Concealment devices and dead letter boxes
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.
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
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 brick, a ballpoint,
a food can,
a picture frame,
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
Electronic dead letter boxes
short-range agent communications
Electronic dead drops
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)  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
secret services in many espionage operations and used an
UHER tape recorder as the storage device.
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
that was reportedly captured in Cuba 
and in the former USSR . These devices are also thought
to have been used in Central and Eastern Europe .
It operated in the VHF-H band and sent bursts of data, encrypted with a
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 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.
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
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.
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© Crypto Museum. Created: Friday 24 October 2014. Last changed: Saturday, 10 June 2017 - 07:33 CET.