de Borde, Duncan (2007-06-28). "Two-factor authentication"(PDF). Siemens Insight Consulting. Archived from the original(PDF) on 2012-01-12. Retrieved 2009-01-14.
de Borde, Duncan (2007-06-28). "Two-factor authentication"(PDF). Siemens Insight Consulting. Archived from the original(PDF) on 2012-01-12. Retrieved 2009-01-14.