System support for strong accountability (2009)

Computer systems not only provide unprecedented efficiency and numerous benefits, but also offer powerful means and tools for abuse. This reality is increasingly more evident as deployed software spans across trust domains and enables the interactions of self-interested participants with potentially conflicting goals. With systems growing more complex and interdependent, there is a growing need to localize, identify, and isolate faults and unfaithful behavior. Conventional techniques for building secure systems, such as secure perimeters and Byzantine fault tolerance, are insufficient to ensure that trusted users and software components are indeed trustworthy. Secure perimeters do not work across trust domains and fail when a participant acts within the limits of the existing security policy and deliberately manipulates the system to her own advantage. Byzantine fault tolerance offers techniques to tolerate misbehavior, but offers no protection when replicas collude or are under the control of a single entity. 
Complex interdependent systems necessitate new mechanisms that complement the existing solutions to identify improper behavior and actions, limit the propagation of incorrect information, and assign responsibility when things go wrong. This thesis addresses the problems of misbehavior and abuse by offering tools and techniques to integrate accountability into computer systems. A system is accountable if it offers means to identify and expose semantic misbehavior by its participants. An accountable system can construct undeniable evidence to demonstrate its correctness—the evidence serves as explicit proof of misbehavior and can be strong enough to be used as a basis for social sanction external to the system. 
Accountability offers strong disincentives for abuse and misbehavior but may have to be designed-in'' to an application's specific protocols, logic, and internal representation; achieving accountability using general techniques is a challenge. Extending responsibility to end users for actions performed by software components on their behalf is not trivial, as it requires an ability to determine whether a component correctly represents a user's intentions. Leaks of private information are yet another concern—even correctly functioning applications can leak sensitive information, for which their owners may be accountable. Important infrastructure services, such as distributed virtual resource economies, offer a range of application-specific issues such as fine-grain resource delegation, virtual currency models, and complex work-flows. 
This thesis work addresses the aforementioned problems by designing, implementing, applying, and evaluating a generic methodology for integrating accountability into network services and applications. Our state-based approach decouples application state management from application logic to enable services to demonstrate that they maintain their state in compliance with user requests, i.e., state changes do take place, and the service presents a consistent view to all clients and observers. Internal state managed in this way, can then be used to feed application-specific verifiers to determine the correctness the service's logic and to identify the responsible party. The state-based approach provides support for strong accountability—any detected violation can be proven to a third party without depending on replication and voting. 
In addition to the generic state-based approach, this thesis explores how to leverage application-specific knowledge to integrate accountability in an example application. We study the invariants and accountability requirements of an example application—a lease-based virtual resource economy. We present the design and implementation of several key elements needed to provide accountability in the system. In particular, we describe solutions to the problems of resource delegation, currency spending, and lease protocol compliance. These solutions illustrate a complementary technique to the general-purpose state-based approach, developed in the earlier parts of this thesis. 
Separating the actions of software and its user is at the heart of the third component of this dissertation. We design, implement, and evaluate an approach to detect information leaks in a commodity operating system. Our novel OS abstraction—a doppelganger process—helps track information flow without requiring application rewrite or instrumentation. Doppelganger processes help identify sensitive data as they are about to leave the confines of the system. Users can then be alerted about the potential breach and can choose to prevent the leak to avoid becoming accountable for the actions of software acting on their behalf.