Check Digit Schemes in General Use
Schemes for the assignment of identification number are extremely varied in
methodology and in the information encoded. Most interesting to mathematicians are
those that incorporate an extra digit for the detection of errors or fraud. Although the
purpose of this paper is to analyze the methods used for driver's license numbers, it is
worthwhile to begin with a brief survey of the methods employed to assign check
digits to the most ubiquitous number in use and to provide a theoretical result that
delineates their limitations.
The simplest and least effective method for assigning a check digit is to use the
remainder or inverse of the remainder of the identification number modulo some
number. For airline tickets, UPS packages, and Federal Express mail the check digit
is the identification number modulo 7. At the bottom of FIGURE 1 we see the airline
ticket number 17000459570 (the airline code 012 is not used in the calculation) is
assigned a check digit 3 since 17000459570 = 3 mod 7.
U.S. postal money orders use the remainder modulo 9 while VISA traveler's checks
use the inverse of the number modulo 9. thus, the check digit for the VISA number
1002044679091 is 2 since 1002044679091 = 7 mod 9.