1. An apparatus for performing exponentiations, comprising: a set of computational devices containing first and second subsets, wherein the first subset has a plurality of members which are chained together such that the devices of the first subset can operate both independently and as members of a first computational chain, and wherein the second subset has a plurality of members which are chained together such that the devices of the second subset can operate both independently and as members of a second computational chain distinct from said first computational chain; and a chaining controller adapted to instruct the first subset of devices to act as a first computational chain when the apparatus is required to perform an exponentiation of a first size, and being further adapted to instruct the second subset of devices to act as a second computational chain when the apparatus is required to perform an exponentiation of a second size distinct from said first size.