Your BT Hub has an inbuilt security system that scrambles the signal it sends. It does this with an encryption code, so the signal doesn't make any sense to other computers. We then give you a unique code (called a wireless key) to enter into your computer so you're the only one who can pick up your Hub's signal.
However, using public wi-fi networks is different. By their very nature, they don't have wireless encryption so people can use them as a public service. This means there's a risk information could be intercepted if a website doesn't secure its data as it passes between your laptop or smartphone and the hotspot.
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