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The author didn't provide a proof. That's why I'm wondering if anyone else has seen this approach. I don't have a good online reference for this, but the methods are readily understandable by anyone with a junior high education level. A similar approach is described at gbbservices.com/math/squarediff.html#Cracking_RSA
The complexity is probably less than that since negative/linguistically impossible combinations also provide evidence. These can be tracked by listing bad choices in 625 columns, and placing an X in the cell if they are not to be considered.
