https://www.fincen.gov/sites/default/files/shared/e-cash.pdf
https://www.fincen.gov/sites/default/files/shared/e-cash.pdf
http://web.archive.org/web/19970610020649/http://www.digicash.com/ecash/about.html
http://www.cs.kent.edu/~javed/DL/papers/web/howdigicashworks.html
https://chaum.com/ecash/memorandum/