Well, we have at least twice as many data points as that. IBM discovered differential cryptanalysis in the early 70s, and the NSA apparently knew about it befor

Well, we have at least twice as many data points as that. IBM discovered differential cryptanalysis in the early 70s, and the NSA apparently knew about it before then, and nobody else found out until the late 80s. I don't know what organization spends the most money on cryptanalysis every year, but the NSA's gotta be near the top. It's reasonable to assume they've found important results that the public won't know of for several years.