CPP 2020: Proceedings of the 9th ACM SIGPLAN International Conference on Certified Programs and Proofs

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SESSION: Invited Talks

Matching logic: the foundation of the K framework (invited talk)

The K framework (kframework.org) is an effort in realizing the ideal language framework, where programming languages must have formal semantics and all language tools are automatically generated from the formal semantics. Until recently, K has been developed as an engineering endeavor driven by challenges such as formalizing the complete semantics of large languages (C, Java, JavaScript, Python, etc), but deriving its semantics from translations to various formalisms, such as rewriting logic, graph transformations, or Coq. This semantics borrowing approach came not only at a notational cost, where the original language meaning was ``lost in translation'', but also at a foundational cost: the target formalisms were more complicated than necessary, yet more restricted due to their prescribed ways to define language semantics.

In this talk I will present matching logic as the new and direct foundation of K. I will also discuss some of its applications in defining constructors, transition systems, modal mu-logic and temporal logic variants, reachability logic and thus Hoare logics, and separation-logic-style recursive predicates and patterns. Matching logic can therefore be regarded as an expressive foundation for programming languages, and K as a best effort implementation. An appealing aspect of matching logic’s Hilbert-style proof system is that it admits a small proof checker, in the order of 200 LOC including parsing.

Proof assistants at the hardware-software interface (invited talk)

Some of the earliest applications of proof assistants were to correctness of digital hardware designs, but the subject doesn’t come up too frequently today at venues like CPP. I will try to make the case that proof assistants are a crucial tool for resolving both classical problems and new ones at the hardware-software interface. That is, it is important to understand exactly what guarantees a processor exports to software, it is important to verify that hardware exports those guarantees correctly, and it is important to prove end-to-end theorems covering both hardware and software. A few social developments make this an exciting time to tackle these problems: open instruction sets and open-source hardware designs are growing in real-world relevance, and surprising new classes of security vulnerabilities have gotten more practitioners thinking about precise hardware-software contracts. I will sketch the state of the research area and go into detail on a few of my own related projects. An ancillary goal is to convey that programming or proving digital hardware is a lot like programming or proving software, with a few fun distinctions, so more of the CPP crowd might want to give it a try!

SESSION: Program Verification

A verified packrat parser interpreter for parsing expression grammars

Parsing expression grammars (PEGs) offer a natural opportunity for building verified parser interpreters based on higher-order parsing combinators. PEGs are expressive, unambiguous, and efficient to parse in a top-down recursive descent style. We use the rich type system of the PVS specification language and verification system to formalize the metatheory of PEGs and define a reference implementation of a recursive parser interpreter for PEGs. In order to ensure termination of parsing, we define a notion of a well-formed grammar. Rather than relying on an inductive definition of parsing, we use abstract syntax trees that represent the computational trace of the parser to provide an effective proof certificate for correct parsing and ensure that parsing properties including soundness and completeness are maintained. The correctness properties are embedded in the types of the operations so that the proofs can be easily constructed from local proof obligations. Building on the reference parser interpreter, we define a packrat parser interpreter as well as an extension that is capable of semantic interpretation. Both these parser interpreters are proved equivalent to the reference one. All of the parsers are executable. The proofs are formalized in mathematical terms so that similar parser interpreters can be defined in any specification language with a type system similar to PVS.

Proof pearl: Braun trees

Braun trees are functional data structures for implementing extensible arrays and priority queues (and sorting functions based on the latter) efficiently. Some well-known functions on Braun trees have not yet been verified, including especially Okasaki’s linear time conversion from lists to Braun trees. We supply the missing proofs and verify all of these algorithms in Isabelle, including non-obvious time complexity claims. In particular we provide the first linear-time conversion from Braun trees to lists. We also state and verify a new characterization of Braun trees as the trees t whose index set is the interval {1, …, size of t}.

FreeSpec: specifying, verifying, and executing impure computations in Coq

FreeSpec is a framework for the Coq theorem prover which allows for specifying and verifying complex systems as hierarchies of components verified both in isolation and in composition. While FreeSpec was originally introduced for reasoning about hardware architectures, in this article we propose a novel iteration of FreeSpec formalism specifically designed to write certified programs and libraries. Then, we present in depth how we use this formalism to verify a static files webserver. We use this opportunity to present FreeSpec proof automation tactics, and to demonstrate how they successfully erase FreeSpec internal definitions to let users focus on the core of their proofs. Finally, we introduce FreeSpec.Exec, a plugin for Coq to seamlessly execute certified programs written with FreeSpec.

