What do you mean by "greater or lesser than the actual key"? If you mean number of bits you can do a simply file size comparison, if you mean binary numerical value a simple c program _should_ be able to handle that without any trouble I think.... although maybe you would need to include some of those 'big number' routines I keep hearing about... and you would have to strip off any header info before computing.
Happy Hunting, -Chris.
What I am looking at is a way to do binary searches in the key space w/ a function that would look at a test key and the result of running RSA on it and then tell me the relative magnitude between the real key and the test key. What this means is that I could take a cypher-text and attempt a de-crypt w/ some conveniently large number and then go up or down from there till I find it. The advantage of this approach is that it allows one to search the key-space w/o having to test each and every possibility. This would significantly(!) reduce the time to crack...