Although all keys generate some string of letters and numbers, the right key will produce a message with reasonable letter frequencies and of course with legible words.
For English, about 13% of letters are "e", so if the frequency is too far off that it is probably not the right key; likewise, if 13% of the letters are "z" it is also not likely to be the right key. By applying frequency checks to the most and least common letters it should be a fairly strong filter for plausible keys.
For English, about 13% of letters are "e", so if the frequency is too far off that it is probably not the right key; likewise, if 13% of the letters are "z" it is also not likely to be the right key. By applying frequency checks to the most and least common letters it should be a fairly strong filter for plausible keys.