Cryptanalysis is the study of taking encrypted data, and trying to unencrypt it without use of the key. The other side of cryptography, cryptanalysis is used to break codes by finding weaknesses within it. In addition to being used by hackers with bad intentions, cryptanalysis is also often used by the military. Cryptanalysis 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 they have. A ciphertext-only attack is one where the cryptanalyst 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 cryptanalyst 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. The "man-in-the-middle" attack is one example. In this attack, 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 however, 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; 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 a brute-force attack.

At its core, cryptanalysis is a science of mathematics, probability and fast computers; cryptanalyst's also usually require some persistence, intuition, guesswork and some general knowledge of the target.

Cryptanalysis 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.