SESSION: Automated Verification and SAT Solving

Verifying x86 instruction implementations

Verification of modern microprocessors is a complex task that requires a substantial allocation of resources. Despite significant progress in formal verification, the goal of complete verification of an industrial design has not been achieved. In this paper, we describe a current contribution of formal methods to the validation of modern x86 microprocessors at Centaur Technology. We focus on proving correctness of instruction implementations, which includes the decoding of an instruction, its translation into a sequence of micro-operations, any subsequent execution of traps to microcode ROM, and the implementation of these micro-operations in execution units. All these tasks are performed within one verification framework, which includes a theorem prover, a verified symbolic simulator, and SAT solvers. We describe the work of defining the needed formal models for both the architecture and micro-architecture in this framework, as well as tools for decomposing the requisite properties into smaller lemmas which can be automatically checked. We additionally cover the advantages and limitations of our approach. To our knowledge, there are no similar results in the verification of implementations of an x86 microprocessor.

Frying the egg, roasting the chicken: unit deletions in DRAT proofs

The clausal proof format DRAT is the standard de facto to certify SAT solvers' unsatisfiability results. DRAT proofs act as logs of clause inferences and clause deletions in the solver. The non-monotonic nature of the proof system makes deletions relevant. State-of-the-art proof checkers ignore deletions of unit clauses, differing from the standard in meaningful ways that require adaptions when proofs are generated or used for purposes other than checking. On the other hand, dealing with unit deletions in the proof checker breaks many of the usual invariants used for efficiency reasons. Furthermore, many SAT solvers introduce spurious unit deletions in proofs. These deletions are never intended to be applied in the checker but are nevertheless introduced, making many proofs generated by state-of-the-art solvers incorrect. We present the first competitive DRAT checker that honors unit deletions, as well as fixes for the spurious deletion issue in proof generation. Our experimental results confirm that unit deletions can be applied with similar average performance to state-of-the-art checkers. We also confirm that a large fraction of the proofs generated during the last SAT solving competition do not respect the DRAT standard. This result was confirmed with proof incorrectness certificates that were independently validated. We find that our proof incorrectness certificates can be of help when debugging SAT solvers and DRAT checkers.

SESSION: Proof Engineering and User Interaction

An equational theory for weak bisimulation via generalized parameterized coinduction

Coinductive reasoning about infinitary structures such as streams is widely applicable. However, practical frameworks for developing coinductive proofs and finding reasoning principles that help structure such proofs remain a challenge, especially in the context of machine-checked formalization.

This paper gives a novel presentation of an equational theory for reasoning about structures up to weak bisimulation. The theory is both compositional, making it suitable for defining general-purpose lemmas, and also incremental, meaning that the bisimulation can be created interactively. To prove the theory’s soundness, this paper also introduces generalized parameterized coinduction, which addresses expressivity problems of earlier works and provides a practical framework for coinductive reasoning. The paper presents the resulting equational theory for streams, but the technique applies to other structures too.

All of the results in this paper have been proved in Coq, and the generalized parameterized coinduction framework is available as a Coq library.

Exploration of neural machine translation in autoformalization of mathematics in Mizar

In this paper we share several experiments trying to automatically translate informal mathematics into formal mathematics. In our context informal mathematics refers to human-written mathematical sentences in the LaTeX format; and formal mathematics refers to statements in the Mizar language. We conducted our experiments against three established neural network-based machine translation models that are known to deliver competitive results on translating between natural languages. To train these models we also prepared four informal-to-formal datasets. We compare and analyze our results according to whether the model is supervised or unsupervised. In order to augment the data available for auto-formalization and improve the results, we develop a custom type-elaboration mechanism and integrate it in the supervised translation.

REPLica: REPL instrumentation for Coq analysis

Proof engineering tools make it easier to develop and maintain large systems verified using interactive theorem provers. Developing useful proof engineering tools hinges on understanding the development processes of proof engineers. This paper breaks down one barrier to achieving that understanding: remotely collecting granular data on proof developments as they happen.

We have built a tool called REPLica that instruments Coq’s interaction model in order to collect fine-grained data on proof developments. It is decoupled from the user interface, and designed in a way that generalizes to other interactive theorem provers with similar interaction models.

We have used REPLica to collect data over the span of a month from a group of intermediate through expert proof engineers—enough data to reconstruct hundreds of interactive sessions. The data reveals patterns in fixing proofs and in changing programs and specifications useful for the improvement of proof engineering tools. Our experiences conducting this study suggest design considerations both at the level of the study and at the level of the interactive theorem prover that can facilitate future studies of this kind.

