Cryptanalysis is the study of taking encrypted data, and trying to unencrypt it without use of the key. The other side of cryptography, it is used to break codes by finding weaknesses within them. In addition to being used by hackers with bad intentions, this discipline is also often used by the military. It is also appropriately used by designers of encryption systems to find, and subsequently correct, any weaknesses that may exist in the system under design.
There are several types of attacks that a cryptanalyst may use to break a code, depending on how much information he or she has. A ciphertext-only attack is one where the analyst has a piece of ciphertext (text that has already been encrypted), with no plaintext (unencrypted text). This is probably the most difficult type of cryptanalysis, and calls for a bit of guesswork. In a known-plaintext attack, the analyst has both a piece of ciphertext and the corresponding piece of plaintext.
Other types of attacks may involve trying to derive a key through trickery or theft, such as in the "man-in-the-middle" attack. In this method, the cryptanalyst places a piece of surveillance software in between two parties that communicate. When the parties' keys are exchanged for secure communication, they exchange their keys with the attacker instead of each other.
The ultimate goal of the cryptanalyst is to derive the key so that all ciphertext can be easily deciphered. A brute-force attack is one way of doing so. In this type of attack, the cryptanalyst tries every possible combination until the correct key is identified. Although using longer keys make the derivation less statistically likely to be successful, faster computers continue to make brute-force attacks feasible. Networking a set of computers together in a grid combines their strength, and their cumulative power can be used to break long keys. The longest keys used, 128-bit keys, remain the strongest, and less likely to be subject to this type of attack.
At its core, cryptanalysis is a science of mathematics, probability, and fast computers. Cryptanalysts also usually require some persistence, intuition, guesswork and some general knowledge of the target. The field also has an interesting historical element; the famous Enigma machine, used by the Germans to send secret messages, was ultimately cracked by members of the Polish resistance and transferred to the British.