There are two papers at ftp.cwi.nl descibing some of the mathemathics, /pub/CWIreports/CS/CS-R9318.ps.Z and /pub/CWIreports/CS/CS-R9323.ps.Z CS-R9318 N. Ferguson "Single Term Off-Line Coins" CS-R9323 Stefan Brands "An Efficient Off-line Electronic Cash System Based On The Representation Problem"