NSA (US Agencies) 

The National Security Agency (NSA) is a cryptologic intelligence agency of the United States Department of Defense responsible for the collection and analysis of foreign communications and foreign signals intelligence, as well as protecting U.S. government communications and information systems, which involves information security and cryptanalysis/cryptography.

The NSA is directed by at least a lieutenant general or vice admiral. NSA is a key component of the U.S. Intelligence Community, which is headed by the Director of National Intelligence. The Central Security Service is a co-located agency created to coordinate intelligence activities and co-operation between NSA and other U.S. military cryptanalysis agencies. The Director of the National Security Agency serves as the Commander of the United States Cyber Command and Chief of the Central Security Service.

By law, NSA's intelligence gathering is limited to foreign communications, although domestic incidents such as the NSA warrantless surveillance controversy have occurred.

Source: Wikipedia