It generally is taken as common wisdom that one day, everyone is going to switch to biometrics so we can finally get rid of those pesky passwords. This post is an attempt to stave off that future. This isn't because I'm in the password cracking business, (I'm sure horse-buggy salesmen thought that cars were a poor substitute as well). Instead it's because biometrics are a really bad solution. In fact, over the long run, biometrics will make it even easier for an attacker to break your authentication schemes. The rest of this post is an attempt to explain why this is the case, along with some other reasons why you probably shouldn't go out and buy that thumbprint reader quite yet.
If Biometrics Are So Bad, Why Do People Still Like Them:
Let's face it, passwords suck. This is a blog pretty much devoted to password cracking. I'm aware of the fact that they suck. Passwords are a pain to use, a pain to remember, and pain to make secure. Who doesn't want to get rid of them? At the same time, biometrics seem like the perfect solution. Just about everyone has something to scan, (we'll make a special use case for the double amputee). You won't forget them. You won't leave them at home. User's can't share them. Oh, and with all that fancy equipment, how's a hacker supposed to break it? In addition, it's an authentication scheme that everyone understands. Try to explain public key cryptography to someone, and all you'll get are blank stares. Everyone knows about fingerprints and iris scans. Heck, we use biometrics every day to distinguish between people like Bob and Alice, (though please stop staring at Alice's chest and calling it an authentication scheme...) So why shouldn't we use biometrics with computers to authenticate people?
Identification, not Authentication:
If you've ever had to suffer through a CISSP prep course, you'll probably remember the instructor droning on about the IAA triad, (Identification, Authentication, and Authorization). The thing is, keeping those three ideas separate really is important, since they perform different jobs. Identification is a way to keep multiple users distinct, and as a statement of who you think that person might be - "Hey that looks like Alice". Authentication is where you verify someone's identity - "Hey that is Alice because she knows the secret codeword". Biometrics are extremely useful for identification. Yes, you heard me right - I'm a fan of biometrics when it comes to identifying people in certain cases. It's using biometrics as a form of authentication that I have a problem with.
My very first internship back in 1997 was working on the National Automated Fingerprint Identification System. It was one of the best summers of my life. From moments such as, "Wait these fingerprints were taken from someone who died in the tub and laid there for three weeks - Ewww", to having to pass through an almost "Get Smart" level of security when going to work, it was amazingly fun. Beyond that though, I gained respect and a better understanding of how biometrics like fingerprints can be useful, (though I also obtained a better understanding of false positives). It's not just law enforcement where biometrics as a form of identification is used. From all accounts, biometric identification has also proven vital to supporting our troops in Iraq and Afghanistan. So I want to state for the record I'm not against using biometrics. As I'll go into much more details in the later sections though, due to certain inherent weaknesses, biometrics perform very poorly when used for authentication in computer systems. In short, biometrics for authentication = fail.
The Number #1 Reason Why Biometrics Fail - It's all Digital:
It's easy to get carried away talking about the lack of false positives, liveness sensors, or the inability to duplicate the blood vessels at the back of an eye when discussing the security of biometrics. In reality, very rarely does any of that matter since they are all factors of the biometric sensor itself. This is important. If you can bypass the sensor, then all you are dealing with is a digital representation of the data. Probably the best example of this was the excellent Defcon 15 talk by Zac Franken - "Biometric and Token Based Access Control Systems: Are you protected by two screws and a plastic cover? Probably". In it, he demonstrated grabbing biometric "signatures" from devices and then replaying those signatures to allow him access. It's a lot like a pass-the-hash attack but easier. This gets even worse for remote authentication, where the sensor is completely out of the control of the defender. Now, a defender can try to make the sensor harder to bypass, for example by using some form of trusted computing, but this is a tough battle to fight. Let me put it this way: If we had trusted computing down pat, no-one would be able to jailbreak their I-Phones...
The fact that an attacker is dealing with a digital representation of a biometric signature means the type of biometric measurement, (fingerprint, iris, voice, etc), they are attacking generally doesn't make a difference. The one caveat I want to mention though is that the type of signature can impact the ability to perform guessing attacks. As I'll explain later though, that becomes largely irrelevant as biometric authentication becomes more prevalent.
