Tim Cook published an op-ed in Time Magazine on Thursday arguing for the US to rein in data brokers in order to give people true privacy when it comes to their data.

Data brokers are firms such as Experian or Oracle which collect huge amounts of information on consumers, then sell that data or insights based on the data to their customers.

Most people aren't aware that data brokers exist, and how they hoover up online information.

Cook called for new federal privacy regulation to shine a light on data brokers' dealings.

Apple CEO Tim Cook came after the "shadow economy" of firms profiting from people's personal data in a column for Time Magazine on Wednesday.

Cook set his sights on data brokers, saying that they need to be reined in. Data brokers are firms which act as middlemen, transferring data between different companies and parties, or as Cook put it, "a company that exists purely to collect your information, package it and sell it to yet another buyer." The biggest data brokers are firms such as Axciom, Experian, and Oracle.

"Right now, all of these secondary markets for your information exist in a shadow economy that's largely unchecked— out of sight of consumers, regulators and lawmakers," the Apple CEO wrote.

Cook called for the US government to create federal privacy legislation to empower consumers and make data brokers register with a specialised "clearinghouse" to increase transparency.

Read more: Apple CEO Tim Cook on privacy: "we have to admit when the free market is not working"

Cook has called for federal regulation of consumer data before, and repeated some of the points he laid out in his attack on data-hoarding tech firms at a privacy conference in Brussels in October.

Cook has kept the pressure up on privacy, consistently criticising other tech companies for hoovering up people's data to feed their business models. Apple recently mounted a large poster in Las Vegas at CES — the world's largest tech show — saying "What happens on your iPhone stays on your iPhone" in what was seen as a direct challenge to Google.

However, others have criticised Cook's privacy crusade as hypocritical. Ex-Facebook security chief Alex Stamos in October pointed to Apple's business in China, which he described as an "ethical blindspot" for the company.

Likewise, while Apple doesn't have an online ad business akin to Google or Facebook's, it is perfectly happy taking billions of dollars so that Google is the default search engine on the Safari browser.