4096 bit keys only make sense if they're asymmetric and those won't capture anything close to 4096 bits of entropy. As for the strength of 256 bits, the Wikipe

4096 bit keys only make sense if they're asymmetric and those won't capture anything close to 4096 bits of entropy. As for the strength of 256 bits, the Wikipedia page on brute force attacks should tell you how infeasible attacking even a 128 bit key is: http://en.wikipedia.org/wiki/Brute-force_attack http://en.wikipedia.org/wiki/Brute-force_attack