The top pin of that pin stack will be trapped above the shear line, the bottom pin will fall freely, and now a new pin stack (the next most misaligned one) prevents further rotation. This will overset the pin. A key that is bitted to the wrong depth in even one pin position will not operate the lock. See Figure 1. (In practice, the cuts are produced by stacking pin segments of particular lengths, not by actually cutting the pins; hence the term "pin stack.") With no key in the lock, all the pin stack cuts rest within the plug. Other classes of attack, not discussed here but at least as worthy of study and scrutiny, include lock decoding, which is concerned with producing a working key based only on access to the external interface of the lock, lock bypass, which aims to unlatch the underlying locking mechanism without operating the lock at all, and forced entry, which, as the term suggests, involves the destructive application of force to the lock or its surroundings. Rotation of the plug within the shell operates the locking mechanism. The basic design consists of a rotatable cylinder tube, called the plug, linked to the underlying locking mechanism.

When a key is inserted into the keyway slot at the front of the plug, the pin stacks are raised within the plug and shell. Left: Cylinder face, the lock's "user interface." Note the keyway, which is cut into the plug, which in turn sits inside the shell. The height (or cut depth) of a key under each pin stack position is called its bitting; the bitting of a key is the "secret" needed to open a lock. If you tried to rotate the plug of such a lock without a key in the keyway, the top pin segment of each pin stack would block the plug at exactly the same number of degrees of rotation; each pin stack would contribute equally to preventing the plug from turning. The basic principles of operation are essentially the same as those of the standard pin tumbler lock, except that the tumblers are exposed at the front of the cylinder and a round ("tubular") key is used. Although combination lock manipulation exploits some of the same kinds of imperfections as pin tumbler lock picking, the principles and techniques are quite different and are well beyond the scope of this document. To determine which pin stacks have these pins, gently push up each "set" pin.

Repeat until lock turns: - Locate the pin stack that's being pinched at the shear line (it resists slightly when pushed up) - Continue to push that pin stack up until its cut reaches the shear line and the plug turns slightly. The plug can rotate freely only if the key lifts every pin stack's cut to align at the border between the plug and shell. Figure 1. A pin tumbler lock cylinder. Figure 2. Pin tumbler lock with a correct key inserted. See Figure 2. The plug will be blocked from rotating if any pin stack is lifted either not far enough (with the cut still in the plug below the shear line) or too far (with the cut pushed above the shear line and into the shell); to rotate, all pin stacks must have a cut at the shear line. Right: Side view, with part of the shell and plug cut away to expose the six pin stacks. Each pin stack is cut in one or more places perpendicular to its length.

The top pin of the most misaligned pin stack becomes "pinched" at the shear line between the plug and the shell. Right: With all of the cuts at the shear line, the plug can rotate freely within the shell. In the locked state the plug is prevented from rotating by a set of movable pin stacks, typically under spring pressure, that protrude from holes in the top of the opening in the shell into corresponding holes drilled into the top of the plug. The rest is just technique -- locating and recognizing the state of each pin stack, manipulating the pins, applying torque to the plug. The basic skills of pin tumbler lock picking include selecting the proper tools, manipulating pins through the keyway, applying torque, and recognizing the state of each pin. Tubular cylinders typically have four to eight pin tumblers arranged in a circular pattern around the circumference of the plug.

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