Twitter has used an "impossible" mathematical problem first discovered by GCHQ to protect its users from electronic snooping.
The group said "perfect forward secrecy" was live on its services, drastically increasing the effort needed to intercept its traffic.
It is thought the move is to make it more difficult for data to be collected on its users without going through legal channels.
Jim Killock, director of the Open Rights Group, said it was a "policy move" driven by revelations about mass surveillance by British eavesdropping agency GCHQ and the American National Security Agency.
He said: "Companies have now realised precisely how vulnerable their information is on the internet. It's no longer a theoretical risk. We know it's been going on now.
"This is about asking users to trust the companies involved and to also force the legal authorities to approach companies directly rather than attempting to seize the data in transit."
In June it was revealed that GCHQ was using a project called Tempora to indiscriminately scoop data from fibre optic cables entering and leaving the UK.
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