I hope this can be a cautionary tale about master keys and safes. Our story starts late one night or early one morning when unknown people* broke into a pharmacy. More accurately, they broke into the bigger building as a way into the pharmacy and damaged five doors before getting to the safe and lifting it. If you are a building manager, this instantly makes this a bad day.
Our shop was called since the long gone safe contained an exterior key and a master key for all internal doors. Doubtless the 'perps' were after drugs or cash, but the keys were in the safe. I asked after the safe a bit and found out it was a microwave sized box from Costco -- a fire safe only. Unanchored, it was easy pickings. This pharmacy does not fill so called 'triplicate prescriptions' including the narcotics. However, there was no external sign saying this to all who walked in or past on the sidewalk. I suggested such a sign and the next safe be burglary rated and anchored.
[If locksmiths and builders are in an arms race with thieves, I always suggest after a break and enter the client try to jump two levels in barrier if possible. If you put back the same hardware as was there, you already know that same criminal can get in. Should they try with just the next level of tool and you only go up one notch in hardware, they will still get in. Sometimes there are practical ways to do this and sometimes there are not.]
Now, back to the keying. The outside of the building was on its own key and keyway. We could do this fast but knew the key distribution is often the problem. Property managers know this is a real hassle too but in this case unavoidable.
The master key was a different problem. We had swept the building less than a year ago and they had just started on a fresh master key. The manager did not want to do that again, but I also found out the public could get into the upper floors for a few hours after the main floor was closed. This generated a great vulnerability to leaving the old master in place. Also, we knew none of the office keys were gone. I was asked if I could change just the master and leave the rest. Without seeing the pinning tables, I only gave a maybe.
Back at the shop, I found the file and had a careful look. It was a 5pin Schlage C system with 4 chambers progressed. It had only two layers -- the master key and the user keys.** Given this, there are 4^4 = 256 theoretical user keys. Having seen the building, I knew it was not even close to using half of these but I did not have the new master yet.
Here is the system with made up numbers.
Master Key 76543 with the 7 held in all locks and all keys. This brand of lock used fixed parity so if the master uses an even number in a cut, the user keys work from the remaining even numbers. Likewise for the odd cuts. The progression counting was kept simple from the tip of the key back.
This gives a KBS (Key Bitting Array) as follows.
-8765 Possible progression cuts
-4321 Sequence of progression
When looking at the issued keys both when the shop had rekeyed the entire place and a few changes since, I found every user key started with 78. This meant the second chamber in every lock had a 2 master wafer and I could change this to get my new master and retire the stolen key. In fact, there were 3 choices for the new key -- 70543, 72543 and 74543. I picked the 72543 and went to the bottom of our computer records for the building and marked all the user keys starting 72 as 'Do Not Cut'. We lost about 64 possible keys but still had 192 theoretical user keys for the future of which only about 40 were already issues.
I had told the manager it would take some luck to be able to find a usable replacement master but in this case it worked out. I went back to the site and visited every lock to pull the stolen master and install the new.
[We do not always get lucky. We set up a simple 5pin Schlage system for a growing business and based on their estimates held one chamber, used two for a master key at each site and the last two for keys within each site. The roll over in managers exceeded estimates and the expansion to new sites was much higher than first planned. This system has hit the wall and all the possible keys have been used up. New plans have been debated.]
* I will use the plural but it could have been just one person.
** In the trade, these are called change keys but thinking I might reach a wider audience I will call them user keys. And yet, the janitors 'use' a key to open all the doors and it is the master key.
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Laux Myth ... Thoughts From a Locksmith
By MartinB, Found @ http://lauxmyth.blogspot.com/