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Key Bumping

Key bumping is a covert entry technique against pin-tumbler locks that uses a specially prepared key to "bump" top pins above the shear-line to allow the plug to rotate. There are two types of key bumping, pull-out and minimal movement, but both produce similar forensic evidence on the bump key and the lock.

Key bumping is similar in function to pick guns.

Key Bumping Principles

To bump a lock, a key is acquired that fits the keyway of the lock. The key is modified so that all cuts are at their lowest depths or lower. This is commonly referred to as a "999 key," because 9 is usually the designated lowest cut depth. If done by hand, a key gauge or micrometer can be used to measure the key and ensure cuts are deep enough. If done with a key machine, the key may be duplicated from a working bump key, or cut by code to the lowest depths.

In the pull-out method, the key is inserted into the lock fully then withdrawn one pin space. In the minimal movement method, the key is further modified by removing material from the tip and shoulder of the key. The minimal-movement key is inserted completely into the lock. In both cases, light tension is applied to the key and a tool (known as a bump hammer) is used to impact the bow of the key, causing the key to be forced into the lock. The impact on the key causes kinetic energy to travel from the key to the top pins, causing the top pins to momentarily jump. If all top pins jump above the shear-line while tension is applied the plug is free to rotate.

As with pick guns, the main source of forensic evidence of bumping is on the pins themselves. The action of striking the bump key into the lock causes distinct dents and scratches on the bottom pins. Bumping also affects the face of the plug, the keyway profile, pin chambers, top pins, and the bump key itself. For forensic examination of bump keys, see the Key Analysis page.

Forensic Evidence

The act of key bumping basically slams the key against the bottom pins to allow for kinetic energy to be transferred from the key to the top pins. Because they are immobile and absorb the kinetic energy, this causes considerable damage to the bottom pins in the form of large dents and scratches.

 Key bumping causes noticeable damage to the bottom pins in the form of dents.

A bump key that is cut by hand, with a low speed key cutter, or made of a considerably stronger material (steel, iron, nickel-silver) than the pins may act as a file as it impacts bottom pins. In this photo, light scratches can be seen traveling through the bumping dent.

 Hand-made bump keys may cause light scratches to also appear on bottom pins.

Alternate lighting may be used to illuminate bumping scratches and dents more efficiently. In this photo, there are dents on the left, center, and top of the pin, as well as scratches. In many cases it is posisble to count the number of times the bump key was used by counting the dents.

 A pin that has been repeatedly bumped, many dents are visible.
Light shearing against the bottoms of the top pins may occur during key bumping.

Bumping is rarely 100% successful, either because bottom pins are bumped above shear line, or top pins are not bumped high enough. When this happens the tension applied will misfire, causing one or more top pins to bind. This causes light shearing against the bottom of the top pins.
Damage to the serrations of a spool pin with serrated edges.

Some top pin designs will be more affected by bumping than others. In this photo, a spool pin with serrated edges is shown. Repeated bumping of this pin has caused the serration to close up (compare with previous photo). In general, situations like this slightly decrease pick resistance.
The bottom of a top pin that has sustained damage do to a variation in the design of the bottom pins.

In some pin designs the bottoms of the top pins will be considerably damaged by bumping. In this case, the bottom pins are lightly rounded on both sides, allowing them to be inserted either way. When bumped repeatedly, the bottoms of the top pin become considerably dented.

The pin chambers within the plug may also be damaged by bumping. When kinetic energy does not properly transfer to the top pin, the pin stack may instead press against the chamber walls (caused by the movement of the bump key). Repeated bumping may cause these areas to distort, stretching in various directions.

 A photo showing the distortion of the chambers in the plug.

One of the most noticeable pieces of evidence from key bumping is damage to the face of the lock. This is caused by the shoulder of the key impacting the area above and below the keyway. The use of modified shoulders may prevent this from happening, see the Anti-Forensics page.

 Damage to the plug above the keyway is common in bumping due to impact from the shoulder of the key.

In the minimal-movement method, material is removed from the tip and shoulder. This makes the method work but also inserts the key far enough that in some cases affects the keyway. This is due to the key material getting thicker as it reaches the bow. In the photo, this can be seen around the edge of the keyway.

 A lock that has been bumped with a minimal-movement key.
A failed bumping attempt against the EVVA 3KS showing damage to the wafer arm.

Many unskilled attackers may attempt to bump unbumpable locks. This will probably damage the lock and borders on breaking the lock completely. In the photo, an EVVA 3KS wafer is shown after an bumping attempt. The wafer arm has broken off and a large dent is present, this lock was non-functional after this.
Damage to the cylinder walls of the EVVA 3KS due to a failed bumping attempt.

In this photo we see the cylinder from the failed bumping of an EVVA 3KS. The serious damage to the walls of the cylinder is due to the wafers slamming against the cylinder when bumping was attempted.

coming soon: Effects of Wear

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