The importance of securing hardware, particularly in the context of the Internet of Things (IoT), cannot be overstated in light of the increasing prevalence of low-level attacks. As the IoT industry continues to expand, security has become a more holistic concern, as evidenced by the wide range of attacks that we observed, from large-scale distributed denial-of-service attacks to data theft through monitoring a device's low-level behavior, such as power consumption. Traditional software-based security measures fall short in defending against the full spectrum of attacks, particularly those involving physical tampering with system hardware.
This underscores the critical importance of proactively integrating attack vectors that encompass both hardware and software domains, with a particular emphasis on considering both the analog and digital characteristics of hardware. This thesis investigates system security from a hardware perspective, specifically examining how low-level circuit behavior and architectural design choices impact SRAM's data remanence and its implications for security. This dissertation not only identifies new vulnerabilities due to SRAM data remanence but also paves the way for novel security solutions in the ongoing "security arms race".
I present an attack, volt boot, that executes cold-boot style short-term data remanence in on-chip SRAM without using temperature effect. This attack exploits the fact that SRAM's power bus is externally accessible and allows data retention using a simple voltage probe. Next, I present a steganography method that hides information in the SRAM exploiting long-term data remanence. This approach leverages aging-induced degradation to imprint data in SRAM's analog domain, ultimately resulting in hidden and plausibly deniable information storage in the hardware.
Finally, I show how an adversary weaponizes SRAM data remanence to develop an attack on a hardware-backed security isolation mechanism. The following provides a brief overview of the three major contributions of this thesis:
1. Volt boot is an attack that demonstrates the vulnerability of on-chip SRAM due to the physical separation common in modern SoCs' power distribution networks. By probing external power pins (to the cache) of an SoC while simultaneously shutting down the main system power, Volt boot creates data retention across power cycles. On-chip SRAM can be a safe memory when the threat model considers traditional off-chip cold-boot-style attacks. This research demonstrates an alternative method for preserving information in on-chip SRAM through power cycles, expanding our understanding of data retention capabilities. Volt boot leverages asymmetrical power states (e.g., on vs. off) to force SRAM state retention across power cycles, eliminating the need for traditional cold boot attack enablers, such as low-temperature or intrinsic data retention time.
2. Invisible Bits is a hardware steganography technique that hides secret messages in the analog domain of SRAM embedded within a computing device. Exploiting accelerated transistor aging, Invisible Bits stores hidden data along with system data in an on-chip cache and provides a plausible deniability guarantee from statistical analysis. Aging changes the transistor's behavior which I exploit to store data permanently (ie long-term data remanence) in an SRAM. Invisible Bits presents unique opportunities for safeguarding electronic devices when subjected to inspections by authorities.
3. UntrustZone utilizes long-term data remanence to exfiltrate secrets from on-chip SRAM.
An attacker application must be able to read retained states in the SRAM upon power cycles, but this needs changing the security privilege. Hardware security schemes, such as ARM TrustZone, erase a memory block before changing its security attributes and releasing it to other applications, making short-term data remanence attacks ineffective. That is, attacks such as Volt boot fail when hardware-backed isolation such as TEE is enforced. UntrustZone unveils a new threat to all forms of on-chip SRAM even when backed by hardware isolation: long-term data remanence. I show how an attacker systematically accelerates data imprinting on SRAM's analog domain to effectively burn in on-chip secrets and bypass TrustZone isolation. / Doctor of Philosophy / In computing systems, hardware serves as the fundamental bulwark against security breaches. The evolution in software security has compelled adversaries to seek potential vulnerabilities in the hardware.The infamous cold boot attack exemplifies such vulnerabilities, showcasing how adversaries exploit hardware to access runtime secrets, even when cryptographic algorithms protect the system's disk. In this attack, volatile main memory (DRAM) is `frozen' at extremely cold temperatures, allowing it to retain information even when disconnected from the victim machine. Subsequently, an adversary transfers this `frozen memory' to another machine to extract the victim's secrets. This classic case is among numerous sophisticated hardware vulnerabilities identified in recent years, highlighting the evolving challenge of securing hardware against ingenious attacks.
This rise in hardware-based attacks across industry and academia underscores the importance of adopting a comprehensive approach to safeguard computing systems. This approach must encompass secure processor design, ensuring a trusted distribution chain, rigorous software security vetting, and protection against runtime side-channel leakage. Consequently, there is a growing emphasis in both industry and academia on prioritizing security in design decisions. My dissertation delves into the low-level hardware behaviors, particularly focusing on the data remanence phenomena of Static Random Access Memory (SRAM). By discovering new security vulnerabilities and proposing effective mitigation strategies, this thesis contributes to the ongoing effort to fortify computing systems against evolving threats that are rooted in the hardware.
SRAM stands as a ubiquitous form of volatile memory found in most processors and microcontrollers, serving as a crucial component for temporary storage of instructions and data to facilitate rapid access.
By design, SRAM forgets its contents upon a processor's power cycle and defaults to a state determined by low-level circuit behavior. However, this dissertation unveils the possibility of retaining on-chip information even after power cycling, leveraging inherent low-level circuit behaviors to create data retention.
This revelation exposes major security implications, resulting in the following three key contributions:
Firstly, I introduce the volt boot attack, which exploits the vulnerability of on-chip SRAM, particularly to physical separation in modern System on Chip (SoC) power distribution networks.
We conventionally assume that on-chip SRAM is secure against off-chip cold-boot attacks, but volt boot demonstrates the feasibility of achieving a similar state without traditional prerequisites such as low temperatures or long intrinsic data retention times.
Subsequently, I propose a data hiding technique---invisible Bits, which leverages accelerated device wear out to embed data into the transistors of SRAM. This method introduces a novel form of hardware-based steganography, concealing data within the analog domain alongside digital system data.
Lastly, I show how accelerated device aging can be weaponized to design a sophisticated attack aimed at extracting secrets from a Trusted Execution Environment (TEE) like ARM TrustZone.
While short-term data remanence attacks such as Volt boot are rendered ineffective against hardware-backed isolation enforced by TEEs, UntrustZone harnesses the methodologies and tools from preceding works to induce long-term data remanence. This poses a new threat to on-chip cryptography that stores secrets on chip, even when fortified by hardware isolation mechanisms, such as ARM TrustZone.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/118742 |
| Date | 02 May 2024 |
| Creators | Mahmod, Jubayer |
| Contributors | Electrical and Computer Engineering, Hicks, Matthew, Xiong, Wenjie, Hsiao, Michael S., Nazhandali, Leyla, Yan, Mengjia |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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