Task-Structured Probabilistic I/O Automata

"May 28, 2009."Modeling frameworks such as Probabilistic I/O Automata (PIOA) and Markov Decision Processes permit both probabilistic and nondeterministic choices. In order to use these frameworks to express claims about probabilities of events, one needs mechanisms for resolving nondeterministic choices. For PIOAs, nondeterministic choices have traditionally been resolved by schedulers that have perfect information about the past execution. However, these schedulers are too powerful for certain settings, such as cryptographic protocol analysis, where information must sometimes be hidden. Here, we propose a new, less powerful nondeterminism-resolution mechanism for PIOAs, consisting of tasks and local schedulers. Tasks are equivalence classes of system actions that are scheduled by oblivious, global task sequences. Local schedulers resolve nondeterminism within system components, based on local information only. The resulting task-PIOA framework yields simple notions of external behavior and implementation, and supports simple compositionality results. We also define a new kind of simulation relation, and show it to be sound for proving implementation. We illustrate the potential of the task-PIOAframework by outlining its use in verifying an Oblivious Transfer protocol

Universally Composable Security With Local Adversaries

The traditional approach to formalizing ideal-model based definitions of security for multi-party protocols models adversaries (both real and ideal) as centralized entities that control all parties that deviate from the protocol. While this centralized-adversary modeling suffices for capturing basic security properties such as secrecy of local inputs and correctness of outputs against coordinated attacks, it turns out to be inadequate for capturing security properties that involve restricting the sharing of information between separate adversarial entities. Indeed, to capture collusion-freeness and and game-theoretic solution concepts, Alwen et.al. [Crypto, 2012] propose a new ideal-model based definitional framework that involves a de-centralized adversary. We propose an alternative framework to that of Alwen et. al. We then observe that our framework allows capturing not only collusion-freeness and game-theoretic solution concepts, but also several other properties that involve the restriction of information flow among adversarial entities. These include some natural flavors of anonymity, deniability, timing separation, and information confinement. We also demonstrate the inability of existing formalisms to capture these properties. We then prove strong composition properties for the proposed framework, and use these properties to demonstrate the security, within the new framework, of two very different protocols for securely evaluating any function of the parties’ inputs