They are necessary for anyone seeking public assistance, like food stamps, or registering for the draft. Congress decreed that the numbers be put on records including professional licenses, marriages licenses and divorce decrees to better track scofflaws of child support orders.

Image A handout from Mrs. Ostergren, who spoke to the Portsmouth Rotary Club. Credit... Gary C. Knapp for The New York Times

The Social Security number took on a second role. It allowed collectors of data to link pieces of information together, like a driver’s license record, credit report data and the information on the warranty card for a toaster. That is a useful tool for marketers and just as useful for criminals.

It was only in 2004 that Congress prohibited states from using the Social Security number on drivers’ licenses. Yet the databases with those numbers still exist. Until 2001, states could sell lists with those numbers, which means that for virtually anyone 22 years or older, the name, address, phone number and Social Security number are in private databases.

The nine-digit string took on a third role — as a password that was supposed to protect all that private information from snoops and criminals. But its ubiquity defeats that purpose, Mr. Hoofnagle said. “It will pass when the business community no longer needs a Social Security number,” he said.

The Social Security Administration’s Office of Inspector General said that 16 percent of the 99,000 fraud cases it investigated in the 12-month period that ended Sept. 30 involved the misuse of Social Security numbers. One involved an identify theft ring in Central Florida. Twelve people were convicted, sentenced to prison and ordered to repay more than $2 million.

About 16,000 incidents are not a lot considering that 240 million numbers are currently in use, and certainly theft and fraud involving credit card numbers are much more pervasive.

But credit card numbers are rarely exposed on documents in public view. And if a credit card is stolen or misused, obtaining a new one is a fairly simple process. A new Social Security number is rarely granted. (Indeed, one is limited to 3 replacements of the green paper Social Security card in a year and 10 over a lifetime.)