The importance of safeguarding individuals’ privacy rights in online activities is unmistakable in today’s anonymity networks. Since the introduction of Mixnet by Chaum, numerous anonymity networks with different objectives and design principles have emerged, providing a diverse range of applications for privacy-conscious users. However, security issues in anonymity networks have persistently existed and continue to impact their survival and development. Each anonymity network presents distinct security challenges, making it more challenging for researchers to achieve a comprehensive and systematic understanding of their security. The current literature exhibits some unavoidable gaps, including the lack of vulnerability perspectives, a unified understanding of diverse attack types, defense perspectives, and theoretical perspectives. To address these gaps, we investigate prevailing attacks targeting anonymity networks from the viewpoint of network designers and operators. We use Tor, I2P, and Freenet (arguably the three most popular anonymity networks) as case studies. Starting with these attacks, we conduct an in-depth analysis of the vulnerabilities underlying them and explore related defense mechanisms and formal security. Specifically, we classify vulnerabilities into external and internal categories, utilizing the protocol stack of an anonymity network to guide the categorization of internal vulnerabilities. Furthermore, we examine their root causes. In addition to these aspects, we emphasize the importance of formal security in researching the security of anonymity networks by integrating the investigated vulnerabilities, attacks, and defenses. Through this comprehensive, thorough, and unified approach, this paper aims to provide insights into the security of anonymity networks and offer general research findings. Finally, we discuss ongoing challenges and future directions in this specific area.

The Ethereum cryptocurrency system becomes prevalent with the increasing number of daily transactions. It is a high-quality channel to secretly transfer information because senders and recipients are pseudonymous and transactions are tamper-resistant. Several existing covert channels have been designed for several years. However, most of the existing covert channels can either be detected by their special patterns or have low embedding rate. In this paper, we first use the VALUE field of a transaction in the Ethereum system to construct a HMAC-based multiple-bit embedding (HMAC-based MBE) scheme. We further proposed a Hash-based multiple-bit embedding (Hash-based MBE) scheme to enhance the covertness. Theoretical and experimental analysis show that both schemes have either higher embedding rate or better covertness than that of the existing works.

Blockchain-based censorship-resistant systems enable the user to access the blocked content through a covert channel while avoiding a suspicious network connection between the user and the proxy. However, state-of-the-art blockchain-based censorship-resistant schemes cannot satisfy both low communication fees and unobservability, and their method of identifying transactions with covert data may inadvertently expose the covert channel. In this paper, we present Hades, a blockchain-based covert channel framework that aims to circumvent censorship. Hades allows users to encode covert data as a transaction field, and identify transactions with covert data by using another transaction field as a label. We also present the security model for Hades, which defines the unobservability of Hades as the indistinguishability of transactions with covert data from normal transactions. We further propose two cost-friendly and unobservable instantiations of Hades: the basic RDSAC and the improved DDSAC. RDSAC uses private keys to encode covert data and utilizes random factors in the signing process as labels, while incurring a communication delay. DDSAC avoids the delay by encoding covert data into random factors and sampling a transaction amount from normal transactions as the label. We implement a prototype system of Hades and evaluate its performance. Experiment results show that our Hades prototype is unobservable, robust, and efficient. RDSAC and DDSAC can identify 1,654 transactions in 6.054 seconds and 0.071 seconds, respectively. Hades supports 1KB data transfer at $0.44 on the Bitcoin mainnet and cost-free data transfer on the Bitcoin testnet.

Covert channels serve the construction of cyberspace security. By realizing the secure transmission of data, it is widely used in political and financial fields. Blockchain covert channels have higher reliability and concealment compared to traditional network-based covert channels. However, existing blockchain covert storage channels need to create a large number of transactions to transmit covert information. Creating transactions requires a transation fee, which means that the implementation of blockchain covert storage channels requires a high cost. Besides, created transactions remain on-chain permanently, leading to the threat of covert information being detected. To overcome these limitations, we propose a blockchain covert timing channel framework. Specifically, we utilize inv and getdata messages in the Bitcoin transaction broadcast as carriers and propose three modulation modes to achieve covert channels without cost and leaving no trace. We evaluate the concealment of our modes by K-S, KLD tests, and machine learning approaches. Experimental results show the indistinguishability between traffic carrying covert information and normal traffic. Our channels promise a capacity of 2.4 bit/s.