Beneath the Attack Surface

Abstract

Computer systems are often analyzed as purely virtual artifacts, a collection of software operating on a Platonic ideal of a computer. When software is executed, it runs on actual hardware: an increasingly complex web of analog physical components and processes, cleverly strung together to present an illusion of pure computation. When an abstract software system is combined with individual hardware instances to form functioning systems, the overall behavior varies subtly with the hardware. These minor variations can change the security and privacy guarantees of the entire system, in both beneficial and harmful ways. We examine several such security effects in this dissertation.

First, we look at the fingerprinting capability of JavaScript and HTML5: when invoking existing features of modern browsers, such as JavaScript execution and 3-D graphics, how are the results affected by underlying hardware, and how distinctive is the resulting fingerprint?

Second, we discuss AES side channel timing attacks, a technique to extract information from AES encryption running on hardware. We present several reasons why we were unable to reproduce this attack against modern hardware and a modern browser.

Third, we examine positive uses of hardware variance: namely, seeding Linux's pseudorandom number generator at kernel initialization time with true entropy gathered during early boot. We examine the utility of these techniques on a variety of embedded devices, and give estimates for the amount of entropy each can generate.

Lastly, we evaluate a cyberphysical system: one which combines physical processes and analog sensors with software control and interpretation. Specifically, we examine the Rapiscan Secure~1000 backscatter X-ray full-body scanner, a device for looking under a scan subject's clothing, discovering any contraband secreted about their person. We present a full security analysis of this system, including its hardware, software, and underlying physics, and show how an adaptive, motivated adversary can completely subvert the scan to smuggle contraband, such as knives, firearms, and plastic explosives, past a Secure~1000 checkpoint. These attacks are entirely based upon understanding the physical processes and sensors which underlie this cyberphysical system, and involve adjusting the contraband's location and shape until it simply disappears.