The card companies have implied through their marketing that the data is encrypted to make sure that a digital eavesdropper cannot get any intelligible information. American Express has said its cards incorporate “128-bit encryption,” and J. P. Morgan Chase has said that its cards, which it calls Blink, use “the highest level of encryption allowed by the U.S. government.”

But in tests on 20 cards from Visa, MasterCard and American Express, the researchers here found that the cardholder’s name and other data was being transmitted without encryption and in plain text. They could skim and store the information from a card with a device the size of a couple of paperback books, which they cobbled together from readily available computer and radio components for $150.

They say they could probably make another one even smaller and cheaper: about the size of a pack of gum for less than $50.