Just a handful of anonymous card transactions can reveal your identity

black and white mastercard held in hand

Spending habits are frighteningly telling. Just four pieces of information gleaned from a shopper’s credit card transactions can be enough to pick them out from a database of anonymous credit card records.

An MIT report, published on Thursday in Science, analysed completely anonymous credit card transactions by 1.1 million people over three months in an unnamed country at an unnamed bank. The researches needed only four bits of secondary information about an individual’s transactions – such as location or timing of the purchase – to identify a unique purchasing pattern for 90 per cent of the people involved.

No names, account numbers or other obvious identifiers were involved, but each transaction was time-stamped with the day of purchase and linked to a store. Once a purchasing pattern was identified, analysts were able to find the name of the credit card holder by matching their movements to other publicly available information on LinkedIn and Facebook profiles, in Twitter messages containing time and locations information, and ‘check-ins’ on social media.

Researchers could also tell men and women apart from how long they lingered in a store, and separate those in higher income brackets, WSJ reported. Ultimately, we can be identified from where or when we bought a jumper or a new pair of shoes.

We expose much more through our digital transactions than we think, and that footprint is only going to grow. Smartphone app usage and mobile calling data also tells a lot about our habits, as will mobile payments when they hit the mainstream.

Research firms, advertisers, retailers and trade associations will be interested in more uses for metadata, and the report’s findings will likely add to debates over personal data collected by government surveillance programmes.

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