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Chapter: Security in Computing : Protection in General-Purpose Operating Systems

Biometrics: Authentication Not Using Passwords

Some sophisticated authentication devices are now available. These devices include handprint detectors, voice recognizers, and identifiers of patterns in the retina.

Biometrics: Authentication Not Using Passwords


Some sophisticated authentication devices are now available. These devices include handprint detectors, voice recognizers, and identifiers of patterns in the retina. Authentication with such devices uses unforgeable physical characteristics to authenticate users. The cost continues to fall as these devices are adopted by major markets; the devices are useful in very high security situations. In this section we consider a few of the approaches available.


Biometrics are biological authenticators, based on some physical characteristic of the human body. The list of biometric authentication technologies is still growing. Now there are devices to recognize the following biometrics: fingerprints, hand geometry (shape and size of fingers), retina and iris (parts of the eye), voice, handwriting, blood vessels in the finger, and face. Authentication with biometrics has advantages over passwords because a biometric cannot be lost, stolen, forgotten, lent, or forged and is always available, always at hand, so to speak.


Identification versus Authentication


Two concepts are easily confused: identification and authentication. Biometrics are very reliable for authentication but much less reliable for authentication. The reason is mathematical. All biometric readers operate in two phases: First, a user registers with the reader, during which time a characteristic of the user (for example, the geometry of the hand) is captured and reduced to a template or pattern. During registration, the user may be asked to present the hand several times so that the registration software can adjust for variations, such as how the hand is positioned. Second, the user later seeks authentication from the system, during which time the system remeasures the hand and compares the new measurements with the stored template. If the new measurement is close enough to the template, the system accepts the authentication; otherwise, the system rejects it. Every template is thus a pattern of some number of measurements.


Unless every template is unique, that is, no two people have the same measured hand geometry, the system cannot uniquely identify subjects. However, as long as it is unlikely that an imposter will have the same biometric template as the real user, the system can authenticate. The difference is between a system that looks at a hand geometry and says "this is Captain Hook" (identification) versus a man who says "I, Captain Hook, present my hand to prove who I am" and the system confirms "this hand matches Captain Hook's template" (authentication). Biometric authentication is feasible today; biometric identification is largely still a research topic.


Problems with Biometrics


There are several problems with biometrics:


  Biometrics are relatively new, and some people find their use intrusive. Hand geometry and face recognition (which can be done from a camera across the room) are scarcely invasive, but people have real concerns about peering into a laser beam or sticking a finger into a slot. (See [SCH06a] for some examples of people resisting biometrics.)


  Biometric recognition devices are costly, although as the devices become more popular, their costs go down. Still, outfitting every user's workstation with a reader can be expensive for a large company with many employees.


  All biometric readers use sampling and establish a threshold for when a match is close enough to accept. The device has to sample the biometric, measure often hundreds of key points, and compare that set of measurements with a template. There is normal variability if, for example, your face is tilted, you press one side of a finger more than another, or your voice is affected by an infection. Variation reduces accuracy.


  Biometrics can become a single point of failure. Consider a retail application in which a biometric recognition is linked to a payment scheme: As one user puts it, "If my credit card fails to register, I can always pull out a second card, but if my fingerprint is not recognized, I have only that one finger." Forgetting a password is a user's fault; failing biometric authentication is not.


Although equipment is improving, there are still false readings. We label a "false positive" or "false accept" a reading that is accepted when it should be rejected (that is, the authenticator does not match) and a "false negative" or "false reject" one that rejects when it should accept. Often, reducing a false positive rate increases false negatives, and vice versa. The consequences for a false negative are usually less than for a false positive, so an acceptable system may have a false positive rate of 0.001 percent but a false negative rate of 1 percent.


  The speed at which a recognition must be done limits accuracy. We might ideally like to take several readings and merge the results or evaluate the closest fit. But authentication is done to allow a user to do something: Authentication is not the end goal but a gate keeping the user from the goal. The user understandably wants to get past the gate and becomes frustrated and irritated if authentication takes too long.


Although we like to think of biometrics as unique parts of an individual, forgeries are possible. The most famous example was an artificial fingerprint produced by researchers in Japan [MAT02]. Although difficult and uncommon, forgery will be an issue whenever the reward for a false positive is high enough.


Sidebar 4-5: Using Cookies for Authentication


On the web, cookies are often used for authentication. A cookie is a pair of data items sent to the web browsing software by the web site's server. The data items consist of a key and a value, designed to represent the current state of a session between a user and a web site. Once the cookie is placed on the user's system (usually in a directory with other cookies), the browser continues to use it for subsequent interaction between the user and that web site. Each cookie is supposed to have an expiration date, but that date can be modified later or even ignored.


For example, The Wall Street Journal 's web site, wsj.com, creates a cookie when a user first logs in. In subsequent transactions, the cookie acts as an identifier; the user no longer needs a password to access that site. (Other sites use the same or a similar approach.)


It is important that users be protected from exposure and forgery. That is, users may not want the rest of the world to know what sites they have visited. Neither will they want someone to examine information or buy merchandise online by impersonation and fraud. However, Sit and Fu [SIT01] point out that cookies were not designed for protection. There is no way to establish or confirm a cookie's integrity, and not all sites encrypt the information in their cookies.


Sit and Fu also point out that a server's operating system must be particularly vigilant to protect against eavesdropping: "Most HTTP exchanges do not use SSL to protect against eavesdropping; anyone on the network between the two computers can overhear the traffic. Unless a server takes strong precautions, an eavesdropper can steal and reuse a cookie, impersonating a user indefinitely."


Sometimes overlooked in the authentication discussion is that credibility is a two-sided issue: The system needs assurance that the user is authentic, but the user needs that same assurance about the system. This second issue has led to a new class of computer fraud called phishing, in which an unsuspecting user submits sensitive information to a malicious system impersonating a trustworthy one. Common targets of phishing attacks are banks and other financial institutions because fraudsters use the sensitive data they obtain from customers to take customers' money from the real institutions. We consider phishing in more detail in Chapter 7.


Authentication is essential for an operating system because accurate user identification is the key to individual access rights. Most operating systems and computing system administrators have applied reasonable but stringent security measures to lock out illegal users before they can access system resources. But, as reported in Sidebar 4-5, sometimes an inappropriate mechanism is forced into use as an authentication device.


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