Combination lock
Encyclopedia
A combination lock is a type of lock
Lock (device)
A lock is a mechanical or electronic fastening device that is released by a physical object or secret information , or combination of more than one of these....

 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.

From a mathematical standpoint, "combination" lock is a misnomer. In mathematics, the term combination
Combination
In mathematics a combination is a way of selecting several things out of a larger group, where order does not matter. In smaller cases it is possible to count the number of combinations...

 represents a set of symbols that can be in any order: 1-2-3 is the same combination as 3-2-1, 2-3-1, and 1-3-2. However, the symbols or numbers for a lock must be entered in the correct sequence: 1-2-3 will not open a lock set to 3-2-1. The correct mathematical term for a set of symbols that must be entered in a specific sequence is a permutation
Permutation
In mathematics, the notion of permutation is used with several slightly different meanings, all related to the act of permuting objects or values. Informally, a permutation of a set of objects is an arrangement of those objects into a particular order...

.

History

The earliest known combination lock was excavated in a Roman
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 period tomb on the Kerameikos, Athens
Athens
Athens , is the capital and largest city of Greece. Athens dominates the Attica region and is one of the world's oldest cities, as its recorded history spans around 3,400 years. Classical Athens was a powerful city-state...

. Attached to a small box, it featured several dials instead of keyholes. In 1206, the Muslim
Muslim
A Muslim, also spelled Moslem, is an adherent of Islam, a monotheistic, Abrahamic religion based on the Quran, which Muslims consider the verbatim word of God as revealed to prophet Muhammad. "Muslim" is the Arabic term for "submitter" .Muslims believe that God is one and incomparable...

 engineer Al-Jazari
Al-Jazari
Abū al-'Iz Ibn Ismā'īl ibn al-Razāz al-Jazarī was a Muslim polymath: a scholar, inventor, mechanical engineer, craftsman, artist, mathematician and astronomer from Al-Jazira, Mesopotamia, who lived during the Islamic Golden Age...

 documented a combination lock in his book al-Ilm Wal-Amal al-Nafi Fi Sina'at al-Hiyal (The Book of Knowledge of Ingenious Mechanical Devices). Muhammad al-Astrulabi (ca. 1200) also made combination locks, two of which are kept in Copenhagen and Boston Museums.

Gerolamo Cardano
Gerolamo Cardano
Gerolamo Cardano was an Italian Renaissance mathematician, physician, astrologer and gambler...

 later described a combination lock in the 16th century.

In 1878 a German man by the name of Joseph Loch was said to have invented the modern combination Lock for Tiffany's Jewelers in New York City, and from the 1870s to the early 1900s made many more improvements in the designs and functions of permutation locks.

Multiple-dial locks

One of the simplest types of combination lock, often seen in low-security bicycle
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....

 locks and in briefcase
Briefcase
A briefcase is a narrow box-shaped bag or case used mainly for carrying papers and other documents and equipped with a handle. Lawyers commonly use briefcases to carry briefs to present to a court, hence the name...

s, 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.

Single-dial locks

Combination locks found on padlock
Padlock
Padlocks are portable locks used to protect against theft, vandalism, sabotage, unauthorized use, and harm. They are designed to protect against some degree of forced and surreptitious entry.- History :...

s or safe
Safe
A safe is a secure lockable box used for securing valuable objects against theft or damage. A safe is usually a hollow cuboid or cylinder, with one face removable or hinged to form a door. The body and door may be cast from metal or formed out of plastic through blow molding...

s 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
Brute force attack
In cryptography, a brute-force attack, or exhaustive key search, is a strategy that can, in theory, be used against any encrypted data. Such an attack might be utilized when it is not possible to take advantage of other weaknesses in an encryption system that would make the task easier...

 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
Master Lock
Master Lock is an American company primarily known for developing and manufacturing padlocks, combination locks and related security products. Now an operating unit of Fortune Brands Home & Security, Inc., Master Lock Company LLC was formed in 1921 by locksmith-inventor Harry Soref, and is...

 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 very short time.

In 1978 a combination lock which could be set by the user to a permutation of their own choosing was invented by Andrew Elliot Rae. At this time the electronic keypad was invented and he was unable to get any manufacturers to back his mechanical lock for lockers, luggage or brief-cases. The silicon chip locks never became popular due to the need for battery power to maintain their integrity. The patent expired and the original mechanical invention was instantly manufactured and sold worldwide mainly for luggage, lockers and hotel safe. It is now a standard part of the luggage used by travellers.

Other designs

Many doors use combination locks which require the user to enter a numeric sequence on a keypad
Keypad
A keypad is a set of buttons arranged in a block or "pad" which usually bear digits, symbols and usually a complete set of alphabetical letters. If it mostly contains numbers then it can also be called a numeric keypad...

 to gain 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 learned 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
Electronic lock
An electronic lock is a locking device which operates by means of electric current. Electric locks are sometimes stand-alone with an electronic control assembly mounted directly to the lock. More often electric locks are connected to an access control system...

, while generally safe from the attacks on their mechanical counterparts, suffer from their own set of flaws. If the arrangement of numbers is fixed, it is easy to determine the lock sequence by viewing several successful accesses. Similarly, the numbers in the combination (but not the actual sequence) may be determined by which keys show signs of recent use. More advanced electronic locks may scramble the numbers' locations randomly to prevent these attacks.

There is a variation of the traditional dial based combination lock wherein the "secret" is encoded in an electronic microcontroller. These are popular for safe and vault doors where tradition tends towards dial locks rather than keys. They allow many valid combinations, one per authorized user, so changing one person's access has no effect on other users. These locks often have auditing features, recording which combination is used at what time for every opening. Power for the lock may be provided by a battery or by a tiny generator set in operation by spinning the dial.

Manufacturers

  • ABUS
    ABUS
    ABUS is a German lock manufacturer founded in 1924, based in the town of Wetter in North Rhine-Westphalia. The name ABUS is an acronym for August Bremicker und Söhne KG....

  • Master Lock
    Master Lock
    Master Lock is an American company primarily known for developing and manufacturing padlocks, combination locks and related security products. Now an operating unit of Fortune Brands Home & Security, Inc., Master Lock Company LLC was formed in 1921 by locksmith-inventor Harry Soref, and is...

  • Sargent and Greenleaf
    Sargent and Greenleaf
    Sargent and Greenleaf, more commonly known among locksmithing circles as S&G, is a U.S. company that manufactures combination locks, key-operated safe and safe deposit box locks, high security military padlocks, and associated equipment....

  • Wordlock
    Wordlock
    Wordlock is a brand of combination locks, made by Wordlock, Inc., that differs from traditional combination locks in that it has letters on its dials instead of numbers. This allows the combination to be a four-letter or five-letter word or name, similar to a password, and therefore potentially...

  • Dudley
  • Conair

External links

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