A Formal Framework for Modelling Coercion Resistance and Receipt Freeness

Coercion resistance and receipt freeness are critical properties for any voting system. However, many different definitions of these properties have been proposed, some formal and some informal; and there has been little attempt to tie these definitions together or identify relations between them. We give here a general framework for specifying different coercion resistance and receipt freeness properties using the process algebra CSP. The framework is general enough to accommodate a wide range of definitions, and strong enough to cover both randomization attacks and forced abstention attacks. We provide models of some simple voting systems, and show how the framework can be used to analyze these models under different definitions of coercion resistance and receipt freeness. Our formalisation highlights the variation between the definitions, and the importance of understanding the relations between them.