A standard hardware token is a small device, typically in the general form factor of a credit card or keychain fob. The simplest hardware tokens look identical to a USB flash drive and contain a small amount of storage holding a certificate or unique identifier, and are often called dongles. More complex hardware tokens incorporate LCD displays, keypads for entering passwords, biometric readers, wireless devices, and additional features to enhance security
Many hardware tokens contain an internal clock that, in combination with the device’s unique identifier, an input PIN or password, and potentially other factors, is used to generate a code, usually output to a display on the token. This code changes on a regular basis, often every 30 s. The infrastructure used to keep track of such tokens can predict, for a given device, what the proper output will be at any given time and can use this to authenticate the user.