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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. 

Ransomware is increasingly prevalent in recent years. To defend against ransomware in computing devices using flash memory as external storage, existing designs extract the entire raw flash memory data to restore the external storage to a good state. However, they cannot allow a fine-grained recovery in terms of user files as raw flash memory data do not have the semantics of "files". In this work, we design FFRecovery, a new ransomware defense strategy that can support fine-grained data recovery after the attacks. Our key idea is, to recover a file corrupted by the ransomware, we can 1) restore its file system metadata via file system forensics, and 2) extract its file data via raw data extraction from the flash translation layer, and 3) assemble the corresponding file system metadata and the file data. A simple prototype of FFRecovery has been developed and some preliminary results are provided. 

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. 

Combating the OS-level malware is a very challenging problem as this type of malware can compromise the operating system, obtaining the kernel privilege and subverting almost all the existing anti-malware tools. This work aims to address this problem in the context of mobile devices. As real-world malware is very heterogeneous, we narrow down the scope of our work by especially focusing on a special type of OS-level malware that always corrupts user data. We have designed mobiDOM, the first framework that can combat the OS-level data corruption malware for mobile computing devices. Our mobiDOM contains two components, a malware detector and a data repairer. The malware detector can securely and timely detect the presence of OS-level malware by fully utilizing the existing hardware features of a mobile device, namely, flash memory and Arm TrustZone. Specifically, we integrate the malware detection into the flash translation layer (FTL), a firmware layer embedded into the flash storage hardware, which is inaccessible to the OS; in addition, we run a trusted application in the Arm TrustZone secure world, which acts as a user-level manager of the malware detector. The FTL-based malware detection and the TrustZone-based manager can communicate with each other stealthily via steganography. The data repairer can allow restoring the external storage to a healthy historical state by taking advantage of the out-of-place-update feature of flash memory and our malware-aware garbage collection in the FTL. Security analysis and experimental evaluation on a real-world testbed confirm the effectiveness of mobiDOM. 

Flash memory has been used extensively as external storage of smartphones, tablets, IoT devices, laptops, etc. Therefore, more and more sensitive or even mission critical data are stored in flash and, once the data turn obsolete, securely deleting them is necessary for both regulation compliance and privacy protection. Traditional secure deletion on flash memory mainly focuses on sanitizing data. However, unique nature of flash memory may cause various data ``remnants'' and, even though the data are removed, the remnants may be utilized by the adversary to recover the deleted data, compromising the secure deletion guarantee. Based on both theoretic analysis and experiments using real-world workloads, we have identified one common type of remnants in the flash memory, namely duplicates, which are caused by unique internal functions of flash storage media including garbage collection, wear leveling, bad block management. We propose RedFlash, a novel secure deletion scheme which can efficiently Remove both the data and the corresponding duplicates towards secure deletion on Flash memory. Security analysis and experimental evaluation show that RedFlash can ensure the secure deletion guarantee, at the cost of a small performance degradation, compared to a regular (non-secure) flash controller. 

Detecting the OS-level malware (e.g., rootkit) is an especially challenging problem, as this type of malware can compromise the OS, and can then easily hide their intrusion behaviors or directly subvert the traditional malware detectors running in either the user or the kernel space. In this work, we propose mobiDOM to solve this problem for mobile computing devices. The key idea of mobiDOM is to securely detect the OS-level malware by fully utilizing the existing secure features of a mobile device in the hardware. Specifically, we integrate a malware detector in the flash translation layer (FTL), a firmware layer embedded into the external flash storage which is inaccessible to the OS; in addition, we build a trusted application in the Arm TrustZone secure world, which acts as a user-level controller of the malware detector. The FTL-based malware detector and the TrustZone-based controller communicate with each other stealthily via steganography. Security analysis and experimental evaluation confirm that mobiDOM can securely and effectively detect the OS-level malware. 

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.