The recent and unexplained reappearance of the retired SIGINT codeword UMBRA has led to some interesting speculation about what may be going on (see here
UMBRA was for many years the overall codeword for Top Secret information in the communications intelligence Sensitive Compartmented Information (SCI) compartment, used for that purpose throughout the Five Eyes community. (Its presence in the name of this blog is no coincidence.)
Other codewords, such as SPOKE and MORAY, were used for Secret and Confidential COMINT-related information.
UMBRA was retired in 1999, along with SPOKE and MORAY. All three were replaced by the term COMINT, and more recently by Special Intelligence (SI).
There seems to be considerable confusion about how long UMBRA was used, with recent commentary suggesting that the codeword had been in use "at least since [the] middle of the 1950s
UMBRA was by far the longest-lived of the codewords for the most sensitive level of COMINT, but its service did not extend as far back as the 1950s.
Unlike the terms COMINT and Special Intelligence, UMBRA and its predecessors were themselves classified and, in the early post-war period, were frequently changed. Later codewords were retained for several years but were still replaced whenever they were publicly compromised.
UMBRA was the last of the classified codewords. It too was compromised within a few years of its introduction, but unlike the others, it was not replaced but remained in service until its retirement in 1999.
Intelligence historian Matthew Aid has kindly provided the following list of the codewords for the Top Secret COMINT compartment and their dates of service:
ULTRA (194? - Mar 1946)
CREAM (Mar 15 1946 - Sep 1947)
GLINT (Sep 1 1947 - Jun 1949)
COPSE (Jul 1 1949 - Aug 1950)
ACORN (Aug 1 1950 - Jun 1951)
SUEDE (Jul 1 1951 - Jun 1952)
CANOE (Jul 1 1952 - Sep 1953)
FROTH (Oct 1 1953 - Dec 1954)
EIDER (Jan 1 1955 - Jun 1959)
DAUNT (Jul 1 1959 - Dec 1960)
DINAR (Jan 1 1961 - Sep 1965)
TRINE (Sep 1 1965 - Dec 1968)
UMBRA (Dec 1 1968 - Oct 1999)