Towards a security model for computational puzzle schemes

In the literature, computational puzzle schemes have been considered as a useful tool for a number of applications, such as constructing timed cryptography, fighting junk emails, and protecting critical infrastructure from denial-of-service attacks. However, there is a lack of a general security model for studying these schemes. In this paper, we propose such a security model and formally define two properties, namely the determinable difficulty property and the parallel computation resistance property. Furthermore, we prove that a variant of the RSW scheme, proposed by Rivest, Shamir, and Wagner, achieves both properties.