A key fob is a type of security token: a small hardware device with built-in authentication mechanisms. Just as the keys held on an ordinary real-world key chain or fob control access to the owner's home or car, the mechanisms in the key fob control access to network services and information. The key fob (and similar devices, such as smart cards) provide two-factor authentication: the user has a personal identification number (PIN), which authenticates them as the device's owner; after the user correctly enters their PIN, the device displays a number which allows them to log on to the network. Because a key fob is a physical object, it is easy for the owner to know if it has been stolen. In comparison, a password can be stolen (or guessed) and used for an extended period before -- if ever -- the theft is detected.