But on the subject of privacy, we are an ambivalent nation. Americans — especially younger Americans, who swim in a sea of shared information — are casual to the point of recklessness about what we put online.

The trick, and I won’t pretend it’s always easy, is to distinguish the reasonable and constructive from the invasive and excessive. We want the sales clerk at the Gap to know our credit card is good, but not to have access to our whole credit history. We want our doctors to share our health histories with one another, but probably not with our employers. We may or may not want retailers to know what kind of books we read, what kind of car we drive, where we are thinking of traveling. We may or may not want those who follow us on the Web to know our real-time location, or our real name.

So imagine that you wanted to design an ID that would effectively control illegal hiring without stirring fears of Big Brother. It would be a single-purpose document, containing only the information that establishes you are eligible to work here. As passports are required for traveling abroad, as library cards are required for checking out books, the ID would be required for starting a job. I’d apply it to future hires only, to avoid forcing employers to be part of a national witch hunt.

You might start with the Social Security card. You would issue a plastic version, and in it you would embed a chip containing biometric information: a fingerprint, an eye scan or a digital photo. The employer would swipe the card and match it to the real you. Unlike your present Social Security card, the new version would be useless to a thief because it would contain your unique identifier. The information would not need to go into a database.

The Government Printing Office already embeds biometric information in passports — 75 million of them so far — and a slew of other documents, such as border-crossing smart cards for Americans who commute to Mexico or Canada, and security passes for the F.B.I. And one major employer is already rolling out a system of biometric IDs for all its millions of workers and contractors: the federal government. This is not exotic technology. I just stayed in a hotel in Barcelona that uses a fingerprint reader in place of a room key.

There would be a significant cost to set up and maintain the system, though it’s reasonable to assume that some of that money could be recouped through modest fees and fines on violators.