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==One-time code==
A one-time code is a prearranged word, phrase or symbol that is intended to be used only once to convey a simple message, often the signal to execute or abort some plan or confirm that it has succeeded or failed.  One-time codes are often designed to be included in what would appear to be an innocent conversation. Done properly they are almost impossible to detect, though a trained analyst monitoring the communications of someone who has already aroused suspicion might be able to recognize a comment like "Aunt Bertha has gone into labor" as having an ominous meaning. Famous example of one time codes include:

*In the Bible, [[Jonathan (1 Samuel)|Jonathan]] prearranges a code with [[King David|David]], who is going into hiding from Jonathan's father, King Saul. If, during archery practice, Jonathan tells the servant retrieving arrows "the arrows are on this side of you," it's safe for David to return to court, if the command is "the arrows are beyond you," David must flee.<ref>I Samuel 20:20-22</ref> 
*"One if by land; two if by sea" in "[[Paul Revere's Ride (poem)|Paul Revere's Ride]]" made famous in the poem by [[Henry Wadsworth Longfellow]]
*"Climb Mount Niitaka" - the signal to Japanese planes to begin the [[attack on Pearl Harbor]]
*During [[World War II]] the [[British Broadcasting Corporation]]'s overseas service frequently included "[[Radio Londres#Coded messages|personal messages]]" as part of its regular broadcast schedule. The seemingly nonsensical stream of messages read out by announcers were actually one time codes intended for [[Special Operations Executive]] (SOE) agents operating behind enemy lines. An example might be "The princess wears red shoes" or "Mimi's cat is asleep under the table". Each code message was read out twice. By such means, the [[French Resistance]] were instructed to start sabotaging rail and other transport links the night before [[D-day]].
*"Over all of Spain, the sky is clear" was a signal (broadcast on radio) to start [[Spanish Civil War#Outbreak of the war|the nationalist military revolt in Spain]] on July 17, 1936.

Sometimes messages are not prearranged and rely on shared knowledge hopefully known only to the recipients. An example is the telegram sent to U.S. President [[Harry Truman]], then at the [[Potsdam Conference]] to meet with Soviet premier [[Joseph Stalin]], informing Truman of the [[Trinity test|first successful test]] of an [[atomic bomb]]. 
:"Operated on this morning. Diagnosis not yet complete but results seem satisfactory and already exceed expectations. Local press release necessary as interest extends great distance. [[Leslie Groves|Dr. Groves]] pleased. He returns tomorrow. I will keep you posted."

''See also'' [[one-time pad]], an unrelated cypher algorithm