FILE - The brand logo of Alphabet Inc's Google is seen outside its office in Beijing, China Aug. 8, 2018.

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Google and Mastercard have had a secret partnership for the past year in which the credit card company has shared U.S. transaction data with the tech giant, according to a report by Bloomberg.

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Last year, Google announced the Store Sales Measurement service, through which Google said it collected approximately 70 per cent of U.S. credit and debit card transactions through third-party partnerships, without saying who those partnerships were with.

We now know that Mastercard was one of the partnerships, with others possibly, as well.

In the partnership, Google tracks whether your offline buying habits are influenced by online ads.

WATCH: Google tracks you without your knowledge

Google will record whether you have clicked an online ad and then purchased the same item within 30 days of clicking on the ad. The company will then tell the advertiser that the item was purchased in a store, although Google and Mastercard say the data is anonymous in order to protect personal identities.

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Both Google and Mastercard did not publicly announce their partnership, which took four years to be negotiated, according to four people with knowledge of the deal. Google reportedly paid Mastercard millions of dollars for the transaction data.

In a statement, Google said that it 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 statement read.

Mastercard released its own statement that also reassured the anonymity of the data. “In processing a transaction, we see the retailer’s name and the total amount of the consumer’s purchase, but not specific items,” the statement read.

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It is possible to opt out of Google collecting data on your Mastercard transactions by toggling off “Web and App Activity” in your Google account.

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In the past, Google has told advertisers when someone who has clicked an ad then visited a physical store using the Location History feature in Google Maps. The advertiser, however, did not know if the shopper made a purchase.

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The ability to connect online and offline behaviour is one that has been sought after by advertisers, who often do not know if online ads correlate to in-store purchases.

Google is also exploring other ways to link the two worlds, such as using Bluetooth devices to track when shoppers enter stores.

— With files from Bloomberg

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