Brute Force Cryptographic Attack

Breaking encryption is not that easy these days.

Here are some resources that may help you getting familiar with the subject.

A brute force attack against a cipher consists of breaking a cipher by trying all possible keys. Statistically, if the keys were originally chosen randomly, the plaintext will become available after about half of the possible keys are tried. The underlying assumption is, of course, that the cipher is known. Since A. Kerckoffs first published it, a fundamental maxim of cryptography has been that security must reside only in the key. As Claude E. Shannon said a few decades later, 'the enemy knows the system'. In practice, it has been excellent advice.
Full article


More resources:
A Mathematical Theory of Communication (PDF paper)
Brute-Force Attacks Explained: How All Encryption is Vulnerable 
How secure is AES against brute force attacks?
Information theory (Wikipedia)