SESSION: Decidability and Complexity

Verified programming of Turing machines in Coq

We present a framework for the verified programming of multi-tape Turing machines in Coq. Improving on prior work by Asperti and Ricciotti in Matita, we implement multiple layers of abstraction. The highest layer allows a user to implement nontrivial algorithms as Turing machines and verify their correctness, as well as time and space complexity compositionally. The user can do so without ever mentioning states, symbols on tapes or transition functions: They write programs in an imperative language with registers containing values of encodable data types, and our framework constructs corresponding Turing machines.

As case studies, we verify a translation from multi-tape to single-tape machines as well as a universal Turing machine, both with polynomial time overhead and constant factor space overhead.

A functional proof pearl: inverting the Ackermann hierarchy

We implement in Gallina a hierarchy of functions that calculate the upper inverses to the hyperoperation/Ackermann hierarchy. Our functions run in Θ(b) for inputs expressed in unary, and in O(b2) for inputs expressed in binary (where b = bitlength). We use our inverses to define linear-time functions—Θ(b) for both unary-represented and binary-represented inputs—that compute the upper inverse of the diagonal Ackermann function A(n). We show that these functions are consistent with the usual definition of the inverse Ackermann function α(n).

Undecidability of higher-order unification formalised in Coq

We formalise undecidability results concerning higher-order unification in the simply-typed λ-calculus with β-conversion in Coq. We prove the undecidability of general higher-order unification by reduction from Hilbert’s tenth problem, the solvability of Diophantine equations, following a proof by Dowek. We sharpen the result by establishing the undecidability of second-order and third-order unification following proofs by Goldfarb and Huet, respectively.

Goldfarb’s proof for second-order unification is by reduction from Hilbert’s tenth problem. Huet’s original proof uses the Post correspondence problem (PCP) to show the undecidability of third-order unification. We simplify and formalise his proof as a reduction from modified PCP. We also verify a decision procedure for first-order unification.

All proofs are carried out in the setting of synthetic undecidability and rely on Coq’s built-in notion of computation.

SESSION: Homotopy Type Theory

Cubical synthetic homotopy theory

Homotopy type theory is an extension of type theory that enables synthetic reasoning about spaces and homotopy theory. This has led to elegant computer formalizations of multiple classical results from homotopy theory. However, many proofs are still surprisingly complicated to formalize. One reason for this is the axiomatic treatment of univalence and higher inductive types which complicates synthetic reasoning as many intermediate steps, that could hold simply by computation, require explicit arguments. Cubical type theory offers a solution to this in the form of a new type theory with native support for both univalence and higher inductive types. In this paper we show how the recent cubical extension of Agda can be used to formalize some of the major results of homotopy type theory in a direct and elegant manner.

Three equivalent ordinal notation systems in cubical Agda

We present three ordinal notation systems representing ordinals below ε0 in type theory, using recent type-theoretical innovations such as mutual inductive-inductive definitions and higher inductive types. We show how ordinal arithmetic can be developed for these systems, and how they admit a transfinite induction principle. We prove that all three notation systems are equivalent, so that we can transport results between them using the univalence principle. All our constructions have been implemented in cubical Agda.

SESSION: Mechanized Metatheory

Coq à la carte: a practical approach to modular syntax with binders

The mechanisation of the meta-theory of programming languages is still considered hard and requires considerable effort. When formalising properties of the extension of a language, one hence wants to reuse definitions and proofs. But type-theoretic proof assistants use inductive types and predicates to formalise syntax and type systems, and these definitions are closed to extensions. Available approaches for modular syntax are either inapplicable to type theory or add a layer of indirectness by requiring complicated encodings of types.

We present a concise, transparent, and accessible approach to modular syntax with binders by adapting Swierstra's Data Types à la Carte approach to the Coq proof assistant. Our approach relies on two phases of code generation: We extend the Autosubst 2 tool and allow users to specify modular syntax with binders in a HOAS-like input language. To state and automatically compose modular functions and lemmas, we implement commands based on MetaCoq. We support modular syntax, functions, predicates, and theorems.

We demonstrate the practicality of our approach by modular proofs of preservation, weak head normalisation, and strong normalisation for several variants of mini-ML.

