To me, preventing this seems like exactly the sort of thing the NSA ought to be for.
No, they've always had both roles. For instance, their experience and knowledge of advanced code-breaking techniques allowed them to strengthen the DES cipher's S-boxes in ways that weren't publicly understood until decades later.
https://en.wikipedia.org/wiki/Data_Encryption_Standard#NSA%2...
This is naught but my opinion, and I understand where others might differ in their reading of the situation presented to us.