A keylogger is a program or piece of hardware that records all keyboard keystrokes to an encrypted file which can then be read later. Based on the order of the keystrokes, it is usually easy to identify the password(s) from the file later. Like the Trojan, this also requires that someone actually type the password. Keyloggers come in two types: hardware and software. A hardware keylogger can be fitted between the keyboard cable and the computer and can be activated with a few keystrokes. It is then left in place until after the password that you are looking to recover is typed. Later it is removed and the file of keystrokes is examined for the password. A hardware keylogger is undectable by anti-virus software. A software keylogger is installed on a system and effectively has the same function, however, it is a little bit more complex to use since it must be installed to run stealthily to be effective. A keylogger could be used to steal a password from someone who is using an office computer or sharing a computer. It is possible that installing and using such a device or piece of software could be illegal depending upon whether the target has a presumption of privacy when using the computer on which the keylogger is installed.