Thoughts On Master Keying

Specifically in the automotive industry we have seen continuing progress in the reduction and elimination of mechanical keyed cylinders. Fewer and fewer cars each year have keyed cylinders in the trunk, doors or glove box. Even the keyed ignition switch is disappearing.

In the commercial and residential world there are multiple advantages to electronic security, but common pin tumbler locks and master keying are still widely used. In addition to electronics, various high security lock manufacturers offer second and third level locking components that further reduce the loss of security, but in many instances it is a matter of expense that determines the continued use of simple pin tumbler locks in a system.

Master keying is still alive and well. For the purposes of this article the term ‘master key’ refers to the top or highest level in the system.

In standard mechanical master keying a primary fundamental is that master keying is a controlled loss of security. Each pin stack consists of a spring, top pin or driver and bottom pin. Master pins are added to this stack to create additional shear lines. (Illustration #1)

When the number of shear lines increases, security decreases. Each additional shear line added further reduces the security level. By keying a cylinder to fit two random keys, you can create a cylinder with dozens of shear lines. In addition to the two intended keys, dozens of other cut combinations may work that lock, resulting in an uncontrolled loss of security. Either by picking, key manipulation or simple key insertion, unauthorized access may be granted. (Illustration #2)

When you are designing, implementing or servicing master keyed locks, it is essential to remember the limitations of a master key system. Following are some things to consider.

System Size

A fully progressed two-step 6-pin system will yield a maximum of 4096 cuts while a fully progressed 5-pin system will result in a maximum array of 1024 cut combinations. Not all of the cuts may be usable. For example, although easily cut, a key with cuts of 1-1-1-1-1-1 isn’t a good key to issue. A key with cuts of 7-3-8-9-0-9 isn’t possible in most systems because of the Maximum Adjacent Cut Specification (MACS) rule, also known as the maximum allowable cut. A nine cut next to a zero will cut off too much of the adjacent material of the key blank to allow a zero pin to seat properly. Many manufacturers specify a MACS of 7 values between adjacent cuts. Know the limitations of the system you are using.

The system size you need is determined by how many individual keys your customer needs now and by allowing for future growth of the system. When a system is initially written, the size of the system should be adequate to fit the customer’s needs today and allow for future re-keying and adding new locks into the system. Ask your customers how often tenants move in and out; see if they are planning on adding new doors or buildings, etc.

You wouldn’t want to write a fully progressed 4096 cut system for a 30-unit office building. A small 64-cut system might be adequate, or if there is a lot of re-keying expected, you might go to the next level with a 256 cut system.

Progression

Some systems today are still hand written on the job. In a hand written system, you may be a couple of pages into it before you discover a large amount of undesirable key cuts. Even in a computerized system the progression sequence must considered. A computerized system will allow you to experiment by changing the number and position of constants carried and the order of progression. By a simple keystroke, you can write and evaluate the feasibility of a new system or rewrite it easily before putting it into use.

If you are initiating a new system, be sure to examine the cuts of the old system. What kind of patterns do you see? Using higher cuts where there were low cuts helps assure that no key from the old system will work any lock in the new system. In a partially progressed system, alternate the chambers carrying constant values. If the old keys had a constant in the first chamber, use the fifth or sixth chamber in the new one for constants.