Harvard Computer Science Technical Reports for 2015

TR-01-15 [tr-01-15.pdf (361 K)]
Aslan Askarov, Scott Moore, Christos Dimoulas, and Stephen Chong 2015. ``Cryptographic Enforcement of Language-Based Erasure''
Information erasure is a formal security requirement that stipulates when sensitive data must be removed from computer systems. In a system that correctly enforces erasure requirements, an attacker who observes the system after sensitive data is required to have been erased cannot deduce anything about the data. Practical obstacles to enforcing information erasure include: (1) correctly determining which data requires erasure; and (2) reliably deleting potentially large volumes of data, despite untrustworthy storage services.

In this paper, we present a novel formalization of language-based information erasure that supports cryptographic enforcement of erasure requirements: sensitive data is encrypted before storage, and upon erasure, only a relatively small set of decryption keys needs to be deleted. This cryptographic technique has been used by a number of systems that implement data deletion to allow the use of untrustworthy storage services. However, these systems provide no support to correctly determine which data requires erasure, nor have the formal semantic properties of these systems been explained or proven to hold. We address these shortcomings. Specifically, we study a programming language extended with primitives for public-key cryptography, and demonstrate how information-flow control mechanisms can automatically track data that requires erasure and provably enforce erasure requirements even when programs employ cryptographic techniques for erasure.

TR-02-15 [tr-02-15.pdf (361 K)]
Aslan Askarov, Stephen Chong, and Heiko Mantel 2015. ``Global and Local Monitors to Enforce Noninterference in Concurrent Programs'' Controlling confidential information in concurrent systems is difficult, due to covert channels resulting from interaction between threads. This problem is exacerbated if threads share resources at fine granularity.

In this work, we propose a novel monitoring framework to enforce strong information security in concurrent programs. Our monitors are hybrid, combining dynamic and static program analysis to enforce security in a sound and rather precise fashion. In our framework, each thread is guarded by its own local monitor, and there is a single global monitor. We instantiate our monitoring framework to support rely-guarantee style reasoning about the use of shared resources, at the granularity of individual memory locations, and then specialize local monitors further to enforce flow-sensitive progress-sensitive information-flow control. Our local monitors exploit rely-guarantee-style reasoning about shared memory to achieve high precision. Soundness of relyguarantee- style reasoning is guaranteed by all monitors cooperatively. The global monitor is invoked only when threads synchronize, and so does not needlessly restrict concurrency. We prove that our hybrid monitoring approach enforces a knowledgebased progress-sensitive noninterference security condition.

Keywords-Language-based security; information-flow control for concurrent systems; hybrid information-flow monitor.