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 : 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 first successful test of an atomic bomb . '' See also '' one-time pad , an unrelated cypher algorithm An '' idiot code '' is a code that is created by the parties using it . This type of communication is akin to the hand signals used by armies in the field
Potsdam Conference
Potsdam Conference
Code (cryptography)