Andrew K. Hirsch

Ashley Samuelson, Andrew K. Hirsch, Ethan Cecchetti

Traditional concurrent-programming techniques require programmers to painstakingly write programs for each participant in a concurrent system. Choreographic programming, in contrast, allows a programmer to write one centralized program and compile it to individual programs. This approach simplifies critical properties like deadlock freedom, but it complicates forking new processes, a core primitive in concurrent programming. This work addresses that gap with the choreographic fork calculus $λ{\pitchfork}$, the first functional choreographic language with process forking. $λ{\pitchfork}$ provides a deadlock-freedom guarantee while allowing programs to dynamically determine when to spawn new processes, what they will do, and who will communicate with them. In doing so, it supports practical applications like load balancers and parallel divide-and-conquer.

Jan Menz, Andrew K. Hirsch, Peixuan Li et al.

To ensure programs do not leak private data, we often want to be able to provide formal guarantees ensuring such data is handled correctly. Often, we cannot keep such data secret entirely; instead programmers specify how private data may be declassified. While security definitions for declassification exist, they mostly do not handle higher-order programs. In fact, in the higher-order setting no compositional security definition exists for intensional information-flow properties such as where declassification, which allows declassification in specific parts of a program. We use logical relations to build a model (and thus security definition) of where declassification. The key insight required for our model is that we must stop enforcing indistinguishability once a \emph{relevant declassification} has occurred. We show that the resulting security definition provides more security than the most related previous definition, which is for the lower-order setting. This paper is an extended version of the paper of the same name published at OOPSLA 2023 ([21]).

Andrew K. Hirsch, Ethan Cecchetti

Type systems designed for information-flow control commonly use a program-counter label to track the sensitivity of the context and rule out data leakage arising from effectful computation in a sensitive context. Currently, type-system designers reason about this label informally except in security proofs, where they use ad-hoc techniques. We develop a framework based on monadic semantics for effects to give semantics to program-counter labels. This framework leads to three results about program-counter labels. First, we develop a new proof technique for noninterference, the core security theorem for information-flow control in effectful languages. Second, we unify notions of security for different types of effects, including state, exceptions, and nontermination. Finally, we formalize the folklore that program-counter labels are a lower bound on effects. We show that, while not universally true, this folklore has a good semantic foundation.

Andrew K. Hirsch, Pedro H. Azevedo de Amorim, Ethan Cecchetti et al.

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.

Andrew K. Hirsch, Michael R. Clarkson

Authorization logics have been used in the theory of computer security to reason about access control decisions. In this work, a formal belief semantics for authorization logics is given. The belief semantics is proved to subsume a standard Kripke semantics. The belief semantics yields a direct representation of principals' beliefs, without resorting to the technical machinery used in Kripke semantics. A proof system is given for the logic; that system is proved sound with respect to the belief and Kripke semantics. The soundness proof for the belief semantics, and for a variant of the Kripke semantics, is mechanized in Coq.