micro-payments where payees may opt for zero-confirmation. When a payee chooses to accept zero-confirmation transactions, it needs to verify the validity of the transaction. In particular, one of the steps is to verify that the input of the transaction is not previously spent. This requires the payee to either trust some full nodes to perform the verification, or download the entire blockchain in order to perform the verification locally. The former is less secure as anyone can be a full node, it cannot be blindly trusted; the latter is not desired since a blockchain is large in size and is ever growing. We propose TICK, a Tiny Client for blockchains, to solve this problem. TICK advances blockchain client in the following aspects. First, TICK allows a mobile device to efficiently and cryptographically verify whether an input of a transaction is previously spent or not. Second, TICK can be adapted without changing the current implementation of the blockchain, such as Bitcoin. Rather, it only requires miners to put an extra hash value in the free field of a block. Last, as a side effect, rather than requiring new miners to download the entire blockchain to performmining,theTICKdesignallowsnewminerstodownload only a small portion of data to start mining. We implement TICK for Bitcoin, and provide an experimental evaluation on its performance by using the current Bitcoin blockchain data. Our result shows that the proof for verifying whether an input of a transaction is spent or not is only less than 2KB. The verification is very fast – a mobile client only needs to compute a few dozens of hash values. In addition, to start mining, new miners only need to download a few GB data, rather than downloading over 200 GB data.