Code 46 : Idiot code

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.
Example: Any sentence where 'day' and 'night' are used means 'attack'. The location mentioned in the following sentence specifies the location to be attacked.

• Plaintext: Attack Gotham.
• Codetext: We walked day and night through the streets but couldn't find it! Tomorrow we'll head into Gotham.

An early use of the term appears to be by George Perrault, a character in the science fiction book Friday^[1] by Robert A. Heinlein:

The simplest sort [of code] and thereby impossible to break. The first ad told the person or persons concerned to carry out number seven or expect number seven or it said something about something designated as seven. This one says the same with respect to code item number ten. But the meaning of the numbers cannot be deduced through statistical analysis because the code can be changed long before a useful statistical universe can be reached. It's an idiot code... and an idiot code can never be broken if the user has the good sense not to go too often to the well.

Terrorism expert Magnus Ranstorp said that the men who carried out the September 11, 2001, attacks on the United States used basic e-mail and what he calls "idiot code" to discuss their plans