Clark Herbert H., Schaefer Edward F. Concealing One’s Meaning from Overhearers

Journal of Memory and Language 26, p. 209-225 (1987). The authors are the linguists of Stanford University.
Two people talking, as at a crowded party, may try to conceal all or part of what they mean from overhearers. To do this, it is proposed, they need to build what they wish to conceal on a private key, a piece of information, such as an event mentioned in an earlier conversation, that is common ground for the two of them and yet not inferable by the overhearers. What they say must be designed so that it cannot be understood without knowledge of that key.