Quantum Multi-Solution Bernoulli Search with Applications to Bitcoin’s Post-Quantum Security

Alexandru Cojocaru1, Juan Garay2, Aggelos Kiayias3, Fang Song4, and Petros Wallden5

1University of Maryland
2Texas A&M University
3University of Edinburgh and IOHK
4Portland State University
5University of Edinburgh

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Abstract

A proof of work (PoW) is an important cryptographic construct enabling a party to convince others that they invested some effort in solving a computational task. Arguably, its main impact has been in the setting of cryptocurrencies such as Bitcoin and its underlying blockchain protocol, which received significant attention in recent years due to its potential for various applications as well as for solving fundamental distributed computing questions in novel threat models. PoWs enable the linking of blocks in the blockchain data structure and thus the problem of interest is the feasibility of obtaining a sequence (chain) of such proofs. In this work, we examine the hardness of finding such chain of PoWs against quantum strategies. We prove that the chain of PoWs problem reduces to a problem we call multi-solution Bernoulli search, for which we establish its quantum query complexity. Effectively, this is an extension of a threshold direct product theorem to an average-case unstructured search problem. Our proof, adding to active recent efforts, simplifies and generalizes the recording technique of Zhandry (Crypto'19). As an application, we revisit the formal treatment of security of the core of the Bitcoin consensus protocol, the Bitcoin backbone (Eurocrypt'15), against quantum adversaries, while honest parties are classical and show that protocol's security holds under a quantum analogue of the classical “honest majority'' assumption. Our analysis indicates that the security of Bitcoin backbone is guaranteed provided the number of adversarial quantum queries is bounded so that each quantum query is worth $O(p^{-1/2})$ classical ones, where $p$ is the success probability of a single classical query to the protocol's underlying hash function. Somewhat surprisingly, the wait time for safe settlement in the case of quantum adversaries matches the safe settlement time in the classical case.

Quantum computers offer computational speed-ups, where the exact speed-up depends on the task examined. The classification of problems to hard/easy, as well as the exact cost it takes to solve a computational task, will change when quantum computing devices scale-up in size and quality. It is well known that this affects cryptography by making the most widely used encryption and signature schemes insecure. What is less explored, is the effect that quantum algorithms have in other cryptographic tasks. Many major blockchains and cryptocurrencies, such as bitcoin, rely on the concept of “Proof of Work” (PoW), where participants/miners demonstrate that they spend some computational time trying to solve a problem and get a reward for this. The core mathematical problem that the security and persistence of the blockchain relies on, is the ability to produce chains of such PoWs.
In our paper we examine how this mathematical problem, chain of PoWs, can be solved by a quantum adversary and provide bounds for their capabilities. Based on this result, we revisit the security of the Bitcoin backbone protocol (a mathematical abstraction capturing the key elements of the Bitcoin protocol), in the setting where all honest parties are classical, and there is a single quantum adversary (controlling all the quantum computational resources of the malicious parties). Our analysis shows that the security could be maintained if the total classical computational power of the honest parties in terms of queries/operations is a very large (but constant) number greater than the adversarial quantum computational power. This is a first step to the full analysis of bitcoin in the quantum era, when all parties would have quantum computational capabilities.

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