Probably the most foolproof biometric measure is the eye’s iris. Its complex pattern of zigzagging lines and random dots is much more distinctive than the whorls of a fingerprint.

In fact, because authorities in a few foreign countries are confident that iris scans can’t be circumvented, they’re starting to allow airlines to use iris scanning at selected airports. For the airlines, it’s a perquisite for frequent fliers: If people register their iris scans, they can bypass the usual security check.

Currently, a person’s eye must be in close to the scanner. Intelligence and law-enforcement agencies hope that some way can be found to scan irises from a distance — or even to spot a suspect in a crowd.