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"