A mechanized formalization of GraphQL

GraphQL is a novel language for specifying and querying web APIs, allowing clients to flexibly and efficiently retrieve data of interest. The GraphQL language specification is unfortunately only available in prose, making it hard to develop robust formal results for this language. Recently, Hartig and Pérez proposed a formal semantics for GraphQL in order to study the complexity of GraphQL queries. The semantics is however not mechanized and leaves certain key aspects unverified. We present GraphCoQL, the first mechanized formalization of GraphQL, developed in the Coq proof assistant. GraphCoQL covers the schema definition DSL, query definitions, validation of both schema and queries, as well as the semantics of queries over a graph data model. We illustrate the application of GraphCoQL by formalizing the key query transformation and interpretation techniques of Hartig and Pérez, and proving them correct, after addressing some imprecisions and minor issues. We hope that GraphCoQL can serve as a solid formal baseline for both language design and verification efforts for GraphQL.

ConCert: a smart contract certification framework in Coq

We present a new way of embedding functional languages into the Coq proof assistant by using meta-programming. This allows us to develop the meta-theory of the language using the deep embedding and provides a convenient way for reasoning about concrete programs using the shallow embedding. We connect the deep and the shallow embeddings by a soundness theorem. As an instance of our approach, we develop an embedding of a core smart contract language into Coq and verify several important properties of a crowdfunding contract based on a previous formalisation of smart contract execution in blockchains.

SESSION: Verified Cryptography

Formalising oblivious transfer in the semi-honest and malicious model in CryptHOL

Multi-Party Computation (MPC) allows multiple parties to compute a function together while keeping their inputs private. Large scale implementations of MPC protocols are becoming practical thus it is important to have strong guarantees for the whole development process, from the underlying cryptography to the implementation. Computer aided proofs are a way to provide such guarantees.

We use CryptHOL to formalise a framework for reasoning about two party protocols using the security definitions for MPC. In particular we consider protocols for 1-out-of-2 Oblivious Transfer (OT21) — a fundamental MPC protocol — in both the semi-honest and malicious models. We then extend our semi-honest formalisation to OT41 which is a building block for our proof of security for the two party GMW protocol — a protocol that can securely compute any Boolean circuit.

The semi-honest OT21 protocol we formalise is constructed from Extended Trapdoor Permutations (ETP), we first prove the general construction secure and then instantiate for the RSA collection of functions — a known ETP. Our general proof assumes only the existence of ETPs, meaning any instantiated results come without needing to prove any security properties, only that the requirements of an ETP are met.

Verified security of BLT signature scheme

The majority of real-world applications of digital signatures use timestamping to ensure non-repudiation in face of possible key revocations. This observation led Buldas, Laanoja, and Truu to a server-assisted digital signature scheme built around cryptographic timestamping.

In this paper, we report on the machine-checked proofs of existential unforgeability under the chosen-message attack (EUF-CMA) of some variations of BLT digital signature scheme. The proofs are developed and verified using the EasyCrypt framework, which provides interactive theorem proving supported by the state-of-the-art SMT solvers.

SESSION: Concurrency and Linearity

Formalizing determinacy of concurrent revisions

Concurrent revisions is a concurrency control model designed to guarantee determinacy, meaning that the outcomes of programs are uniquely determined. This paper describes an Isabelle/HOL formalization of the model’s operational semantics and proof of determinacy. We discuss and resolve subtle ambiguities in the operational semantics and simplify the proof of determinacy. Although our findings do not appear to correspond to bugs in implementations, the formalization highlights some of the challenges involved in the design and verification of concurrency control models.

Formalizing 𝜋-calculus in guarded cubical Agda

Dependent type theories with guarded recursion have shown themselves suitable for the development of denotational semantics of programming languages. In particular Ticked Cubical Type Theory (TCTT) has been used to show that for guarded labelled transition systems (GLTS) interpretation into the denotational semantics maps bisimilar processes to equal values. In fact the two notions are proved equivalent, allowing one to reason about equality in place of bisimilarity.

We extend that result to the π-calculus, picking early congruence as the syntactic notion of equivalence between processes, showing that denotational models based on guarded recursive types can handle the dynamic creation of channels that goes beyond the scope of GLTSs.

Hence we present a fully abstract denotational model for the early π-calculus, formalized as an extended example for Guarded Cubical Agda: a novel implementation of Ticked Cubical Type Theory based on Cubical Agda.

Intrinsically-typed definitional interpreters for linear, session-typed languages

An intrinsically-typed definitional interpreter is a concise specification of dynamic semantics, that is executable and type safe by construction. Unfortunately, scaling intrinsically-typed definitional interpreters to more complicated object languages often results in definitions that are cluttered with manual proof work. For linearly-typed languages (including session-typed languages) one has to prove that the interpreter, as well as all the operations on semantic components, treat values linearly. We present new methods and tools that make it possible to implement intrinsically-typed definitional interpreters for linearly-typed languages in a way that hides the majority of the manual proof work. Inspired by separation logic, we develop reusable and composable abstractions for programming with linear operations using dependent types. Using these abstractions, we define interpreters for linear lambda calculi with strong references, concurrency, and session-typed communication in Agda.

