For some problems in Computer Science, it can be proven that there are no "efficient" ways of solving them. In this case, the only resort is brute force. Brute force means that you let the computer try out every possible --and we really mean every single, possible and potential-- value that solves the problem, until you succeed.

Several cryptographic algorithms are based on such problems where the secret key can only be guessed or found by exhaustively trying out all possible values, which could take a few billion years. Confronted with such time scales, attackers find themselves in a very weak position.