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If you read the article you linked to you will understand that the NSA approached Linus Torvalds (the inventor of Linux and still one of the developers) to make a backdoor - but he didn't. That's simply because Linus doesn't own Linux - he can't decide on his own what to change. And Linux is open-source and devoloped by a huge load of developers - nobody, not even Linus, can implement a backdoor without anyone noticing.
That's why Linux is safe and will stay so.
They pressured/intimidated him into denying NSA attempts.
He nodded yes.
If they still have the ability to control what he says, how can you be so sure?
Have you personally reviewed ALL Linux kernel source code?
If they can pressure Linus, they can pressure anybody who discovers and attempts to divulge the backdoor in the source.
No, I haven't. But the great thing about Linux and open-source in general is that it's huge - Linux is developed by hundreds of developers all around the world. And not even the NSA has the power to intimidate a developer in Russia (most notably Russia), Switzerland, Norway or elsewhere. It's the same principle that makes Bitcoin so safe - decentralization.