> Even assuming just upper/lowercase and 20 characters you are looking at 114 bits of entropy.
Upper/lowercase are already 52 characters. If we add digits (0-9) we are already looking at log_2(62^20) = 119 bits of entropy.
On that note: Let's assume the password were indeed purely random and had 119 genuine bits of entropy. If you're the attacker and your goal is to do 1000 (≃ 2^10) rounds of SHA1 for each of those 2^119 potential passwords, couldn't you then just try to brute-force the 128-bit key directly, since 119 + 10 >= 128?
Put differently, wouldn't the statement "A fully random 20-character alphanumeric password has been brute-forced" amount to saying "AES-128 can be brute-forced"?
Your calculation is correct. My point still stands (2 million years is plenty of time), but my number given was completely off. I can't edit it anymore though.
2^114/350e18/3600/24/365 = 1.9e6
Even 2^114 itself is only 2 * 10^34