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1. 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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2. TrustZone Enhanced Plausibly Deniable Encryption System for Mobile Devices

   https://doi.org/10.1145/3453142.3493512  

   Liao, Jinghui; Chen, Bo; Shi, Weisong (December 2021, 2021 IEEE/ACM Symposium on Edge Computing (SEC))

   Modern mobile devices are increasingly used to store and process sensitive data. In order to prevent the sensitive data from being leaked, one of the best ways of protecting them and their owner is to hide the data with plausible deniability. Plausibly Deniable Encryption (PDE) has been designed for such purpose. The existing PDE systems for mobile devices however, have suffered from significant drawbacks as they either ignore the deniability compromises present in the special underlying storage media of mobile devices or are vulnerable to various new attacks such as side-channel attacks. In this work, we propose a new PDE system design for mobile devices which takes advantage of the hardware features equipped in the mainstream mobile devices. Our preliminary design has two major component: First, we strictly isolate the hidden and the public data in the flash layer, so that a multi-snapshot adversary is not able to identify the existence of the hidden sensitive data when having access to the low layer storage medium of the device. Second, we incorporate software and operating system level deniability into ARM TrustZone. With this TrustZone-enhanced isolation, our PDE system is immune to side-channel attacks at the operating system layer. 

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3. TruzCall: Secure VoIP Calling on Android using ARM TrustZone

   https://doi.org/10.1109/MobiSecServ48690.2020.9042945  

   Ahlawat, A; Du, W (February 2020, 2020 Sixth International Conference on Mobile And Secure Services)

   Use of mobile phones today has become pervasive throughout society. A common use of a phone involves calling another person using VoIP apps. However the OSes on mobile devices are prone to compromise creating a risk for users who want to have private conversations when calling someone. Mobile devices today provide a hardware-protected mode called trusted execution environment (TEE) to protect users from a compromised OS. In this paper we propose a design to allow a user to make a secure end-to-end protected VoIP call from a compromised mobile phone. We implemented our design, TruzCall using Android OS and TrustZone TEE running OP-TEE OS. We built a prototype using the TrustZone-enabled Hikey development board and tested our design using the open source VoIP app Linphone. Our testing utilizes a simulation based environment that allows a Hikey board to use a real phone for audio hardware. 

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4. TruZ-Droid: Integrating TrustZone with Mobile Operating System

   Ying, Kailiang; Ahlawat, Amit; Alsharifi, Bilal; Jiang, Yuexin; Thavai, Priyank; Du, Wenliang (June 2018, The 16th ACM International Conference on Mobile Systems, Applications, and Services (MobiSys 2018))

   Mobile devices today provide a hardware-protected mode called Trusted Execution Environment (TEE) to help protect users from a compromised OS and hypervisor. Today TEE can only be leveraged either by vendor apps or by developers who work with the vendor. Since vendors consider third-party app code untrusted inside the TEE, to allow an app to leverage TEE, app developers have to write the app code in a tailored way to work with the vendor’s SDK. We proposed a novel design to integrate TEE with mobile OS to allow any app to leverage the TEE. Our design incorporates TEE support at the OS level, allowing apps to leverage the TEE without adding app-specific code into the TEE, and while using existing interface to interact with the mobile OS. We implemented our design, called TruZ-Droid, by integrating TrustZone TEE with the Android OS. TruZ-Droid allows apps to leverage the TEE to protect the following: (i) user’s secret input and confirmation, and (ii) sending of user’s secrets to the authorized server. We built a prototype using the TrustZone-enabled HiKey board to evaluate our design. We demonstrated TruZ-Droid’s effectiveness by adding new security features to existing apps to protect user’s sensitive information and attest user’s confirmation. TruZ-Droid’s real-world use case evaluation shows that apps can leverage TrustZone while using existing OS APIs. Our usability study proves that users can correctly interact with TruZ-Droid to protect their security sensitive activities and data. 

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5. 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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