Master-Keyed Lock Vulnerability
16 January 2003, Revised 27 January 2003
In a recent research paper, we describe weaknesses in most master-keyed lock systems, such as those used by offices, schools, and businesses as well as by some residential facilities (particularly apartment complexes, dormitories, and condominiums). These weaknesses allow anyone with access to the key to a single lock to create easily the "master" key that opens every lock in the entire system. Creating such a key requires little skill, leaves behind no evidence, and does not entail engaging in recognizably suspicious behavior. The only materials required are a metal file and a small number of blank keys, which for many locks are readily available.
Needless to say, the ability for any keyholder to obtain system-wide access represents a serious potential threat to the security of master keyed installations. Individuals and institutions that depend on such locks to protect their safety and property should be aware of these risks and consider alternatives to eliminate or reduce their exposure to this threat.
Who is vulnerable?
A master keyed lock system is one in which locks are designed not only to be opened by their individual keys, but also by special "master" keys that open some or all other locks in the system. They are commonly found in commercial, industrial, educational and government facilities as well as in some centrally managed residences. Master keying is used because it allows those who must have access to many locks (maintenance workers, managers, etc.) to carry only a few keys. (Note that master keying is unrelated to whether the locks are sold under the "Master ®" brand name.) It is not usually possible to tell by inspecting a key or a lock from the outside whether it is part of a master system. Individuals should ask their locksmith, building management, or maintenance office whether their locks are master keyed.
This research demonstrates that virtually all master keyed mechanical lock systems are at least theoretically vulnerable; the practical seriousness of the threat to any particular system depends on a number of factors:
Alternatives and countermeasures
Unfortunately, at this time there is no simple or completely effective countermeasure that prevents exploitation of this vulnerability short of replacing a master keyed system with a non-mastered one. Residential facilities and safety-critical or high-value environments are strongly urged to consider whether the risks of master keying outweigh the convenience benefits in light of this vulnerability. Lock users should evaluate these risks before purchasing or installing new master keyed systems.
Depending on individual circumstances, a range of defenses may be appropriate:
Why is this information being made available?
Since this research was completed last Fall, we have been quietly circulating details of the vulnerability to the lock, law enforcement, and security communities. However, there is some evidence that the details are now circulating in the underground world. At this point we believe that it is no longer possible to keep the vulnerability secret and that more good than harm would now be done by warning the wider community. Several correspondents have noted that this attack, and similar techniques, have been passed down as folklore in both the locksmithing and underground communities.
Technical details and resources
The vulnerability was discovered by applying the techniques of cryptanalysis, ordinarily used to break secret codes, to the analysis of mechanical lock design. The research paper describing this analysis and the discovered vulnerability can be found (in PDF format) on the world wide web at http://www.crypto.com/papers/mk.pdf. (Note -- this file is rather large, about 4MB and is not suitable for download over dialup or slow connections). A version of the paper will appear in the IEEE journal Security and Privacy.
The author strongly suggests that facility managers and concerned individuals consult with a competent security professional or locksmith to discuss the vulnerability of their particular installations.
Neither the author nor AT&T endorses or recommends specific lock products or security services. However, the following resources and organizations may be helpful for locating an appropriate security specialist or as a source of technical information:
This fact sheet will be available on the world wide web at http://www.crypto.com/masterkey.html.