“Surely You’re Joking, Mr. Feynman! Chapter 18: Safecracker Meets Safecracker”, 1985 (; backlinks; similar):
[In this chapter, Feynman discusses his fascination with locks and safes, and the ways he learns to crack different locks and safes. He also talks about the locks and safes there were at Los Alamos (the ones that held the secrets of the atomic bomb).
Generally speaking, the safes containing this material, which is (understandably) considered top secret, are startlingly easy to crack. When Feynman and his colleagues first arrive at Los Alamos, the construction of the facility is not even complete yet, and there is almost no security at all for the “top secret” information.
Even later, when the information is stored in safes and locked file cabinets, the locks are generally cheap, and it is easy for Feynman to determine what the combination is, as combinations were often reused. He can memorize common passwords, or watch people dialing carelessly to deduce several digits, and then brute-force the rest; often, people do not even change the factory default.]