Colin Aitken1, Anders Nordgaard2,3. 1. School of Mathematics, The University of Edinburgh, Peter Guthrie Tait Road, Edinburgh, EH9 3FD, U.K. 2. Swedish Police Authority - National Forensic Centre, SE-58194, Linköping, Sweden. 3. Department of Computer and Information Science, Linköping University, SE-58183, Linköping, Sweden.