The forbidden zone

It sounds like the title of either a fantasy or an erotic novel. With locks, however, something else is meant: Combination locks have the technical peculiarity that certain numbers may not be used as a code on the last wheel. If this is not observed, the internal mechanics will get stuck. The lock does not open and can even suffer permanent damage if you try to turn the spindle by force. This range of numbers is called the forbidden zone. The numbers for the individual combination locks are specified in the respective instructions for changing the code.


1.000.000 Codes?

The smaller the “forbidden” area, the greater the selection of possible codes.

Example 1: Manuf. information Forbidden Zone 00 to 20 on the last wheel results in:

Forbidden Zone Wheel 1 Wheel 2 Wheel3
00 to 20 00-99 00-99 21-99
  100      x   100        x    79     = 790.000 possible codes


Example 2: Manuf. information Forbidden Zone 00 to 09 on the last wheel results in:

Forbidden Zone Wheel 1 Wheel 2 Wheel 3
00 to 09 00-99 00-99 10-99
  100      x   100        x    90     = 900.000 possible codes


The size of the forbidden zone therefore has a significant impact on the security of a mechanical combination lock. Because the more variations are possible, the safer your actual code is.

One can read often in brochures and web pages of safe dealers (but also some lock manufacturers) the statment: “1,000,000 codes to choose from!”.

Pure ignorance speaks from this. You now know how to calculate the number of actual code variations. A look at the manufacturers instructions is enough …