In authentication, we generally talk about three “factors” for determining identity. A “factor” is a broad category for establishing that you are who you claim to be. The three types of authentication factor are:

Something you know (a password, a PIN, the answer to a “security question”, etc.)

Something you have (an ATM card, a smart card, a one-time-password token, etc.)

Something you are (your fingerprint, retinal pattern, DNA)

Historically, most people have used the first of these three forms most commonly. Whenever you’ve logged into Facebook, you’re entering something you know: your username and password. If you’ve ever used Google’s two-factor authentication to log in, you probably used a code stored on your smartphone to do so.

One of the less common, but growing, authentication methods are the biometrics. A couple years ago, a major PC manufacturer ran a number of television commercials advertising their laptop models with a fingerprint scanner. The claim was that it was easy and secure to unlock the machine with a swipe of a finger. Similarly, Google introduced a service to unlock an Android smart phone by using facial recognition with the built-in camera.

Pay attention folks, because I’m about to remove the scales from your eyes. Those three factors I listed above? I listed them in decreasing order of security. “But how can that be?” you may ask. “How can my unchangeable physical attributes be less secure than a password? Everyone knows passwords aren’t secure.”

The confusion here is due to subtle but important definitions in the meaning of “security”. Most common passwords these days are considered “insecure” because people tend to use short passwords which by definition have a limited entropy pool (meaning it takes a smaller amount of time to run through all the possible combinations in order to brute-force the password or run through a password dictionary). However, the pure computational complexity of the authentication mechanism is not the only contributor to security.

The second factor above, “something you have” (known as a token), is almost always of significantly higher entropy than anything you would ever use as a password. This is to eliminate the brute-force vulnerability of passwords. But it comes with a significant downside as well: something you have is also something that can be physically removed from you. Where a well-chosen password can only be removed from you by social engineering (tricking you into giving it to an inappropriate recipient), a token might be slipped off your desk while you are at lunch.

Both passwords and tokens have an important side-effect that most people never think about until an intrusion has been caught: remediation. When someone has successfully learned your password or stolen your token, you can call up your helpdesk and immediately ask them to reset the password or disable the cryptographic seed in the token. Your security is now restored and you can choose a new password and have a new token sent to you.

However, this is not the case with a biometric system. By its very nature, it is dependent upon something that you cannot change. Moreover, the nature of its supposed security derives from this very fact. The problem here is that it’s significantly easier to acquire a copy of someone’s fingerprint, retinal scan or even blood for a DNA test than it is to steal a password or token device and in many cases it can even be done without the victim knowing.

Many consumer retinal scanners can be fooled by a simple reasonably-high-resolution photograph of the person’s eye (which is extremely easy to accomplish with today’s cameras). Some of the more expensive models will also require a moving picture, but today’s high-resolution smartphone cameras and displays can defeat many of these mechanisms as well. It’s well-documented that Android’s face-unlock feature can be beaten by a simple photograph.

These are all technological limitations and as such it’s plausible that they can be overcome over time with more sensitive equipment. However, the real problem with biometric security lies with its inability to replace a compromised authentication device. Once someone has a copy of your ten fingerprints, or a drop of your blood from a stolen blood-sugar test or a close-up video of your eye from a scoped video camera, there is no way to change this data out. You can’t ask helpdesk to send you new fingers, an eyeball or DNA. Therefore, I contend that I lied to you above. There is no full third factor for authentication, because, given a sufficient amount of time, any use of biometrics will eventually degenerate into a non-factor.

Given this serious limitation, one should never under any circumstances use biometrics as the sole form of authentication for any purpose whatsoever.

One other thought: have you ever heard the argument that you should never use the same password on multiple websites because if it’s stolen on one, they have access to the others? Well, the same is true of your retina. If someone sticks malware on your cellphone to copy an image of your eye that you were using for “face unlock”, guess what? They can probably use that to get into your lab too.

The moral of the story is this: biometrics are minimally useful, since they are only viable until the first exposure across all sites where they are used. As a result, if you are considering initiating a biometric-based security model, I encourage you to save your money (those scanners are expensive!) and look into a two-factor solution involving passwords and a token of some kind.