In September of 1958, James Dayton, Jr. posted a method of master key system deconstruction. At the time,most locksmiths did little or no master keying, so it only atttracted the attention of the master keying experts, who kept it to themselves, passing it to new experts as a sort of rite of passage.
In about 2003, (sorry, I don't have the actual date in front of me right now) one new expert, in his zeal, passed it on to Professor Matt Blaze (then of AT&T) who went public with it and posted it everywhere, including the front page of the New York Times, as well as teaching it to his computer students and putting it on a website with allegedly 100,000 hits per week. His supervisor at University of Pennsylvania has responded to mail and email with full support of these actions, as well as others such as teaching bumping of high security lock cylinders.
Today, it has been in so many places that it must be considered public knowledge, and steps should be taken to reduce its threat. There are several defences against it, inluding patented or restricted or high security blanks, proper key control procedures fully enorced, and vigilance by all concerned at every level.
The method is simple if you have access to code cutting equipment or can accurately hand file a key. Assuming six cuts on the key, each position typically has either one or two cuts which will allow that position to operate the plug at the operating shearline. Knowing one set allows you to correctly deduce the other set by working one position at a time, and leaving the other positions on the known cuts. In that one position you begin with the shallowest possibiity and work down toward the deepest until you find the other working cut. Then you go to the next position and do the same. In under six blanks for TPP systems or under 24 for RCM systems, the system will be decoded for standard master keyed locksets.
This is a useful technique for working in reverse, when the master key bitting is known but the individual key for a closet or room has been unused for so long as to no longer be recorded.
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