“Digital wallets” that let consumers pay with the swipe of a smartphone could make the plastic credit card obsolete. But the technology also could chip away at consumers’ privacy—and tempt them to spend more than they otherwise would.Using a technology known as “near-field communications,” or NFC, consumers will be able to buy items simply by passing their phones in front of a sensor at the checkout counter. The problem, some critics say, is that the ability of merchants, coupon services and others to extract more information about where people shop and what they purchase amounts to an invasion of privacy.

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