Math problems

I think, the inappropriate dimension in the question indicates that the questioner didn't ever calculate this, because it can be answered without calculating.

Of course, the question only makes sense if you know at least the algorithm. The machine does 10 million decryptions of the file per second. If you don't know anything "about the format, language or content", how would you recognize the correct clear text? It can be encrypted itself. Theoretically, while you get a meaningful plain text in English for one key, another key can lead to the actual encrypted clear text.