Nice, thanks. So for 512 bits, that's 3.78e151, and for 600 bits, that's 9.98e177. Thus the key space is 2.6e26 times bigger for a 600 bit key, which doesn't explain why it's only 25 times harder to crack, per what kmf said earlier. In fact, 2.6e26 is 2^88, which is the same difference as if these were symmetric keys. There must be something more to it, because if 1% of the numbers are primes, then it would only be 100 times harder to brute force a 512 bit symmetric key compared to a 512 bit asymmetric key, and that's clearly not the case. The real difference is more like TENS of orders of magnitude.