The ability to understand the secret communications of our foreign adversaries while protecting our own communications -- a capability in which the United States leads the world -- gives our nation a unique advantage.
Executive Order 12333 of 4 December 1981 describes in more detail the responsibilities of the National Security Agency. The resources of NSA/CSS are organized for the accomplishment of two national missions:
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The Information Assurance mission provides the solutions, products and services, and conducts defensive information operations, to achieve information assurance for information infrastructures critical to U.S. national security interests.
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The foreign signals intelligence or SIGINT mission allows for an effective, unified organization and control of all the foreign signals collection and processing activities of the United States. NSA is authorized to produce SIGINT in accordance with objectives, requirements and priorities established by the Director of Central Intelligence with the advice of the National Foreign Intelligence Board.
Emphasis theirs. From the NSA website at www.nsa.gov.