Security researchers say that it is easy for a technically savvy hacker to make a fake payment card that gives them unlimited free parking. To prove their point, they will talk about how they built just such a card in about three days at a computer security conference Thursday.

According to Joe Grand, owner of Grand Idea Studio, San Francisco's parking meters have no way of telling the difference between a and a fake. These cards can be used to pay 23,000 meters citywide.

Grand, who hadn't worked much with smart cards, said that the work wasn't particularly hard to do. His card simply replays the same signals used by genuine cards to the meter. Although he never actually used the card to get free parking, Grand said he was able to build a card with a balance of US$999.99 -- the maximum possible -- that would never run out of funds.

"If I found this problem, chances are somebody else knows about the problem and possibly is exploiting it," he said. "That's costing all of us taxpayers money."

To figure out how the payment system worked, Grand hooked up an oscilloscope to a parking meter and monitored what happened when he used a genuine payment card. He then analyzed that data by hand, and wrote a software program that would emulate the smart card. After some trial and error, he finally figured out what his program needed to say to the meter in order to work. Then he built a card that would replay the same data, using a programmable smart card called a