Rather than going after the entire keyspace, the codebreaker will try and find flaws in the algorithm that causes it to be reduced to a problem of decreased complexity. Most modern encryption schemes are totally open for peer review, and as such, most new algorithms are open to whoever is interested in seeing them. This allows all of the closet cipher-crackers as well as the seasoned vets holed up at the NSA to try and find a flaw or vulnerability with an algorithm. Once a flaw has been found, the encryption scheme is either modified to compensate for it, or is discarded. However, one should not assume, just because an algorithm has been carefully checked through peer review, that any implementation that you receive will be properly secure.
Decisions made by implementers can have detrimental effects on the success of an algorithm.
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