I have come across the following question in a Security+ exam guide

You are tasked with creating a high-security authentication system for physical access control to a military installation. Which of the following authentication systems would be most appropriate?

  1. Biometric eye scan
  2. Security badge
  3. Smart card and PIN
  4. Encrypted login and password

The answer at the end of the chapter was:

For high-security installations, biometrics is an extremely secure method to authenticate users based on unique physical characteristics.

My question is

How is it possible the answer was not number 3?

Bad guys will have to steal the smart card and somehow get the PIN from the authorized personal which is TWO jobs

while in a biometric eye scan they have to do ONE job

  • 7
    Biometrics are not magical portals to security. They are as good as they are. So far, some of the best and top market leaders in biometrics have repeatedly been defeated by photo copies, licking, blowing. These gizmos aren't based on real science yet. Commented Aug 16, 2014 at 12:50
  • 2
    If you can beat the pin out of them with a lead pipe, why would getting the card require a pipe of different elemental composition?
    – jjanes
    Commented Aug 17, 2014 at 22:48
  • If he does not have the card on him you'll need more than a pipe, thats where TWO is more secure than ONE
    – Ulkoma
    Commented Aug 18, 2014 at 11:16
  • 3
    To be honest, this is a poorly-scoped question. Neither is inherently more secure; they simply have different security properties. Biometrics can never be revoked, but when paired with a human observer they can be effective. Smart cards are likely a better choice when unobserved, but which technique is best depends upon the types of threats you want to protect against and what other security measures you intend to employ. Commented Aug 19, 2014 at 23:55
  • 3
    I don't know about you, but bad guys who point a gun at me and demand my smart card and my pin, get it.
    – ddyer
    Commented Aug 20, 2014 at 0:18

5 Answers 5


Biometrics can be effective as authentication or as identification, but not both at the same time.

According to Wikipedia, retinal scans are accurate to approximately one in one million, meaning on the earth today there are approximately 7,000 individuals who will be identified as you in a retinal scan. Assuming no further authentication is necessary, these people will be both identified and authenticated as you in a single (mis-)step.

But if coupled with an external identification step, biometric authentication becomes a second factor. Typically identification is accomplished by an identification badge ("something you have") while authentication happens through biometrics ("something you are"). So it does turn out to be true two-factor authentication.

This is inherently better than card-and-pin because of the three available factors, "something you know" is the easiest to fake.

So assuming identification happens using a token of some sort (which is very typical in biometric authentication installations, but not explicitly stated here), then the question is poorly-worded but has at least the right idea. Assuming instead that the eye-scan stands alone as both identification and authentication, then answer 3 would be correct.

Either way, the question could use some clarification. It refers to authentication alone without even a word about identification; but that's an absolutely critical factor in evaluating the security of the system and definitely makes a difference here. It's left to the reader to divine the contents of the question-writer's head, which is poor form in writing standardized tests.

  • Biometrics (something you are) are not authentication, they are only identification. Something you have, on the other hand can be both identification and authentication.
    – Gudradain
    Commented Aug 20, 2014 at 14:21
  • Something you know is actually a very important thing as it is the hardest to steal. Stealing biometrics can be easy, stealing phone can be easy. Forcing people to reveal something only them know can be very hard.
    – Gudradain
    Commented Aug 20, 2014 at 14:26
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    @Gudradain eh, no. Not buying it. Something you know is dramatically easier to steal than something like an iris scan. Fingerprint sensors are exceptional because you leave the traces everywhere, but they're explicitly and intentionally not relevant to this question. But it's easy to trick someone into revealing their password or surreptitiously gathering that information. Surreptitiously gathering an iris scan is significantly less probable, and reproducing it even more so - very nearly zero. Passwords are compromised far more often than eye scans.
    – tylerl
    Commented Aug 20, 2014 at 23:17
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    @Gudradain Both the biometrics and the token together establish identity; neither does on its own. The likelihood that both will be stolen is sufficiently low to ensure security. Whether you call the biometric component identification or authentication is immaterial, as it can't do either job on its own. On the backend, biometrics are poor at the job of account selection because they provide a fuzzy match, while other forms are strictly deterministic, hence the authentication/identification distinction as described. But if you want to switch them around, it's largely just semantics.
    – tylerl
    Commented Aug 21, 2014 at 21:24
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    Biometric sensors are inherently and catastrophically vulnerable to replay attacks - if you can produce the same inputs the system is expecting, you can spoof the authorized user and gain access. As such, they are only viable as a secondary factor, and should never be used alone.
    – Stephanie
    Commented Aug 22, 2014 at 4:50

I see you as having two questions here:

  1. Is the answer provided by the study guide technically correct?
  2. Is it possible this sample question doesn't have a clear answer and is a bad question?

With regards to your first question I think you have a valid argument that smart card / PIN offers at least equivalent security to a biometric authenticator. But part of this depends on how both systems are implemented.

Is the smart card simply storing a static value that is presented, along with the PIN, to the authenticating system? Or does it contain a protected private key that signs a one-time challenge and will only do so when it directly receives the proper PIN (meaning the auth system never knows the PIN)? Does the "eye scan" (retina, iris or other?) capture enough data points from the image to represent a significantly large number of possible patterns? Has the eye biometric system been configured with an appropriate accuracy to reject close, but not exact, readings (false acceptance rate)?

But the exam guide provides none of that information and thus expects you to make a fairly uninformed decision about which one is best. And that brings us to the second question.

I'm not sure if this is your first technical certification or not, but expect to be frustrated. In an attempt to assess your knowledge of a wide variety of topics you will run across questions like this on real exams that do not have a single clear answer. They may rely on your knowledge of the exam creator's specific guidance on the subject, which might be published in their official study guide or supporting material.

