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1. A Cross-layer Plausibly Deniable Encryption System for Mobile Devices

   https://doi.org/10.1007/978-3-031-25538-0_9  

   Chen, Niusen; Chen, Bo; Shi, Weisong (February 2023, 18th EAI International Conference on Security and Privacy in Communication Networks (SecureComm 2022))

   Li, Fengjun; Liang, Kaitai; Lin, Zhiqiang; Katsikas, Sokratis K. (Ed.)

   Mobile computing devices have been used to store and process sensitive or even mission critical data. To protect sensitive data in mobile devices, encryption is usually incorporated into major mobile operating systems. However, traditional encryption can not defend against coercive attacks in which victims are forced to disclose the key used to decrypt the sensitive data. To combat the coercive attackers, plausibly deniable encryption (PDE) has been introduced which can allow the victims to deny the existence of the sensitive data. However, the existing PDE systems designed for mobile devices are either insecure (i.e., suffering from deniability compromises) or impractical (i.e., unable to be compatible with the storage architecture of mainstream mobile devices, not lightweight, or not user-oriented). In this work, we design CrossPDE, the first cross-layer mobile PDE system which is secure, being compatible with the storage architecture of mainstream mobile devices, lightweight as well as user-oriented. Our key idea is to intercept major layers of a mobile storage system, including the file system layer (preventing loss of hidden sensitive data and enabling users to use the hidden mode), the block layer (taking care of expensive encryption and decryption), and the flash translation layer (eliminating traces caused by the hidden sensitive data). Experimental evaluation on our real-world prototype shows that CrossPDE can ensure deniability with a modest decrease in throughput. 

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2. The Block-Based Mobile PDE Systems are Not Secure - Experimental Attacks

   https://doi.org/10.1007/978-3-031-17081-2_9  

   Chen, Niusen; Chen, Bo; Shi, Weisong (October 2022, 2022 EAI International Conference on Applied Cryptography in Computer and Communications (EAI AC3 2022))

   Lin, Jingqiang; Tang, Qiang (Ed.)

   Nowadays, mobile devices have been used broadly to store and process sensitive data. To ensure confidentiality of the sensitive data, Full Disk Encryption (FDE) is often integrated in mainstream mobile operating systems like Android and iOS. FDE however cannot defend against coercive attacks in which the adversary can force the device owner to disclose the decryption key. To combat the coercive attacks, Plausibly Deniable Encryption (PDE) is leveraged to plausibly deny the very existence of sensitive data. However, most of the existing PDE systems for mobile devices are deployed at the block layer and suffer from deniability compromises. Having observed that none of existing works in the literature have experimentally demonstrated the aforementioned compromises, our work bridges this gap by experimentally confirming the deniability compromises of the block-layer mobile PDE systems. We have built a mobile device testbed, which consists of a host computing device and a flash storage device. Additionally, we have deployed both the hidden volume-based PDE and the steganographic file system-based PDE at the block layer of our testbed and performed disk forensics to assess potential compromises on the raw NAND flash. Our experimental results confirm it is indeed possible for the adversary to compromise the block-layer PDE systems when the adversary can have access to the raw NAND flash in real world. We also discuss practical issues when performing such attacks in practice. 

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3. FSPDE: A Full Stack Plausibly Deniable Encryption System for Mobile Devices

   https://doi.org/10.1145/3626232.3653262  

   Liao, Jinghui; Chen, Niusen; Xia, Lichen; Chen, Bo; Shi, Weisong (June 2024, Proceedings of the Fourteenth ACM Conference on Data and Application Security and Privacy (CODASPY '24))

