A Sound Type System for Secure Currency Flow

In this article, we focus on TinySol, a minimal calculus for Solidity smart contracts, introduced by Bartoletti, Galletta and Murgia. We start by rephrasing its syntax (to emphasise its object-oriented flavour) and give a new big-step operational semantics for that language. We then use it to define two security properties, namely call integrity and noninterference. These two properties have some similarities in their definition, in that they both require that some part of a program is not influenced by the other part. However, we show that the two properties are actually incomparable. Nevertheless, we provide a type system that statically ensures both noninterference and call integrity; hence, well-typed programs satisfy both properties. We finally discuss the practical usability of the type system and its limitations by means of some simple examples.