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Dead Drop
Dead letter boxes and concealment devices

In espionage tradecraft, a dead drop or dead letter box (DLB) is a method of passing messages between two persons without the requirement for them to meet directly (live drop). It allows a case handler and an agent to exchange messages and objects without compromising security.
A dead drop is made in a secret location that has been agreed in advance by both parties, allowing them to drop and collect the secret item, without attracting the attention of the enemy's security forces, the police or a member of the public.

The places that are used as a drop location are often quite common, e.g. under a stone or a bench in a park, behind a loose brick in a wall, or inside a hollow tree. In addition, the dead drop can be an electronic device that collects information from a passing agent and delivers it to the passing handler at a different moment.
Dead drop spike

In most cases, dead drops are simple containers that are used to pass small items like money, secret objects, instructions, microfilm, audio tapes, documents, cryptographic key tables and frequency lists. Such a container is also known as a concealment device, a microcache or a stash. A good example of a concealment device is the dead drop spike: a water-tight container with a sharp tip that is pushed into the ground, waiting to be picked up by the other party.

Virtually any common object can be converted into a concealment device. For example: a metal bolt, a coin, a stone, a brick, a ballpoint, a food can, a picture frame, a razor, a cut-out book or even a piece of food like a walnut. Below are some popular examples. Click them for further info.
Dead drops and concealment devices
Dead drop spike Dead drop cylinder Walnut concealment device with two OTP booklets Commonly looking bolt which is actually a concealment device 32GB Micro-SD memory card inside a 50 Euro-cent coin At first glance: a normal stone Travel kit with concealment area for passport and OTP

Electronic dead letter boxes   short-range agent communications
Czechoslovakian UHER-based electronic dead letter box Wanted item: American short-range agent transmitter

Electronic dead drops   SRAC
In addition to physical dead drops, it is also possible to use an electronic variant, which is commonly known as an electronic dead drop (EDD) or an electronic dead letter box (EDLB). This method of delivering a message is also known as short-range agent communications (SRAC) [3] and is in fact one of the first practicle implementations of Near Field Communication (NFC).
The first use of SRAC dates back to the Cold War in the early 1960s, and was exploited by both sides. It usually involved small electronic devices that were capable of making burst transmissions in the VHF radio band, sometimes even with data encryption. The devices were used to deliver a message at an electronic dead letter box (EDLB).

An early example of an SCRAC device is the VHF FM transmitter that is shown in the image on the right. It was used by the Czechoslovakian secret services in many espionage operations and used an UHER tape recorder as the storage device.
UHER 206 EDLB transmitter - interior

In this case, the tape recorder was used to play back a verbal message at the highest possible speed, but there are also examples of (Western) devices that were as small as a pack of cigarettes and that were able to record and playback encrypted digital messages at very high speed (burst).

One example is a US-made set, known as CDS-501, that was reportedly captured in Cuba [3] and in the former USSR [4]. These devices are also thought to have been used in Central and Eastern Europe [4]. It operated in the VHF-H band and sent bursts of data, encrypted with a CK-42 device, from an agent to a receiving station located in a Western diplomatic facility (embassy) in the host country. Other known sets are the RT-517, the RT-519, the CDR-701 (RX only), and the RS-804, with the latter being the satellite variant of the CDS-501.
In addition to hiding the secret container at the dead drop location, the agent also has to send a signal to his handler, indicating that the dead drop contained the requires item. This is often done in a public place, so that the handler can regularly check it without attracting attention.

Such a signal can be a mark with a piece of chalk somewhere on a wall or, say, a public mail box, but it can also be a very common mark at the agent's own house. He could, for example, turn the blinds of the main window in a certain position, or leave the lights on in the bathroom. In the same vein, the handler would leave another signal to inform the agent that the item has been picked up.
Live drop
In contrast to a dead drop, a live drop does require physical contact between the agent and his handler. Popular examples of this are the scenes in spy movies where two people, each carrying an identical briefcase, approach each other at the train station, put down their briefcases and shake hands. Once they have finished talking, they pickup the other person's briefcase and walk away. Once briefcase may contain money, whilst the other one could contain secret documents.

Another example of a live drop is where one person sits on a bench in a park eating his lunch. A stranger walks up to the bench and take a seat next to him. Without looking at each other, they exchange a secret pass-phrase, after which the stranger walks away again, leaving his briefcase or an unobtrusive envelope behind. The first person picks it up and leaves in a different direction. Needless to say that live drops are far more dangerous for the agent than dead drops.
A stash is a secret hiding place in which personal objects, such as money, ID papers, frequency lists, drop pick locations and code material, are hidden. When an object is used as a stash, it is also known as a concealment device, much like a dead drop. In fact, many concealment devices are suitable as a dead drop as well as a stash, which is why we have listed them together here.
  • Dead Drop
  • Dead Letter Box
  • Concealment container
  • Concealment device
  • Electronic Dead Drop
  • Electronic Dead Letter Box
  • Short-range agent communications
  • Microcache
  • Toter Briefkasten
  • Dode brievenbus
  1. Wikipedia, Dead drop
    Retrieved October 2014.

  2. Wikipedia, Concealment device
    Retrieved August 2015.

  3. Wikipedia, Short-range agent communications
    Retrieved September 2015.

  4. Jan Bury, Project Kalina: The Lotos Operation Conundrum
    Cryptologia 36:2, April 2012, pp. 119-128.

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Crypto Museum. Created: Friday 24 October 2014. Last changed: Wednesday, 07 September 2016 - 06:28 CET.
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