A key is a device which is used to open a lock. A typical key consist of two parts: the blade, which slides into the keyway of the lock and distinguishes between different keys, and the bow, which is left protruding so that torque can be applied by the user. The blade is usually designed to open one specific lock, although master keys are designed to open sets of similar locks.
Keys provide an inexpensive, though imperfect, method of authentication for access to properties like buildings and vehicles. As such, keys are an essential feature of modern living in the developed world, and are common around the globe. It is common for people to carry the set of keys they need for their daily activities around with them, often linked by a keyring adorned by key fobs and known as a keychain.
The more recent form is that for a pin tumbler cylinder lock. When held upright as if to open a door, a series of grooves on either side of the key (the key's profile) limits the type of lock cylinder the key can slide into. As the key slides into the lock, a series of pointed teeth and notches allow pins to move up and down until those pins are in line with the shear line of the cylinder, allowing that cylinder to rotate freely inside the lock and the lock to open. These predominate in, for example, the United States of America.
A car key or an automobile key is a key used to open and/or start an automobile, often identified with the logo of the car company at the head. Modern key designs are usually symmetrical, and some use grooves on both sides, rather than a cut edge, to actuate the lock. It has multiple uses for the automobile with which it was sold. A car key can open the doors, as well as start the ignition, open the glove compartment and also open the trunk (boot) of the car. Some cars come with an additional key known as a valet key that starts the ignition and opens the drivers side door but prevents the valet from gaining access to valuables that are located in the trunk or the glove box. Some valet keys, particularly those to high-performance vehicles, go so far as to restrict the engine's power output to prevent joyriding. Recently, features such as coded immobilizers have been implemented in newer vehicles. More sophisticated systems make ignition dependent on electronic devices, rather than the mechanical keyswitch. Ignition switches/locks are combined with security locking of the steering column (in many modern vehicles) or the gear lever (Saab Automobile). In the latter, the switch is between the seats, preventing damage to the driver's knee in the event of a collision.
Some keys are high-tech in order to prevent the theft of a car. Mercedes-Benz uses a key that, rather than have a cut metal piece to start the car, uses an encoded infrared beam that communicates with the car's computer. If the codes match, the car can be started. These keys can be expensive to replace, if lost, and can cost up to US$400. Some car manufacturers like Land Rover and Volkswagen use a 'switchblade' key where the key is spring-loaded out of the fob when a button is pressed. This eliminates the need for a separate key fob. This type of key has also been known to be confiscated by airport security officials.
Larger organizations, with more complex "grandmaster key" systems, may have several masterkey systems where the top level grandmaster key works in all of the locks in the system.
A practical attack exists to create a working master key for an entire system given only access to a single master-keyed lock, its associated change key, a supply of appropriate key blanks, and the ability to cut new keys. This is described in Cryptology and Physical Security: Rights Amplification in Master-Keyed Mechanical Locks
Locksmiths may also determine cuts for a replacement master key, when given several different key examples from a given system.
A skeleton key (or passkey) is a very simple design of key which usually has a cylindrical shaft (sometimes called a shank) and a single, minimal flat, rectangular tooth or bit. Skeleton keys are also usually distinguished by their bow, or the part one would grasp when inserting the key, which can be either very plain or extremely ornate. A skeleton key is designed to circumvent the wards in warded locks. Warded locks and their keys provide minimal security and only a slight deterrent as any key with a shaft and tooth that has the same or smaller dimensions will open the lock. However, warded keys were designed to only fit a matching lock and the skeleton key would often fit many. Many other objects which can fit into the lock may also be able to open it. Due to its limited usefulness, this type of lock fell out of use after more complicated types became easier to manufacture. In modern usage, the term "skeleton key" is often misapplied to ordinary bit keys and barrel keys, rather than the correct definition: a key, usually with minimal features, which can open all or most of a type of badly designed lock. Bit keys and barrel keys can be newly-minted (and sold by restoration hardware companies) or antiques. They were most popular in the late 1800s, although they continued to be used well into the 20th century and can still be found today in use, albeit in vintage homes and antique furniture. A bit key is distinguished from a barrel key in that a bit key usually has a solid shank, whereas a barrel shafted key can be made either by drilling out the shank from the bit end or by folding metal into a barrel shape when forging the key.