But you may just be left trying to reverse engineer the question writer's undocumented perceptions that led them to choose one answer over another. In other words, even if you don't agree yourself, is there an answer that from one point of view might be the obvious one?

Some testing organizations put a good deal of time and effort into making sure their questions are both well founded in industry consensus and psychometrically sound. But our industry also has a lot of questions that require answers to start with "well, it depends..." A few testing organizations try to simplify this complexity for the sake of expanding their repository of questions and do us a disservice in pretending there is a clear answer.

My experience taking and passing the Security+ exam some 7+ years ago was that they were included in this latter group. So, good luck to you and try not to get too frustrated in the process!


I think that there's an unstated assumption in the question: that the military installation will have a guard at the entry. Every attack on a biometric system I'm aware of assumes an unprotected or compromised scanner. If there's a guard standing there at the gate making sure that you're using your eyeball (not a gouged-out one, not a photograph, not a dummy scanner connected in place of the real one, and so on), a biometric eye scan is a reasonable technique.

In such a situation, a smart card can be stolen and a PIN can be beaten out of the victim, but there's no way you're going to be able to trick the guard into letting you use someone else's eyeball.


Some sources out there will disagree with the following statement but it has to be said.

Biometrics are NOT authentication. Biometrics are IDENTIFICATION.

Do NOT use biometric for authentication, but you can use them for identification just fine!

There is 3 important concepts for security :

  • Identification : Who you are
  • Authentication : How you prove who you are
  • Authorization : What access you get based on who you are

The big problem with biometric is that you are not able to change them. If you are not able to change them, what happen when they get stolen?

For identification, you can reuse the same username or biometric everywhere. It doesn't matter that you reuse the same thing since it's who you are. It's also not a problem if it cannot change since who you are will never change.

For authentication on the other hand, you need to be able to change things in case they get stolen. You are able to change what you know, passwords, and you are able to change what you have, phone/token device/email address. But, you are not able to change what you are, bimotric.

Hence, biometric is the worst pick for authentication as it's really identification.

The good part

This does not mean that biometrics are useless. In fact, they can provide very good identification as it will be harder for an attacker to steal the biometric of one person than to simply write his public user name.

But, it's just identication. You still need authentication and yes multi-factor authentication is better than single authentication.

There are 3 things you can use for identication / authentication.

  • Something you know : password
  • Something you have : phone, token device, email address
  • Something you are : biometrics, username

If you want the best security, you need to combine them this way :

  • Identification : Something you are + Something you have
  • Authentication : Something you know + Something you have

Something you have is the only thing that can be use as both identification and authentication since it's assumed that you are the only one with it.

Something you are cannot be used as authentication as it cannot change

Something you know cannot be used as identification as you cannot reveal it

The answer

If you really have to pick only one then #3 (smart card + pin) should be the answer.

If you can pick more than one then #1 (iris scan) + #3 (smart card + pin) provide the best security

Some sources

http://technet.microsoft.com/en-us/library/cc512578.aspx (great article on the subject)

Proper biometrics are identity only and will be accompanied, like all good identifiers, by a secret of some kind -- a PIN, a private key on a smart card, or, yes, even a password.




  • 1
    Care to comment your downvote?
    – Gudradain
    Commented Aug 20, 2014 at 15:26
  • 1
    I wasn't responsible for the downvote, but I strongly disagree with your comment that "biometrics are not authentication". You don't get to arbitrarily rewrite several decades of consensus in our industry because you don't agree with it. Biometrics certainly do have weaknesses, but they can also be used to effectively authenticate users. The same way that passwords have weaknesses and are used to authenticate. Can they offer better security when combined, sure, but that doesn't mean the use of any one factor by itself is invalid. Your logic isn't consistent in comparing factors.
    – PwdRsch
    Commented Aug 21, 2014 at 20:08
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    I was one of the downvotes and @PwdRsch is right. The definition of authentication is "the process of verifying identity." The claim that authentication necessarily requires mutability is bizarre, and that biometric factors cannot be used as authenticators is just wrong. Yes, using only a biometric factor as both an identifer and an authenticator is bad. That doesn't mean that it doesn't or can't happen. If that were true, single factor systems based on a biometric couldn't exist, by your definition. One unrelated point, a username is "something you know" not "something you are."
    – Xander
    Commented Aug 21, 2014 at 20:29
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    @Gudradain Biometrics are not guaranteed to be public. Some types can be observed or captured given the right circumstances, and that is indeed one of their weaknesses. But my biometric data isn't sitting out on the Internet in a public database. An attacker has to take steps to collect it, some of which will be more difficult than others. Good biometric product vendors also typically implement 'liveness' checks that attempt to detect and reject copies of biometric data, which either thwarts this type of attack or raises the cost of carrying out a successful attack.
    – PwdRsch
    Commented Aug 21, 2014 at 20:57
  • 1
    @Gudradain Resistance to disclosure and duplication are properties of authenticators, and their success at accomplishing this does definitely depend on which one you're discussing. I am worried that widespread biometric use would increase the risk of accidental or malicious disclosure that leads to someone impersonating me. But that still doesn't make them any more public than you giving the same credit card number to multiple companies. And, back to the original point, that doesn't invalidate them for authentication.
    – PwdRsch
    Commented Aug 21, 2014 at 22:05

It would be easier to defeat a smartcard and pin -- all you would have to do is use a pinhole camera to record your mark entering the pin and then steal their smartcard. Defeating a retinal scan requires fabricating some kind of device that can pass for a real human eye.

Now think which would look more odd to a bored security guard watching the video feeds -- someone inserting a smartcard and entering a pin like everyone else, or someone holding up an odd contraption to the retinal scanner? It's certainly a lot harder to look inconspicuous when you're trying to defeat a biometric device (unless it's el-cheapo fingerprint kind).


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