A Semantic Hierarchy for Erasure Policies

We consider the problem of logical data erasure, contrasting with physical erasure in the same way that end-to-end information flow control contrasts with access control. We present a semantic hierarchy for erasure policies, using a possibilistic knowledge-based semantics to define policy satisfaction such that there is an intuitively clear upper bound on what information an erasure policy permits to be retained. Our hierarchy allows a rich class of erasure policies to be expressed, taking account of the power of the attacker, how much information may be retained, and under what conditions it may be retained. While our main aim is to specify erasure policies, the semantic framework allows quite general information-flow policies to be formulated for a variety of semantic notions of secrecy.Comment: 18 pages, ICISS 201

First-Order Logic for Flow-Limited Authorization

We present the Flow-Limited Authorization First-Order Logic (FLAFOL), a logic for reasoning about authorization decisions in the presence of information-flow policies. We formalize the FLAFOL proof system, characterize its proof-theoretic properties, and develop its security guarantees. In particular, FLAFOL is the first logic to provide a non-interference guarantee while supporting all connectives of first-order logic. Furthermore, this guarantee is the first to combine the notions of non-interference from both authorization logic and information-flow systems. All theorems in this paper are proven in Coq.Comment: Coq code can be found at https://github.com/FLAFOL/flafol-co

Stateful Declassification Policies for Event-Driven Programs

International audience—We propose a novel mechanism for enforcing information flow policies with support for declassification on event-driven programs. Declassification policies consist of two functions. First, a projection function specifies for each confidential event what information in the event can be declassified directly. This generalizes the traditional security labelling of inputs. Second, a stateful release function specifies the aggregate information about all confidential events seen so far that can be declassified. We provide evidence that such declassification policies are useful in the context of JavaScript web applications. An enforcement mechanism for our policies is presented and its soundness and precision is proven. Finally, we give evidence of practicality by implementing and evaluating the mechanism in a browser

Quantitative information-flow tracking for real systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 99-105).An information-flow security policy constrains a computer system's end-to-end use of information, even as it is transformed in computation. For instance, a policy would not just restrict what secret data could be revealed directly, but restrict any output that might allow inferences about the secret. Expressing such a policy quantitatively, in terms of a specific number of bits of information, is often an effective program independent way of distinguishing what scenarios should be allowed and disallowed. This thesis describes a family of new techniques for measuring how much information about a program's secret inputs is revealed by its public outputs on a particular execution, in order to check a quantitative policy on realistic systems. Our approach builds on dynamic tainting, tracking at runtime which bits might contain secret in formation, and also uses static control-flow regions to soundly account for implicit flows via branches and pointer operations. We introduce a new graph model that bounds information flow by the maximum flow between inputs and outputs in a flow network representation of an execution. The flow bounds obtained with maximum flow are much more precise than those based on tainting alone (which is equivalent to graph reachability). The bounds are a conservative estimate of channel capacity: the amount of information that could be transmitted by an adversary making an arbitrary choice of secret inputs. We describe an implementation named Flowcheck, built using the Valgrind framework for x86/Linux binaries, and use it to perform case studies on six real C, C++, and Objective C programs, three of which have more than 250,000 lines of code. We used the tool to check the confidentiality of a different kind of information appropriate to each program. Its results either verified that the information was appropriately kept secret on the examined executions, or revealed unacceptable leaks, in one case due to a previously unknown bug.by Stephen Andrew McCamant.Ph.D

End-to-End Enforcement of Erasure and Declassification

Declassification occurs when the confidentiality of information is weakened; erasure occurs when the confidentiality of information is strengthened, perhaps to the point of completely removing the information from the system. This paper shows how to enforce erasure and declassification policies. A combination of a type system that controls information flow and a simple runtime mechanism to overwrite data ensures end-to-end enforcement of policies. We prove that well-typed programs satisfy the semantic security condition noninterference according to policy. We extend the Jif programming language with erasure and declassification enforcement mechanisms and use the resulting language in a large case study of a voting system.