SAN FRANCISCO – Google has long contended that its online ads drive customers into brick-and-mortar stores—and to make purchases. Now it will start mining credit- and debit-card transactions to prove it, the Washington Post reports.

This means Google will soon be able to tell merchants how many sales can be tied to specific digital ad campaigns, a leap to what industry experts have long touted as “the holy grail” of digital ads. If this pans out, Google could have an enormous edge over its competitors—and siphon off television ad dollars to digital campaigns.

“Google—and also Facebook—believe that in order to get digital dollars from advertisers who are still primarily spending on TV, they need to prove that digital works,” said Amit Jain, chief executive of Bridg.

This announcement will likely spark new concerns over how much technology companies and their partners know about people, and if they disclose how that data is collected and used. Google receives tons of personal information from desktop computers and smartphones, including location data, search terms and browsing habits. That info becomes integrated with real identities of users when they are connected with Google services.

Google officials indicated the privacy of consumers is shielded by its proprietary search formulas, which transforms identities into numbers. “Through a mathematical property we can do double-blind matching between their data and our data,” said Jerry Dischler, vice president of product management for AdWords. “Neither gets to the see the encrypted data that the other side brings.”