Using fewer master pins makes a system more secure. By Ralph Forrest-Ball and fewer master pins means less vulnerability. First, let’s talk about the problem, and then I’ll share three examples of minimalist master keying. For pin-tumbler locks, the most common technique of master keying is “split-pin M master keying,” which involves putting master pins between the bottom pins and driver pins. Using master pins in more than one chamber almost always leads to “incidental” keys. Incidental keys work because each of their cuts is identical to a corresponding cut on one of the operating keys for that lock. As a simple illustration, imagine a lock that is pinned to work with the keys 333333 and 555555. A key cut to 333555, or 353553, or any combination of 3s and 5s, will automatically operate that lock. This is both a blessing and a curse. The upside is that we can take advantage of the incidental master keys to create three-level master key systems (change keys, mas- ter keys and grand master key) without adding any extra master pins. A lock that is pinned to the change key and grand master key automatically works with the inci- dental master key. The downside is that all those incidental keys are vulnerabilities for possible key interchange. Key interchange is when a key operates a lock it wasn’t intended to. There are basi- cally three ways this can happen. First, a random outside key might operate one of our locks because it just happens to be identical to that change key, a master key or one of the incidental keys for that lock. Second, the lock could be manipulated by wiggling or jiggling the key. This typi- cally happens when the offending key is deeper, in every position, than one of the operating keys. A bump key is an example of manipulation. Third, and most serious, is when a key from somewhere in our system operates a lock in our system that it was not intended to operate. A well-designed and properly implemented master key system should have no internal key interchange. But you can never completely eliminate the risk of outside keys and manipulation. So, how do we fight back? The way to reduce the risk is to use fewer master pins. In 40 KEYNOTES JANUARY 2022 aster keying has been described as the careful destruction of security. Actually, master keying has several advantages, such as limiting the damage caused by lost or stolen keys. The trick is to find the right balance between the advantages and disadvantages. Generally speaking, more master pins means more vulnerability, the example above, all three keys have a 3 cut in the first position. Where the change key is the same as the top master key, that’s called a constant. The lock has no master pins in that chamber. Constants are good. Constants are your friends. ANSI/BHMA A156.28 (the industry standard for master keying) says we should use as many con- stants as possible to get the fewest master pins per cylinder, which minimizes the in- cidental keys. Typically, adding one more constant reduces the risk of key interchange by half. I’ll show you some examples with very few master pins, or none at all. Three Examples of Minimalist Master Keying Example #1: Warehouse. Chris and Dana Thompson own a small business with six employees. Typically, the manager unlocks the building in the mornings, and the assistant manager locks up at night. The other four employees don’t have keys to the building. The problem is that, sometimes, an employee needs access to the warehouse early in the morning. But the owners don’t want the employee to have access to the business office, where confidential records are kept. They have asked us if we can make a key that only opens the warehouse, not the business office. The locks are WWW.ALOA.ORG