My Mother Always Said, "Don't Use Your Online Banking Password Everywhere":
I think we all agree that password reuse is a real problem. When singles.org was hacked, some of the 4chan'ers tried the disclosed e-mail/passwords on facebook, and then proceeded to create a lot of embarrassing fake posts. Some of those posts are hilarious; until you realize that this happened to real people. I think we can all agree - using the same password for your online bank account and every other site is a bad idea. But what about biometric signatures? I only have ten fingers. What happens when a site that has a copy of my biometric signature gets hacked? Now an attacker has a digital copy of my fingerprint. That can't be good...
The sad fact is that biometric authentication is only somewhat effective as long as it is a nitch market. The second everyone starts using it, you fingerprint/iris-scan/whatever will get disclosed. I wish I was exaggerating, but if that wasn't the case, we wouldn't have any problems with password reuse either. Heck I know my password has been stolen at least once when a gaming website I used to play on was broken into. In reality, my password probably has been stolen more than that, (the Cain&Able forum may have been hacked, the FSU Alumni database was broken into a couple of years ago, etc). Since a biometric signature is digital and can be used in a replay attack, the user needs to have at least the option to select different forms of authentication across different sites.
This is Going to Hurt - Lack of Revocation in Biometric Authentication
This dovetails with the previous point. What happens when a hacker gets your biometric signature? You can't change your fingerprints, (well you can, but it'll probably be fairly painful). How do you stop someone from logging in if they have your password and you can't choose a new one? That's not a rhetorical question. I really don't know the answer to it, and I don't think anyone else does either. In short, even if there is a form of biometric authentication where it is hard/impossible to guess someone's signature, it really doesn't matter because if it achieves widespread deployment your signature is going to get disclosed.
I Was Framed - Lack of Attribution, But Don't Count on the Jury Knowing That:
There's a lot of privacy issues with using biometrics, but I'm going to skip over that. It's not that the privacy issues are unimportant, but that they can lead into conversations/arguments that wildly diverge from the subject at hand. Instead, let me ask you this question. If you were on a jury, would you convict someone if the only evidence against them was fingerprints and/or DNA samples? I hope the answer is no, but I think a vast majority of people would say yes. What happens to Bob when Alice uses his biometric signature to break into the server room and trashes the place? As much as we want them too, biometrics don't provide attribution on their own, and this fact can be used by an attacker to frame someone.
Trusting Fuzzy Hashing to Encrypt Your Harddrive - This Is Going To End Well:
The great thing about passwords is if you don't enter them in exactly, they don't work. Ok, you might not think it's great when you spent ten minutes trying to log in only to find that caps lock was on, but from a cryptographic standpoint it's really nice. You might have some trouble decrypting your AES256 encrypted hard-drive if your password is constantly changing. To combat that, something called fuzzy hashing has been proposed. What fuzzy hashing attempts to do is return the exact same output hash even when the input values are changing slightly from time to time. This fuzzy hash can then be used as the key for your encryption algorithm. The problem is that current fuzzy hashing techniques aren't that good, and have been shown to have multiple problems that make them extremely vulnerable to guessing attacks, (the attacker only needs to get close). In fact, here are two papers on this very subject: 1, 2. Even if that wasn't the case, good luck trying to keep the cops from obtaining your fingerprint. That's actually one of the fundamental flaws with biometrics. Since they are based on something you are, instead of something you know, it becomes very hard to keep them secret. At least with a RSA key, you can hide it under the floorboards.
Finally, fuzzy hashes may not be able to take into account drastic changes in someone's biometric signature. If you cut your finger, do you have to wait for it to heal before you can decrypt your hard-drive? In short, biometrics and encryption don't play very well with each other.
There were quite a few additional arguments I could have made, (hey I didn't even mention this Mythbusters episode), but I'm probably beating a dead horse by now. It's actually gotten to the point where I'm not even a fan of using biometrics as one-half of a two factor authentication setup. It's not that they don't provide some additional security, but that there are better solutions out there. No form of authentication is perfect, but I'm fairly well convinced that biometrics are not the way to go. I hoped this post proved convincing, and if not, please use the comments section below as I'm always open to new ideas.