Yahoo will encrypt all users personal data by March next year, the web firm's CEO Marissa Mayer has said.

Yahoo, along with other web firms including Apple, Microsoft and Google, have been subject to thousands of data requests under the US government's Prism surveillance programme.

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However, documents revealed by Prism whistleblower Edward Snowden last month showed that the National Security Agency had also secretly accessed Yahoo's datacentres to gather information on users without alerting the company. It's for that reason Yahoo has decided to encrypt information about all of its users.

"As you know, there have been a number of reports over the last six months about the US government secretly accessing user data without the knowledge of tech companies, including Yahoo," Mayer wrote in a Tumblr post titled Our Commitment to Protecting Your Information

"I want to reiterate what we have said in the past: Yahoo has never given access to our data centers to the NSA or to any other government agency. Ever," she said.

Indeed, Yahoo won a court battle earlier this year which allowed the firm to demonstrate how it fought against demands to hand over data about its users to the US government, and now Mayer wants to make sure it's more difficult for agencies to snoop on Yahoo users.

"There is nothing more important to us than protecting our users' privacy," she continued.

"To that end, we recently announced that we will make Yahoo Mail even more secure by introducing https (SSL - Secure Sockets Layer) encryption with a 2048-bit key across our network by January 8, 2014."

Yahoo also aims to encrypt all information that moves between its datacenters by the end of March 2014, as well as offering users the opportunity to encrypt all data flow to and from Yahoo by the end of the first quarter.

"As we have said before, we will continue to evaluate how we can protect our users' privacy and their data. We appreciate, and certainly do not take for granted, the trust our users place in us," Mayer added.