Something you have

Example...

1) One Time Password Generator

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A one time password generator is a small device that shares the same algorithm with a computer or log-on system. It generates a password after a certain amount of time passes, and the changes are synced with the system as well.
Positives for using a one time password generator is that the password is hard to obtain due to frequent changes. Also, it is only accessible for one person per device. Memorizing the password is not necessary as well.
Negatives for using a one time password generator is that once an individual loses its device, it will become almost impossible to enter a building using the device since they do not know the password. Even though there follows a possibility where the person may have memorized it, since the password changes, and the changes are synced to the system, once time passes, then the person will no longer be able to enter.