# Blockchain-Based Multistage Continuous Authentication for Smart Devices
## 1 Introduction  
With the rapid developments of communication and intelligent technologies, mobile devices have become prevalent in number in the Internet of Vehicles [1]. People prefer to store valuable personal information on smart devices for convenient storage and rapid access. Unfortunately, these devices are exposed to the leakage of private data and could be imposed by unauthorized access. Traditional access control mechanisms are based on explicit authentication methods such as passcode [2], fingerprint [3], and personal identification number (PIN) [4]. Many studies have shown the drawbacks of these one-time authentication mechanisms due to the inability to exercise adequate vigilance after initial access. To overcome the weaknesses, researchers propose continuous authentication [5] as a supplement to the existing solutions. The proposed authentication technique can continuously sense the user's interaction with the smart devices. Table 1 summarizes the distinctions between one-time authentication schemes and continuous authentication schemes. More recently, gait recognition [6] has received academic attention as an emerging biometrics technique. Moreover, each individual's gait behavior is unique and stable withina certain time frame. As a result, gait-based authentication methods can identify users by capturing signals from how they walk. Biometric information as a signing key [7] has been successfully applied in the authentication system. In fact, the gait signal can be affected by noise in the process of acquisition [8]. Moreover, there are some errors in the sampling results. It can be seen that keeping a signing key securely and accurately is a burden for the user. Therefore, how to remove noise interference is the key to realizing biometrics recognition. In 2004, [PERSON] proposed the fuzzy extractor technique [9]. This technique can extract two biometric traits within the margin of error to generate an identical random key. This provides a security measure for the authentication process. Therefore, we propose a security model using the fuzzy extractor technique in this paper. In this security model, the fuzzy extractor extracts the gait features and generates a randomly distributed string and auxiliary public information. In the authentication phase, the same randomized string can be generated based on the auxiliary public information and biometric information, which is close to the original input. In this authentication process, the biometric real data are not stored and transmitted in any form. Currently, common identity data storage method adopts centralized storage in a central server [10]. Nevertheless, this method has a very obvious single-point problem [11]. Moreover, authentication based on a single-server architecture is difficult to adapt to the increasingly complex network environment. To overcome these issues, extensive efforts have been devoted to decentralized technology. In 2008, [PERSON] published a paper titled \"Bitcoin: A Peer-to-Peer Electronic Cash System\" [12]. The emergence of blockchain technology has provided new solutions for establishing distributed authentication schemes [13] since then. On the one hand, the blockchain can solve the security problems brought on by the centralized management mentioned above. On the other hand, the open and transparent characteristics of the blockchain make it possible to share information between different entities. Consequently, this paper proposes a blockchain-based multistage continuous authentication model. This model implements on-chain and off-chain authentication of user identity by combining blockchain technology and the IPFS. In addition, the IPFS can solve the limited storage problem of the blockchain. To the best of our knowledge, this is the first work to address continuous authentication with decentralization consideration via blockchain technology for smart devices. The main contributions of this paper are summarized below:  
1. We propose a multistage authentication system that combines blockchain and the IPFS.