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Password Security 101
Two Factor Authentication

Layered Authentication

Two Factor Authentication refers to a second layer of authentication which is required to access a system. This method protects sensitive data and systems when a single factor has been compromised. Most of the time, one factor will consist of something you know and the other factor will consist of  something you have. It is not uncommon, but less desirable to use two things you know. Due to possible loss or theft, using only two things you have is not desirable at all, except, of course, in the case of biometrics!

Some examples of things you might know (challenge-response authentication):

The latter example might be considered something your have, but your must know it to enter it. Portable devices are also used for this purpose.

Some examples of things you might have (token authentication):

The latter two are examples of biometrics, increasingly common in use for authentication as well as identification purposes.

In addition to things you have and things you know, other factors can be layered on top of these. Some examples:

With the epidemic of online fraudulent activities, such as phishing scams, we must all get used to the additional burdens of multiple layers of security.

A Two Factor Fable
The Wolf, The Goat and The Kid

As an old Goat was going forth to pasture, she carefully latched her door, and bid her kid not to open it to anyone who could not give this password: Beware of the Wolf and all his race. A Wolf happened to be passing, and overheard what the old Goat had said. When she was gone, he went to the door, and, knocking, said: "Beware of the Wolf and all his race." But the Kid, peeping through a crack, said: "Show me a white paw and I will open the door." As the Wolf could not do this, he had to depart, no better than he came.

Moral: Two sureties are better than one.

Aesop, 620 - 560 B.C.
Aesop's Fables
Litscape.com -- Classic Literature Online

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