They don't intend to provide full technical details on how they built their fake card, however, as that information could be misused to bilk San Francisco.

They say that if San Francisco wants to avoid fraud on its system, it should look at securing the meters by developing a better way to authenticate smart cards. This may involve making the meters even smarter, so that they can communicate with each other and process digital signatures during each transaction.

"Hardware devices should not be trusted, they need to be analyzed before they're deployed," Grand said. "These devices are not secure."