Assessing Existing Master Key Systems

Two powerful techniques can assist locksmiths when assessing existing master key systems. These techniques work on all types of pin-tumbler locks, regardless of chamber counts, or progressions under the TMK.


Most facility managers or building owners rely on locksmiths to maintain their keying systems. In most cases these keying systems will involve master keying. The details and plans of master keying are complicated and sometimes arcane, and always non-intelligible to the customers of...


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Expert locksmith technique

Figure 2 shows how to cut the “diagnostic keys” to decode master keyed lock cylinders without disassembling the cylinders.
First determine the TMK. It isn’t necessary to dissemble the lock cylinder, but it is necessary to know the original cuts of one change key that operates a lock cylinder. For example, let’s use a Schlage six-pin change key bitted to: 345058.

Follow the process in Figure 2, decoding a chamber at a time. The process involves progressing and individual cut and then trying the modified key in the lock to see if it turns. If the key turns, it then reveals the cut of the TMK in that same position.

If the maximum cut in a chamber is cut, tried, and doesn’t operate the lock cylinder, start with a new key by code where the cut in the relative position is either a 0 (when the change key is even in that chamber) or 1 (when the change key is odd in that chamber).

Once the TMK cut in a chamber is discovered, repeat the process until the TMK is fully decoded.

Now that the TMK is known, cut 24 diagnostic keys. See Figure 3; the 24 keys are grouped using six rings. Each ring is used to test a specific chamber in the six-pin lock cylinder.

Note: Diagnostic keys are not intended for general use and may exceed the normal safety factor. When cutting diagnostic keys, use a key code machine and blade intended for the purpose of exceeding the safety factor.
Each key has a large number stamped on the left of the key head that designates the chamber being tested. The large number stamped on the right of the key head designates the cut of the key being tested in that chamber (see Figure 5).

Every diagnostic key has five of the six cuts in common with the TMK. The cut not in common with the TMK represents the change key. In this manner the change key can be isolated and independently tested.
This works as each chamber of the master keyed cylinder has a bottom pin and a master wafer installed allowing either a master cut or change cut to operate that chamber. Five of the chambers are lifted to the operating position, leaving the sixth chamber to be tested for a specific “change” cut.

Figure 4 is a closeup of pins in the lock cylinder. Inside each chamber are both a green bottom pin and gold master wafer. Each chamber is designed so that only the TMK or the change key operates there.
Figure 5 displays the diagnostic keys used to test chambers three and four. Notice the keys that test the third chamber are cut exactly the same except regarding the third chamber. The number on the right of each key is the cut that is being tested for. This is the same for the fourth chamber where those keys are cut the same except for the fourth position.

Armed with these keys, anyone can simply note which keys operate a lock cylinder. For instance, when testing all 24 keys, the only keys that operate the lock are stamped: 13, 24, 37, 42, 53, and 66; therefore the cuts of the change key that was originally intended for the lock cylinder are 347236.

Notice that two cuts operated the lock cylinder in the third chamber. This is an indication that the lock cylinder was cross-keyed so that both 347236 and 349236 can operate the lock.
Another test revealed that the only diagnostic keys that operated a lock cylinder were stamped 13, 53, and 66. This lock cylinder was keyed so that the lowest level key operation key is the sub-master 301836 and the highest level key is the TMK.

The example keys above represent diagnostic keys for a six-pin, two-step progression system. Cuts in any given chamber will either be part of the subset of (0, 2, 4, 6, or 8} or (1, 3, 5, 7, or 9}. Four additional keys are needed for seven-pin, two-step progression systems.

Single-step progression systems, where cuts in a given chamber will be part of the subset {1, 2, 3, 4, 5, or 6}, will require 36 diagnostic keys for six-pin systems, and 42 diagnostic keys for a seven-pin system.
Diagnostic keys are an effective tool to positively prove compliance to manufacturing specifications.

The key bitting array (KBA), recreated by using diagnostic keys, is displayed in Figure 1. The order of “progressions” was determined by the order in which the buildings of the campus were keyed. The first cuts that were decoded were entered into a blank KBA form. Eventually the KBA was totally filled in. The sequence was determined as some chambers changed from room-to-room testing, indicating that those chambers sequenced first.

Appending key systems from incomplete records
I was asked to give a preliminary assessment regarding a master key system that was being transferred from one lock company to the next. The only known records were two pages left from the factory when the building was originally rekeyed (Figures 6 and 7), and a single page of instructions created by the last locksmith which revealed how future combinations would be generated (see Figure 8).

The request was to develop 100 more usable combinations to be used for the next year and until the building would be freshly rekeyed. New combinations could not interchange with existing lock cylinders and existing operating keys were not to operate future rekeyed lock cylinders. For the interim, all future and existing lock cylinders were to operate on the existing TMK.

The last locksmith kept no records as to what combinations were used from his expansion scheme (see Figure 9). To make sure work was not being repeated, all of the expansion combinations developed by the last locksmith needed to be eliminated from future use.

Making matters worse, it wasn’t clear if the last locksmith created operating keys with a cut in common with the TMK.

Clearly the objective was to develop 100 operating keys that would not operate existing cylinders while making sure that distributed operating keys would not access future rekey.

To accomplish this, the following steps were taken: system parameters were defined; the KBA was recreated; an analysis determining the operability of distributed keys was made; groups of unique change keys were developed; and everything was documented for future use.

Figure 9 represents the different steps needed to recreate the KBA:

Figure 8 reveals that the TMK is: 452351 and Figures 6 and 7 determine the possible cuts in each position to be part of the subset, {1,2,3,4,5,6}. This will be a six-by-six key bitting array.
The second array in Figure 9 records the cuts used in each position in Figure 6.

The third array in Figure 9 records additional cuts used in each position in Figure 7.

The fourth array in Figure 9 records additional cuts used in each position in Figure 8.

Numbers that are printed in inverse (the last array in Figure 9) were never used.

Once the KBA was recreated and how existing keys were developed, assumptions could be made to develop unique operating keys.

The first assumption is that all distributed keys other than the TMK were cut with a 4 in the first position. This cut was in common with the TMK; therefore every key issued was a sub-master key. To guarantee that new keys would not interchange with existing lock cylinders, new operating keys would be never be cut with a 4 in the first position.

The second assumption is that all new operating keys would contain a cut in common with the TMK in either the second, third, or fifth position. This would guarantee that existing keys could not interface with new lock cylinders.

Armed with these assumptions and the information derived from the last array in Figure 9, the following new combinations can be generated. Each of the arrays in Figure 10 represents a matrix of 625 unique combinations. All conditions were satisfied including the fact that all existing and future lock cylinders can be operated by the TMK.

In Summary
This article demonstrates two powerful techniques that can assist locksmiths when assessing existing master key systems. These techniques work on all types of pin-tumbler locks, regardless of chamber counts, or progressions under the TMK.

Decoding the TMK and creating diagnostic keys can work for any brand pin-tumbler lock providing the master keying methods used were according to factory specifications and key stock is available to cut these special keys.

For institutional locksmiths, diagnostic keys should be regularly used to verify that lock cylinders are properly master keyed.

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