I am looking for a way to put standard $9\times9$ Sudoku problems into a standard form by renaming the numbers ($9!$ possibilities), permuting the rows $6^4$ possibilities, and permuting the columns $6^4$ possibilities. This will be a tool to find out how difficult problems are different from easy problems. What features do difficult puzzles have different from easy ones?
By difficult, I mean difficult for MY tired ancient brain, not for any kind of algorithm. We are talking psychology, not computational complexity theory.