Quantum-resistance in blockchain networks: Blockchain and the LACChain Blockchain Network

Table of Links

Abstract and 1. Introduction

2. Context

2.1. Quantum computing as a threat to cryptography

2.2. Current approaches for quantum-safe cryptography

2.3. Blockchain and the LACChain Blockchain Network

3. The vulnerabilities of blockchain technology with the advent of quantum computing

4. A Proposal for a Quantum-Safe Blockchain Network

5. Implementation and 5.1 Generation and distribution of quantum entropy

5.2. Generation of Post-Quantum Certificates

5.3. Encapsulation of the communication between nodes using quantum-safe cryptography

5.4. Signature of transactions using post-quantum keys

5.5. On-chain verification of post-quantum signatures

6. Conclusions and next steps, Acknowledgements, and References

2.3 Blockchain and the LACChain Blockchain Network

Blockchain is a technology that allows one to build decentralized ledgers in which different entities can register transactions that are grouped into blocks that are linked using hashes [21]. The immutability of the transactions stored in blockchain networks is guaranteed because it is impossible to tamper with the ledger without being detected. As any entity can, in principle, have a synchronized copy of the ledger and transactions that are validated according to predefined rules, the history cannot be rewritten. The integrity of the transactions is guaranteed by digital signatures because every transaction is signed by the sender, and the immutability of the chain is guaranteed by hash functions [21].

Our work analyzes vulnerabilities of hash functions and cryptographic algorithms. The security of these core elements of blockchain networks will be threatened when quantum computers become robust enough. This applies to most blockchain networks and it is a critical concern that the blockchain community has not yet properly addressed.

Vitalik Buterin, one of the founders of the Ethereum blockchain technology, acknowledged the quantum threat back in 2015 and suggested moving towards Lamport signatures eventually [53]. Prior to our work, the University of Waterloo and Microsoft Research estimated that the number of logical qubits necessary to implement quantum algorithms that can break 256 bit-long digital signatures generated with (EC)DSA, typically used in current blockchain networks, are 1500 [54] and 2330 [55], respectively. It is still unclear how many physical qubits would be needed for that purpose. Another study by researchers in Singapore, Australia, and France claimed in 2017 that quantum computers will be large and robust enough to break Bitcoin keys in 10 minutes by 2017 [56]. In 2018, three groups of scientists from Russia and Canada achieved an implementation of a quantum-secured blockchain based on an exchange of keys using QKD techniques [57], but their scalability is limited by the limitations of the channels for QKD exchange. Additional work has been published since these initial analyses [58–63]. However, we are not aware of any scalable implementation of a quantum-safe blockchain network prior to our work.

We have designed a solution that can be deployed in different blockchain networks. As a key component to show the viability of our proposal, we have implemented it in the LACChain Consensys Quorum (a.k.a. Besu) Network. LACChain is a blockchain infrastructure led by the Innovation Lab of the Inter-American Development Bank (IDB Lab) in Global Alliance with some of the entities leading the development of blockchain technology in the world [64]. The main goal of LACChain is to enable a robust and scalable blockchain network that can host multipurpose use cases with social, economic, and financial impact. Hyperledger Besu is an Ethereum client originally developed by Consensys and now maintained by the Ethereum community, including Consensys [65].

Blockchain can be thought of as a computational system with a distributed state shared among a network of nodes, of which consistency can be verified by any participant. The state is dynamically updated through messages, called transactions, that are broadcasted by the nodes, and each participant can have a verified and verifiable copy of the state and the transaction history. These transactions allow users to deploy executable code to the network, a.k.a. smart contracts, and interact with them.

In order for a new state to be agreed upon by the network, a subset of nodes, called validator or producer nodes, apply a consensus protocol. There are different types of consensus protocols and each network decides which type of consensus protocol they implement. Essentially, every consensus protocol consists of a set of rules that establish how these nodes will accomplish a computational validation of the latest transactions replicated across the network. The validator or producer nodes propose a package, called a block, which contains the transaction, block number, nonce, block hash, previous block hash, and signatures of the block validators or producers. With this, a new block is cryptographically sealed and, once appended to the blockchain, it cannot be undone or tampered with.

In Ethereum Networks, the code deployed in the network is a stream of bytes representing operation codes from the Ethereum Virtual Machine (a.k.a. EVM). This set of operations can be considered Turing complete and are executed as a stack machine with a depth of 1024 items. The EVM is then the runtime environment where any state transformation takes place [66]. Every smart contract has its own memory space and can be changed or updated by a transaction, which is recorded in the transaction history and implies a modification of the current distributed state. Additionally, each operation has an associated cost, which is an abstraction of the computational power required to perform the requested action by an ideal computer. The cost is called gas and serves as a metric for the amount of computation required to process each block.