Multiparty session types are a type system that can ensure the safety and liveness of distributed peers via the global specification of their interactions. To construct a global specification from a set of distributed uncontrolled behaviours, this paper explores the problem of fully characterising multiparty session types in terms of communicating automata. We equip global and local session types with labelled transition systems (LTSs) that faithfully represent asynchronous communications through unbounded buffered channels. Using the equivalence between the two LTSs, we identify a class of communicating automata that exactly correspond to the projected local types. We exhibit an algorithm to synthesise a global type from a collection of communicating automata. The key property of our findings is the notion of multiparty compatibility which non-trivially extends the duality condition for binary session types.

Abstract. Communicating finite state machines (CFSMs) represent processes which communicate by asynchronous exchanges of messages via FIFO channels. Their major impact has been in characterising essential properties of communications such as freedom from deadlock and communication error, and buffer boundedness. CFSMs are known to be computationally hard: most of these properties are undecidable even in restricted cases. At the same time, multiparty session types are a recent typed framework whose main feature is its ability to efficiently enforce these properties for mobile processes and programming languages. This paper ties the links between the two frameworks to achieve a two-fold goal. On one hand, we present a generalised variant of multiparty session types that have a direct semantical correspondence to CFSMs. Our calculus can treat expressive forking, merging and joining protocols that are absent from existing session frameworks, and our typing system can ensure properties such as safety, boundedness and liveness on distributed processes by a polynomial time type checking. On the other hand, multiparty session types allow us to identify a new class of CFSMs that automatically enjoy the aforementioned properties, generalising Gouda et al's work [13] (for two machines) to an arbitrary number of machines.

Behavioural type systems, usually associated to concurrent or distributed computations, encompass concepts such as interfaces, communication protocols, and contracts, in addition to the traditional input/output operations. The behavioural type of a software component specifies its expected patterns of interaction using expressive type languages, so types can be used to determine automatically whether the component interacts correctly with other components. Two related important notions of behavioural types are those of session types and behavioural contracts. This article surveys the main accomplishments of the last 20 years within these two approaches.

We present a compiler for generating custom cryptographic protocols from high-level multiparty sessions.Sessions specify pre-arranged patterns of message exchanges between distributed participants and their data accesses to a shared store. We define integrity and confidentiality properties of sessions, in a setting where the network and arbitrary compromised parties may be controlled by an adversary. Our compiler enforces these security properties by guarding the sending and receiving of session messages by efficient cryptographic operations and checks.Given a session, our compiler generates an ML module and an interface that exposes send and receive functions that can be called by application code for each party. We prove that this generated code is secure by relying on a recent refinement type system for ML. Functions in the module interface are given dependent types that express invariants of the session state. We typecheck the program against this interface, and complete the proof by a brief, handcrafted argument.We illustrate and evaluate our implementation on a series of typical protocols, inspired by web services. In comparison with prior work, our source language is more expressive, our implementation more efficient, and our proof technique novel. Most of the proof is performed by mechanized type checking of the generated code, and does not rely on the correctness of our compiler. We obtain the strongest session security guarantees to date in a model that accounts for the actual details of protocol code. Security by compilationTaking advantage of modern programming tools and generic wire formats, one can sometimes design, develop, and deploy complex distributed protocols in a matter of hours-relying on automated proxy generators, for example, to rapidly expose existing applications as networked services. The situation is less favorable when attempting to ensure application security subject to realistic assumptions on the network and remote parties. The problem is that most widely-available protocols for cryptographic communications (say TLS or IPSEC) operate at a lower level; they provide authenticity and confidentiality guarantees only for messages exchanged between two endpoints (URLs or IP addresses), but they leave the interpretation of these messages and endpoints to the application programmer. Hence, any guarantee that involves more than two parties (say, a multi-tiered web application with a client, a gateway, and two servers) must be carefully established by linking lowerlevel guarantees on related messages, or by layering ad hoc cryptographic mechanisms on top of communications, for instance by embedding certificates in message payloads.When considering protocols between participants in an open, networked environment, each participant may belong to a different domain, with its own configuration and security policy; although all participants are willing to run the protocol, they may not trust one another. Although it is straightforward to protect the participants of the protocol from netw...

Multiparty session types enforce structured safe communications between several participants, as long as their number is fixed when the session starts. In order to handle common distributed interaction patterns such as cloud algorithms or peer-to-peer protocols, we propose a new role-based multiparty session type theory where roles are defined as classes of local behaviours that an arbitrary number of participants can dynamically join and leave. We offer programmers a polling operation that gives access to the current set of a role's participants in order to fork processes. Our type system with universal types for polling can handle this dynamism and retain type safety. A multiparty locking mechanism is introduced to provide communication safety, but also to ensure a stronger progress property for joining participants that has never been guaranteed in previous systems. Finally, we present some implementation mechanisms used in our prototype extension of ML.