I've watched Mr. Robot lately and can't stop thinking why it was so hard to decrypt files encrypted using AES encryption with a 256-bit key.
Let us say the only method to find the key is through brute force.
Can't we set a computer to brute force starting from the first possible key, and another to begin from the last possible key, and perhaps a few computers to try the keys in the middle?
Wouldn't that reduce time dramatically?
Sure it's possible, but it doesn't really help. The number of possibilities is just too large.

Consider that a 256-bit key has 2256 possible values. That's 12✕1076, or 12 followed by 76 zeroes. If we generously assume that a computer can test a trillion (that's 1012) possible keys a second, and that we have a trillion computers (where will we get them from?) performing the key search, it will take 12✕1076/(1012✕1012) seconds to search the entire keyspace. That's 12✕1052 seconds. As there are only 3,155,760,000 seconds in a century, it will take approximately 4✕1043 centuries to try all possible keys. There's a 50-50 chance that you'll find the key in only half that time.

That's the way encryption is designed. The number of possibilities are just too large to be cracked in time that is interesting for humans.
