Combination lock

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This article is about a type of locking mechanism. For other uses, see Combination Lock (disambiguation).

A letter combination lock.

A combination lock is a type of lock in which a sequence of numbers or symbols is used to open the lock. The sequence may be entered using a single rotating dial which interacts with several discs or cams, by using a set of several rotating discs with inscribed numerals which directly interact with the locking mechanism, or through an electronic or mechanical keypad.

Despite its name, a combination lock requires the correct permutation, not merely the correct combination of digits.

Exploded view of the rotating discs. The notches on the disc correspond to the numerals in the correct combination. In this case, the combination is 9-2-4.

The discs are mounted on one side of the lock, which may in turn be attached to the end of a chain or cable. The other side of the lock, or the other end of the cable, has a pin with several protruding teeth.

When the toothed pin is inserted and the discs are rotated to an incorrect combination, the inner faces of the discs block the pin from being extracted.

1 History
2 Design
3 Manufacturers
4 References
5 External links

[edit] History

The first known combination lock was invented in 1206 by the Arab scholar, inventor and mechanical engineer al-Jazari. He documented the device in his book al-Ilm Wal-Amal al-Nafi Fi Sina'at al-Hiyal (The Book of Knowledge of Ingenious Mechanical Devices).^[1] Gerolamo Cardano later described a combination lock in the 16th century. It is also said that in 1862 Linus Yale Jr. invented the modern combination lock.

[edit] Design

[edit] Multiple-dial locks

One of the simplest types of combination lock, often seen in low-security bicycle locks and in briefcases, uses several rotating discs with notches cut into them. The lock is secured by a pin with several teeth on it which hook into the rotating discs. When the notches in the discs align with the teeth on the pin, the lock can be opened.

This lock is considered to be one of the least secure types of combination lock; many locks of this type can be quickly opened without knowledge of the correct combination. Opening one in this fashion depends on slight irregularities in the machining of the parts. Unless the lock is machined precisely, when the pin is pulled outward, one of the teeth will pull more strongly than the others on its corresponding disc. This disc is then rotated until a slight click is heard, indicating that the tooth has settled into the notch. The procedure is repeated for the remaining discs, resulting in an open lock, and a correct combination, in very little time.

A single-dial padlock.

[edit] Single-dial locks

Combination locks found on padlocks or safes may use a single dial which interacts with several parallel discs or cams. Customarily, a lock of this type is opened by rotating the dial clockwise to the first numeral, counterclockwise to the second, and so on in an alternating fashion until the last numeral is reached; the cams typically have an indentation or notch, and when the correct combination is entered, the notches align, allowing the latch to fit into them and open the lock.

Depending on the quality of the lock, some single-dial combination locks can also be defeated relatively easily. Typical padlocks are manufactured with generous tolerances, allowing two, three or even more digits of 'play' in the correct access sequence. Given a 60-number dial with three cams and three digits of play, the search space is reduced from 60 × 60 × 60 to 20 × 20 × 20, a 96% reduction in potential combinations.

Additionally, if testing the mechanism to open the lock does not modify the state of the lock, multiple combinations can be tried sequentially, drastically reducing the brute force search time. The first two digits are entered normally once, then, starting from the second digit, the dial is rotated sequentially through the digits, testing the lock on each. If it takes three seconds to input the first digit, two seconds for the second digit, and one second for the third digit, then the normal search time for a 60-number dial with three cams would be (3 + 2 + 1) × 60³. The reduced search time would be (3 + 2 + 60) × 60², a reduction of nearly 82% from 360 hours to 65 hours. This strategy can be extended to the second digit as well, slightly reducing the search time further.

When these two strategies are combined on a lock with the properties given above, the brute force search time is reduced by greater than 99%; a brute force search that would have taken 360 hours is reduced to an achievable 2.78 hours. This is still significantly better security than multiple-dial locks and many keyed locks, but unacceptable for high security applications.

Inexpensive padlocks are often also susceptible to direct mechanical attacks, such as the use of a padlock shim which can release the shackle without entering a combination. Early combination padlocks made by Master lock could be cracked by pulling on the shackle of the lock and turning the dial until it stopped; each numeral in the combination could be revealed in this manner. More recent models of Master padlock with a 40-position dial have a mechanical weakness that can give away the last numeral in the combination, and the first two numerals have a mathematical relationship with the last number. This weakness reduces the number of possible combinations from 64,000 to a mere 100, which can be tried in a relatively short time.

[edit] Other designs

Many doors use combination locks which require the user to enter a numeric sequence on a keypad to facilitate entry. These special locks usually require the additional use of electronic circuitry. The chief advantage of this system is that if used for the door of a large office, each employee can be told the code number without having to supply a key to each person. However, if the code number is learnt by someone outside the desired group or is not changed regularly, it could allow easy access to a potential intruder (the same could be said for all combination and keyed locks, and many other security measures, however).

Electronic combination locks, while generally safe from the attacks on their mechanical counterparts, suffer from their own set of flaws. It is much easier to determine the lock sequence by viewing several successful accesses, since the arrangement of numbers is fixed. Similarly, the numbers in the combination (but not the actual permutation) can be determined by which keys show signs of wear and tear due to constant use.