Cryptography is all about preventing attackers from doing malicious things. Security systems that use cryptography are designed to prevent bad people from having their way with your data, impersonating you, or changing documents without being detected. These attackers may attempt to compromise the security of a cryptographic design in two basic ways: either by deeply understanding the mathematics of the algorithms and protocols and using that knowledge to look for and exploit a flaw in the design, or by using brute force.
If you use well-vetted algorithms and protocols, and use them the way they were designed to be used, your design will probably be immune to the first type of attacks. That is why you need to take care to use accepted algorithms and protocols in your design. In a brute-force attack, the attacker tries every possible key, input, or password, trying to guess the secrets used to protect the design.
Cryptographers don't like to claim that anything is impossible. Rather, they say something is “computationally infeasible,” meaning it would take an impractical amount of time to attack the cryptography by trying every combination. For the Data Encryption Standard (DES), 2^56 possible keys could be used. This number is large: 72,057,594,037,927,936.
Although breaking the DES algorithm might seem to be computationally infeasible, in 1998, a machine was developed that did exactly that. 
Passwords are also often attacked using brute force—first dictionary words are tried, then combinations of words and numbers, and even special characters. Attackers may even try Klingon words or words from the Harry Potter books or The Lord of the Rings!
RainbowCrack (project-rainbowcrack.com/) is a well-known program used to crack even fairly long passwords using brute force.
A cryptographic algorithm is well designed if the strength of the algorithm is not dependent on keeping the algorithm secret. The algorithm is infeasible to break if this is true and if it is infeasible to guess the algorithm secret, commonly called a key. Defending against brute force attacks is about picking key sizes so large that it is infeasible to try them all, or reducing the number of attacks that can be performed in a period of time.
TCG assumes that all the algorithms it approves are well designed. This may not be true. Because the specification is not wedded to a particular algorithm, if one is found to be weak at some point in the future, then instead of the specification having to be rewritten, just that algorithm can be removed from the list of approved algorithms.
The strength of an algorithm that is well designed is measured by its immunity to mathematical attacks. The strength of an implementation depends on both the type of algorithm and the size of the key used.
-  en.wikipedia.org/wiki/Data_Encryption_Standard#Chronology.