Something You Know, Have, or Are

Methods for authenticating people differ significantly from those for authenticating machines and programs, and this is because of the major differences in the capabilities of people versus computers. Computers are great at doing large calculations quickly and correctly, and they have large memories into which they can store and later retrieve Gigabytes of information. Humans don't. So we need to use different methods to authenticate people. In particular, the cryptographic protocols we've already discussed are not well suited if the principal being authenticated is a person (with all the associated limitations).

All approaches for human authentication rely on at least one of the following:

• Something you know (eg. a password). This is the most common kind of authentication used for humans. We use passwords every day to access our systems. Unfortunately, something that you know can become something you just forgot. And if you write it down, then other people might find it.

• Something you have (eg. a smart card). This form of human authentication removes the problem of forgetting something you know, but some object now must be with you any time you want to be authenticated. And such an object might be stolen and then becomes something the attacker has.

• Something you are (eg. a fingerprint). Base authentication on something intrinsic to the principal being authenticated. It's much harder to lose a fingerprint than a wallet. Unfortunately, biometric sensors are fairly expensive and (at present) not very accurate.

Something You Have

Instead of basing authentication on something a principal knows and can forget, maybe we should base it on something the principal has. Various token/card technologies support authentication along these lines. For all, 2-factor authentication becomes important --- an authentication process that involves 2 independent means of authenticating the principal. So, we might require that a principal not only possess a device but also know some secret password (often known as a PIN, or personal identification number). Without 2-factor authentication, stealing the device would allow an attacker to impersonate the owner of the device; with 2-factor authentication, the attacker would still have another authentication burden to overcome.

Here are examples of technologies for authentication based on something a principal might possess:

• A magnetic strip card. (eg. Cornell ID, credit card) One serious problem with these cards is that they are fairly easy to duplicate. It only costs about $50 to buy a writer, and it's easy to get your hands on cards to copy them. To get around these problems, banks implement 2-factor authentication by requiring knowledge of a 4 to 7 character PIN whenever the card is used.

  Short PINs are problematic. First, they admit guessing attacks. Banks defend against this by limiting the number of guesses before they will confiscate the card. Second there is the matter of how to check if a PIN that has been entered is the correct one. Storing the PIN on the card's magnetic stripe is not a good idea because a thief who steals the card can easily determine the associated PIN (and then subvert the 2-factor authentication protocol). Storing an encrypted copy of the PIN on the card's magnetic stripe does not exhibit this vulnerability, though.

• Proximity card or RFID. These cards transmit stored information to a monitor via RF. There is currently a debate in this country as to the merits of using RF proximity cards (RFID tags) for identification of people and products. Walmart speaks about puttung RFID tags on every product they shelve, and both the German and U.S. governments are including them in passports. With RFID tags on Walmart products, for example. then somebody with a suitable receiver could tell what you have purchased (even though your purchase is hidden in a bag) --- and this is seen by some as a privacy violation. With RFID tags in passports, somebody with a suitable receiver could remotely identify on the street citizens of a given country and single them out for "special treatment" (likely unpleasant).

  There are two types of RF proximity cards: passive and active. The former is not powered, and use the RF energy from the requester to reply with whatever information is being stored by the card. The latter is powered and broadcasts information, allowing anyone who is in range and has a receiver to query the card. You could imagine that if RF tags are put into passports, then some people might start carrying them in special Faraday-cage passport holders, because now an interloper can learn about someone without the victim's knowledge (or permission).

• Challenge/Response cards and Cryptographic Calculators. These are also called smart cards and perform some sort of cryptographic calculation. Sometimes the card will have memory, and sometimes it will have an associated PIN. A smart card transforms the authentication problem for humans, because we are no longer constrained by stringent computational and storage limitations. Unfortunately, today's smart cards are vulnerable to power-analysis attacks. Furthermore, one must exercise care in using a cryptographic calculator --- if it is used to generate digital signatures, for example, then somehow the device owner must be made aware of what documents are being signed.

  One prevalent form of smartcard is the RSA secure id. It continuously displays encrypted time; and each RSA secure id encrypts with a different key. Whoever has an RSA secure id card responds to server challenges by typing the encrypted time (so, in effect, it is secret) --- a server, knowing what key is associated with each user's card, can then authenticate a user. (The server must be somewhat generous with respect to what times it will accept. Accept too many and replay attacks become possible; accept too few and message delivery delays and execution times prevent people from authenticating themselves).

Something You Are

Since people forget things and lose things, one might contemplate basing an authentication scheme for humans on something that a person is. After all, we recognize people we interact with not because of some password protocol but because of how they look or how they sound --- "something they are". Authentication based on "something you are" will employ behavioral and physiological characteristics of the principal. These characteristics must be easily measured accurately and preferably are things that are difficult to spoof. For example, we might use

• Retinal scan
• Fingerprint reader
• Handprint reader
• Voice print
• Keystroke timing
• Signature

Methods to subvert a fingerprint reader give some indication of the difficulties of deploying unsupervised biometric sensors as the sole means of authenticating humans. Attacks include:

• Steal a finger. Difficult to do without the owner of the finger noticing. Good supervision of the biometric sensor defends against this attack.

• Steal a fingerprint. Lifting a fingerprint is not that hard (at least, according to those TV crime-drama shows). Again, though, good human supervision of the biometric sensor defends against this attack because a guard will notice if somebody is not inserting a naked finger into the reader.

• Replace the biometric sensor. At first glance, this type of attack might seem even more difficult to execute than the two above. Social enginnering might be easier for the attacker to employ, here, though. It suffices that the guard believe that the senor should be changed (maybe because the the old one is "broken").

There are several well known problems with biometric-based authentication schemes:

• Reliability of the method. Similarity of physical features (faces, hands, or fingerprints) and inaccuracy of measurement may together conspire to create an unacceptably high false acceptance rate (FAR).

• Cost and availability. Currently, some readers cost $40-50 and more. Are end users willing to pay that much for an authentication method that does not work as well as passwords?

• Unwillingness or inability to interact with biometric input devices. Some people are uncomfortable putting a body part into a machine; some are uncomfortable having lasers shined in their eyes for a retinal scans; and some don't have fingers or eyes to be measured.

• Compromise the biometric database or system. It might be possible to circumvent the system's biometric sensor and provide an "input" from another source. The sensor is, after all, connected to a system and hijacking that channel might be possible. Knowledge of the stored representation for a characteristic would then allow an attacker to inject the correct characteristic and impersonate anyone.

• Revocation. What does it mean to revoke a fingerprint?

The literature on biometric authentication uses the following vocabulary to characterize what a scheme does and how well it works:

• FAR: (false acceptance rate). This is the probability that the system will fail to reject an impostor (aka FMR: false match rate)
• FRR: (false reject rate). This is the probability that the system will reject a bona fide principal. (aka FNMR: false non-match rate)
• One-to-one matching: Compare live template with a specific stored template in the system. This corresponds to authentication.
• One-to-many matching: Compare live templates with all stored templates in the system. This corresponds to identification.

Summary

Having looked at all these methods for authentication, we can see that as a secondary form of authentication (but not identification!) biometrics might be promising. The most likely form of authentication in the future, however, will be a combination of something you have and something you know. Passwords will be around for a long time yet.