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Unpatchable vulnerability in Apple chip leaks secret encryption keys
(arstechnica.com)
This is a most excellent place for technology news and articles.
The more probable answer is that the NSA asked for the backdoor to be left in. They do all the time, it's public knowledge at this point. AMD and Intel chips have the requisite backdoors by design, and so does Apple. The Chinese and Russian designed chips using the same architecture models, do not. Hmmmm... They have other backdoors of course.
It's all about security theatre for the public but decrypted data for large organizational consumption.
I don't believe that explanation is more probable. If the NSA had the power to compell Apple to place a backdoor in their chip, it would probably be a proper backdoor. It wouldn't be a side channel in the cache that is exploitable only in specific conditions.
The exploit page mentions that the Intel DMP is robust because it is more selective. So this is likely just a simple design error of making the system a little too trigger-happy.
They do have the power and they do compel US companies to do exactly this. When discovered publicly they usually limit it to the first level of the "vulnerability" until more is discovered later.
It is not conjecture, there is leaked documents that prove it. And anyone who works in semiconductor design (cough cough) is very much aware.