SAN FRANCISCO: Google, the internet giant, is to use billions of credit card transactions in an effort to prove that ads shown to users are driving purchase, even in offline stores, in a move that has sparked criticism for its impact on user privacy.

Google Attribution, announced a blog post, will make it possible "for every marketer to measure the impact of their marketing across devices and cross-channel - all in one place".

Though the company already analyzes browsing, search, and location history from across its apps and products, which it uses to attribute the efficacy of ads, the integration of transaction data will connect Google's understanding of the path to purchase like never before.

However, privacy advocates say that there is a lack of understanding among consumers about how data this sensitive is being used; people could be uncomfortable with this intrusion, despite Google's assurances about the safety of user data, the Washington Post reported.

"While we developed the concept for this product years ago, it required years of effort to develop a solution that could meet our stringent user privacy requirements," Google said.

"To accomplish this, we developed a new, custom encryption technology that ensures users' data remains private, secure, and anonymous."

With the move from location tracking to transaction tracking, Google will be able to boast that its platform will 'close the loop' on attribution, with access to roughly 70% of US credit and debit card sales thanks to new partnerships, the Seattle Times revealed.

So far, Google has been relatively unscathed by attacks from hackers, though critics suggest that Google's new capabilities could make it a more enticing target.

"The privacy implications of this are pretty massive," said Miro Copic, a professor of marketing at San Diego State University. "Google needs to tread very carefully."

Google is recovering from an advertising boycott of its video service, YouTube, following the appearance of reputable brands' ads next to inappropriate content, since admitting that it still "can't guarantee" ad safety.

Data sourced from Google, Washington Post, Seattle Times; additional content by WARC staff