System Support for Strong Accountability
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
<italic>trustworthy</italic>. 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 <italic>accountability</italic> into computer systems. A
system is accountable if it offers means to identify and expose
<italic>semantic</italic> 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
<italic>state-based</italic> 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 <italic>strong</italic> 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 <italic>doppelganger</italic> 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.
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