Dead drop

A dead drop or dead letter box is a method of espionage tradecraft used to pass items or information between two individuals (e.g., a case officer and an agent, or two agents) using a secret location. By avoiding direct meetings, individuals can maintain operational security. This method stands in contrast to the live drop, so-called because two persons meet to exchange items or information. Spies and their handlers have been known to perform dead drops using various techniques to hide items (such as money, secrets or instructions) and to signal that the drop has been made. Although the signal and location by necessity must be agreed upon in advance, the signal may or may not be located close to the dead drop itself. The operatives may not necessarily know one another or ever meet. The location and nature of the dead drop must enable retrieval of the hidden item without the operatives being spotted by a member of the public, the police, or other security forces—therefore, common everyday items and behavior are used to avoid arousing suspicion. Any hidden location could serve, although often a cut-out device is used, such as a loose brick in a wall, a (cut-out) library book, or a hole in a tree. A dead drop spike is a concealment device similar to a microcache. It has been used since the late 1960s to hide money, maps, documents, microfilm, and other items. The spike is water- and mildew-proof and can be pushed into the ground or placed in a shallow stream to be retrieved at a later time. Signaling devices can include a chalk mark on a wall, a piece of chewing gum on a lamppost, or a newspaper left on a park bench. Alternatively, the signal can be made from inside the agent's own home, by, for example, hanging a distinctively-colored towel from a balcony, or placing a potted plant on a window sill where it is visible to anyone on the street. While the dead drop method is useful in preventing the instantaneous capture of either an operative/handler pair or an entire espionage network, it is not without disadvantages.