Rights Amplification in Master-Keyed Mechanical Locks – This paper describes a relatively unknown procedure for obtaining a master key if given access to a tumbler based master keyed lock and any low level key in the system. No special skill or equipment beyond a small number of blank keys and a file is needed, and the attacker does not need to engage in any suspicious behavior at the locks location. Countermeasures are described with provide limited protection under certain circumstances.
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Source: https://packetstormsecurity.com/files/30749/mk.pdf.html