   In today's digital landscape, the ubiquity of mobile devices underscores the urgent need for stringent security protocols in both data transmission and storage. Plausibly deniable encryption (PDE) stands out as a pivotal solution, particularly in jurisdictions marked by rigorous regulations or increased vulnerabilities of personal data. However, the existing PDE systems for mobile platforms have evident limitations. These include vulnerabilities to multi-snapshot attacks over RAM and flash memory, an undue dependence on non-secure operating systems, traceable PDE entry point, and a conspicuous PDE application prone to reverse engineering. To address these limitations, we have introduced FSPDE, the first Full-Stack mobile PDE system design which can mitigate PDE compromises present at both the execution and the storage layers of mobile stack as well as the cross-layer communication. Utilizing the resilient security features of ARM TrustZone and collaborating multiple storage sub-layers (block device, flash translation layer, etc.), FSPDE offers a suite of improvements. At the heart of our design, the MUTE and MIST protocols serve both as fortifications against emerging threats and as tools to mask sensitive data, including the PDE access point. A real-world prototype of FSPDE was developed using OP-TEE, a leading open-source Trusted Execution Environment, in tandem with an open-sourced NAND flash controller. Security analysis and experimental evaluations justify both the security and the practicality of our design. To address these limitations, we have introduced FSPDE, the first Full-Stack mobile PDE system design which can mitigate PDE compromises present at both the execution and the storage layers of mobile stack as well as the cross-layer communication. Utilizing the resilient security features of ARM TrustZone and collaborating multiple storage sub-layers (block device, flash translation layer, etc.), FSPDE offers a suite of improvements. At the heart of our design, the MUTE and MIST protocols serve both as fortifications against emerging threats and as tools to mask sensitive data, including the PDE access point. A real-world prototype of FSPDE was developed using OP-TEE, a leading open-source Trusted Execution Environment, in tandem with an open-sourced NAND flash controller. Security analysis and experimental evaluations justify both the security and the practicality of our design. 

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4. A Simple Mobile Plausibly Deniable System Using Image Steganography and Secure Hardware

   https://doi.org/10.1145/3643650.3658607  

   Xia, Lichen; Liao, Jinghui; Chen, Niusen; Chen, Bo; Shi, Weisong (June 2024, Proceedings of the 2024 ACM Workshop on Secure and Trustworthy Cyber-Physical Systems (SaT-CPS '24))

   Traditional encryption methods cannot defend against coercive attacks in which the adversary captures both the user and the possessed computing device, and forces the user to disclose the decryption keys. Plausibly deniable encryption (PDE) has been designed to defend against this strong coercive attacker. At its core, PDE allows the victim to plausibly deny the very existence of hidden sensitive data and the corresponding decryption keys upon being coerced. Designing an efficient PDE system for a mobile platform, however, is challenging due to various design constraints bound to the mobile systems. Leveraging image steganography and the built-in hardware security feature of mobile devices, namely TrustZone, we have designed a Simple Mobile Plausibly Deniable Encryption (SMPDE) system which can combat coercive adversaries and, meanwhile, is able to overcome unique design constraints. In our design, the encoding/decoding process of image steganography is bounded together with Arm TrustZone. In this manner, the coercive adversary will be given a decoy key, which can only activate a DUMMY trusted application that will instead sanitize the sensitive information stored hidden in the stego-image upon decoding. On the contrary, the actual user can be given the true key, which can activate the PDE trusted application that can really extract the sensitive information from the stego-image upon decoding. Security analysis and experimental evaluation justify both the security and the efficiency of our design. 

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5. MobiWear: A Plausibly Deniable Encryption System for Wearable Mobile Devices

   https://doi.org/10.1007/978-3-030-80851-8_10  

   Chen, Niusen; Chen, Bo; Shi, Weisong (May 2021, EAI International Conference on Applied Cryptography in Computer and Communications)

   Mobile computing devices are widely used in our daily life. With their increased use, a large amount of sensitive data are collected, stored, and managed in the mobile devices. To protect sensitive data, encryption is often used but, traditional encryption is vulnerable to coercive attacks in which the device owner is coerced by the adversary to disclose the decryption key. To defend against the coercive attacks, Plausibly Deniable Encryption (PDE) has been designed which can allow the victim user to deny the existence of hidden sensitive data. The PDE systems have been explored broadly for smartphones. However, the PDE systems which are suitable for wearable mobile devices are still missing in the literature. In this work, we design MobiWear, the first PDE system specifically for wearable mobile devices. To accommodate the hardware nature of wearable devices, MobiWear: 1) uses image steganography to achieve PDE, which suits the resource-limited wearable devices; and 2) relies on various sensors equipped with the wearable devices to input passwords, rather than requiring users to enter them via a keyboard or a touchscreen. Security analysis and experimental evaluation using a real-world prototype (ported to an LG G smartwatch) show that MobiWear can ensure deniability with a small computational overhead as well as a small decrease of image quality. 

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