The Holy Grail of advertising data is offline sales. Every marketer wants to know if an ad for something online led to someone going into a physical store and buying it. And data companies have long offered services trying to measure this. Nielsen Catalina, for instance, uses data from things like store membership cards to track user purchases. And big guns like Facebook have long used these services.

It turns out Google is doing it, too. A new Bloomberg report describes a partnership with Mastercard in which the tech giant gets access to offline sales data. Essentially, this program gives Google data that would connect the dots between someone interacting with an ad for a product and then purchasing it in a store within 30 days.

Both Google and Mastercard didn’t comment on the specific partnership to Bloomberg, but they both highlighted that they do not share identifiable customer data with any third parties.”Before we launched this beta product last year, we built a new, double-blind encryption technology that prevents both Google and our partners from viewing our respective users’ personally identifiable information. We do not have access to any personal information from our partners’ credit and debit cards, nor do we share any personal information with our partners,” the company said in a statement.

Google has boasted that this new feature for advertisers, called Store Sales Measurement, gives the company access to about 70% of U.S. credit and debit card data. The company hasn’t named who its partners are, but it’s likely more than just Mastercard. It’s currently being tested with a small group of advertisers, reports Bloomberg. Google adds that users can opt out of this kind of tracking via its “Web and App Activity” setting.

You can read the full report here.