The National Security Agency (NSA) is a United States Government Agency responsible for the collection and analysis of forms of communication through radio broadcasting, whether commercial or personal, Internet, and otherwise intercepted forms of communication. For many years it was an organization whose existence was not admitted by the US government.
Intended for the interception (and decryption if possible) of foreign communications, the NSA has been heavily involved in research into cryptography, crediting the breaking of WWII Japanese codes to a predecessor organization's team of cryptographers. The NSA is also responsible for assisting in securing the U.S.'s governmental, military, and even private (to some extent) communications infrastructure from the efforts of agencies like itself working for foriegn powers. For instance, the agency recommended changes to the IBM submission to NIST during the process which produced the DES encryption algorithm in the first half of the 1970s. Subsequently, it was widely believed that those changes were made so as to make it easier for the NSA to break the cypher when desired. However, the public rediscovery of differential cryptanalysis showed that the one of the changes were actually likely to have been suggested to harden the algorithm against this cryptanalytic technique not then publicly known. It remained publicly unknown until Biham and Shamir idependently discovered it and published some decades later. However, the shortening of the 112-bit key used by the IBM submission to an effective 56 bits has never been explained as anything other than a weakening of the algorithm.
The NSA, in combination with corresponding agencies in the United Kingdom, Canada, Australia, and New Zealand, is believed to be responsible for, among other things, the operation of the Echelon system, whose capabilities are suspected to include the ability to monitor a large proportion of the civilian phone and data traffic tranmitted around the world.
NSA official site http://www.nsa.gov/