SESSION: Formalized Mathematics 1

Formalising perfectoid spaces

Perfectoid spaces are sophisticated objects in arithmetic geometry introduced by Peter Scholze in 2012. We formalised enough definitions and theorems in topology, algebra and geometry to define perfectoid spaces in the Lean theorem prover. This experiment confirms that a proof assistant can handle complexity in that direction, which is rather different from formalising a long proof about simple objects. It also confirms that mathematicians with no computer science training can become proficient users of a proof assistant in a relatively short period of time. Finally, we observe that formalising a piece of mathematics that is a trending topic boosts the visibility of proof assistants amongst pure mathematicians.

A constructive formalization of the weak perfect graph theorem

The Perfect Graph Theorems are important results in graph theory describing the relationship between clique number ω(G) and chromatic number χ(G) of a graph G. A graph G is called perfect if χ(H)=ω(H) for every induced subgraph H of G. The Strong Perfect Graph Theorem (SPGT) states that a graph is perfect if and only if it does not contain an odd hole (or an odd anti-hole) as its induced subgraph. The Weak Perfect Graph Theorem (WPGT) states that a graph is perfect if and only if its complement is perfect. In this paper, we present a formal framework for working with finite simple graphs. We model finite simple graphs in the Coq Proof Assistant by representing its vertices as a finite set over a countably infinite domain. We argue that this approach provides a formal framework in which it is convenient to work with different types of graph constructions (or expansions) involved in the proof of the Lovász Replication Lemma (LRL), which is also the key result used in the proof of Weak Perfect Graph Theorem. Finally, we use this setting to develop a constructive formalization of the Weak Perfect Graph Theorem.

Completeness of an axiomatization of graph isomorphism via graph rewriting in Coq

The labeled multigraphs of treewidth at most two can be described using a simple term language over which isomorphism of the denoted graphs can be finitely axiomatized. We formally verify soundness and completeness of such an axiomatization using Coq and the mathematical components library. The completeness proof is based on a normalizing and confluent rewrite system on term-labeled graphs. While for most of the development a dependently typed representation of graphs based on finite types of vertices and edges is most convenient, we switch to a graph representation employing a fixed type of vertices shared among all graphs for establishing confluence of the rewrite system. The completeness result is then obtained by transferring confluence from the fixed-type setting to the dependently typed setting.

SESSION: Formalized Mathematics 2

The Poincaré-Bendixson theorem in Isabelle/HOL

The Poincaré-Bendixson theorem is a classical result in the study of (continuous) dynamical systems. Colloquially, it restricts the possible behaviors of planar dynamical systems: such systems cannot be chaotic. In practice, it is a useful tool for proving the existence of (limiting) periodic behavior in planar systems. The theorem is an interesting and challenging benchmark for formalized mathematics because proofs in the literature rely on geometric sketches and only hint at symmetric cases. It also requires a substantial background of mathematical theories, e.g., the Jordan curve theorem, real analysis, ordinary differential equations, and limiting (long-term) behavior of dynamical systems.

We present a proof of the theorem in Isabelle/HOL and highlight the main challenges, which include: i) combining large and independently developed mathematical libraries, namely the Jordan curve theorem and ordinary differential equations, ii) formalizing fundamental concepts for the study of dynamical systems, namely the α, ω-limit sets, and periodic orbits, iii) providing formally rigorous arguments for the geometric sketches paramount in the literature, and iv) managing the complexity of our formalization throughout the proof, e.g., appropriately handling symmetric cases.

A formal proof of the independence of the continuum hypothesis

We describe a formal proof of the independence of the continuum hypothesis (CH) in the Lean theorem prover. We use Boolean-valued models to give forcing arguments for both directions, using Cohen forcing for the consistency of ¬ CH and a σ-closed forcing for the consistency of CH.

The lean mathematical library

This paper describes mathlib, a community-driven effort to build a unified library of mathematics formalized in the Lean proof assistant. Among proof assistant libraries, it is distinguished by its dependently typed foundations, focus on classical mathematics, extensive hierarchy of structures, use of large- and small-scale automation, and distributed organization. We explain the architecture and design decisions of the library and the social organization that has led to its development.