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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.
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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 officers, 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.
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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 containers are also known as a
concealment device,
a microcache or a
stash.
A good example of a dead drop container is the
black spike shown above:
a water-tight container with a sharp tip that can be pushed into the ground,
waiting to be picked up by the other party.
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Dead drop containers on this website
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Electronic dead letter boxes
short-range agent communications
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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 contains the requires item. This is often done in a public place,
so that the handler can regularly check it without attracting attention.
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.
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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. One 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 sits down 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 both persons – than dead drops.
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Electronic dead drops
SRAC
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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).
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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 for delivering
messages 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.
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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).
➤ More about Short Range Agent Communication (SRAC)
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© Crypto Museum. Created: Friday 24 October 2014. Last changed: Thursday, 25 November 2021 - 18:01 CET.
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