Signals intelligence

Signals intelligence (SIGINT) is intelligence-gathering by interception of signals, whether communications between people (communications intelligence—abbreviated to COMINT) or from electronic signals not directly used in communication (electronic intelligence—abbreviated to ELINT). Signals intelligence is a subset of intelligence collection management. As sensitive information is often encrypted, signals intelligence in turn involves the use of cryptanalysis to decipher the messages. Traffic analysis—the study of who is signaling whom and in what quantity—is also used to integrate information again.[citation needed]

RAF Menwith Hill, a large site in the United Kingdom, part of ECHELON and the UKUSA Agreement, 2005
A German message intercepted by the British during World War II, signaling Germany's unconditional surrender