This is one of the oldest conspiracy theories on the Internet. It has been comprehensively and repeatedly debunked. It doesn't even make sense as a backdoor, given its function. Here's the best link on HN about it:

https://news.ycombinator.com/item?id=9297787

You also didn't need the source code to find it; you could have found it with "strings".

That was just a naming convention. When asked why it was called NSAKEY, Microsoft said this:

>This is simply an unfortunate name. The NSA performs the technical review for all US cryptographic export requests. The keys in question are the ones that allow us to ensure compliance with the NSA's technical review. Therefore, they came to be known within Microsoft as "the NSA keys", and this was used as a variable name for one of the keys. However, Microsoft holds these keys and does not share them with anyone, including the NSA.

> https://en.wikipedia.org/wiki/NSAKEY

Wow.. How come nobody is bringing THIS up whenever the FBI tries to assure us that the bypass they want from Apple will never get leaked into the wrong hands?