Jefferson invented a wheel-based cryptosystem in the late 1790s. His invention would later surface as the US army mechanism for encryption over 150 years later, called the M-94 (see next page). Each wheel has a random set of letters. The key was actually the arrangement of the 36 disks, which has a complexity of n factorial, very secure for the time (36! = 3.7x1041).
(Check Out This IT Video: Build A Secure Future In Information Security)