Lol I mess around with Keys.lol for like 1 hour one time. I stumbled upon it by accident. I would have to say, In the little time that I have tinkered around with it. I would have to say it IS PRETTY HARD LMAO,
From the math I have read here it's probably not even worth the time. lol
The program that I posted here
https://bitcointalk.org/index.php?topic=4345245.msg51294948#msg51294948 will, when paired with brainflayer, search random private keys at a rate of about 120,000 Bitcoin keys per second on a single i7 core. That's just over 10 billion per day.
The chances of ever finding a match for a random 256 bit key, even when checking 10 billion keys a day, even if you're using multiple cores to double/triple/quadruple etc that rate, even if you're using a GPU assisted program that focuses on a small set of addresses and runs at 500 times that rate, is still virtually zero.
It's absolutely not worth the time.