A tubular key (sometimes referred to as a barrel key when describing a vintage or antique model) is one that is designed to open a tubular pin tumbler lock. It has a hollow, cylindrical shaft which is usually much shorter and has a larger diameter than most conventional keys. Antique or vintage-style barrel keys often closely resemble the more traditional skeleton key but are a more recent innovation in keymaking. In modern keys of this type, a number of grooves of varying length are built into the outer surface at the end of the shaft. These grooves are parallel to the shaft and allow the pins in the lock to slide to the end of the groove. A small tab on the outer surface of the shaft prevents the pins in the lock from pushing the key out and works with the hollow center to guide the key as it is turned.
The modern version of this type of key is harder to duplicate as it is less common and requires a different machine from regular keys. These keys are most often seen in home alarm systems, in the United States.
A Do Not Duplicate key (or DND key, for short) is one which has been stamped do not duplicate , duplication prohibited or similar by a locksmith or manufacturer as a passive deterrent to prevent a retail key cutting service from duplicating a key without authorization or without contacting the locksmith or manufacturer who originally cut the key. More importantly, this is a control system for whoever is the owner of the key, such as a maintenance person or security guard, to identify keys that should not be freely distributed or used without authorization. Though it is intended to prevent unauthorized key duplication, copying restricted keys remains a common security problem. There is no direct legal implication in the US for someone who copies a key that is stamped do not duplicate (unless it is a government owned key), but there are patent restrictions on some key designs (see "restricted keys"). The Associated Locksmiths of America calls DND keys "not effective security", and "deceptive because it provides a false sense of security."
Bar code technology is not a secure form of a key, as the bar code can be copied in a photocopier and often read by the optical reader.
Magnetic stripe keycards are becoming increasingly easy to copy, but have the security advantage that one may change the stored key in a magnetic swipe card in case the current key may be compromised. This immediate change of the "key" information can be applied to other media, but this media probably offers the least expensive option, and the most convenient to users and managers of systems that use this media. Example: If you own a car with this system, you can change your keys anytime you want. You can buy new media anywhere a gift card is sold. At least at this point in time, you could buy a gift card for a penny, then use that as the media for the keys to your car. If the system uses digital environmental data samples to create the "key" string, every car can have a set of keys that no one else has. If a card is stolen, or copied without authorization, the card can be remade, and the car security system can be synchronized with the new card, and no longer activationally responsive to the copy of the old card. This approach can empower the system controller (owner/individual or centralized administration of a business).
Computerized authentication systems, such as key cards, raise privacy concerns, since they enable computer surveillance of each entry. Currently RFID cards and key fobs are becoming more and more popular due to its ease of use. Many modern households have installed digital locks that make use of key cards, in combination with biometric fingerprint and keypad PIN options.
Wooden locks and keys were in use as early as 4,000 years ago in Egypt. It is also said that key was invented by Theodore of Samos in the 6th century BC.
A machine permitting rapid duplication of flat metal keys, which contributed to the proliferation of their use during the 20th century, may have been first invented in the United States in 1917 (image to the left):
The key to be duplicated is placed in one vise and the blank key to be cut in a corresponding vise under the cutting disk. The vise carriage is then into such position by means of a lateral-feed clutch that the shoulders of both the pattern and blank keys just touch the guide disk and cutter respectively. The lateral-feed clutch on the top of the machine is then thrown, and the vertical feed rod released into action and power applied through the combination hand-crank power wheel on the right of the machine, until the cutter has passed over the entire length at the blank. A duplicate of the pattern key is obtained in about one minute.|10px||"Man And His Machines", The World's Work XXXIII:6 April, 1917
Key duplication in a fashion similar to this is performed in modern times with powered milling or grinding equipment in many retail hardware stores and of course as a service of the specialized locksmith.