Tuesday, September 10, 2013

Tradecraft: Dead Letter Boxes


Being an avid reader of Andy McNab's spy novels and having a long-time interest into security and cryptography, I decided to add a new category of articles to this blog: tradecraft. In other words, security-related or spy-related topics.

Dead Letter Boxes (DLBs)

DLBs are a means of transmitting items or messages from 1 person to another without being present in the same place at the same time. Typically during the Cold War, this was a favorite method of communication between spies and their handlers. The big benefit of avoiding to meet is obvious: it makes it more difficult for third-parties (counter-intelligence services) to intercept the communication and uncover spies.

The concept of DLB relies on 2 things, which will be agreed-upon in advance:

  • the box itself which will contain an item or message
  • an agreed-upon signal which means "I've put a message in the DLB. Go and check it!".

The box is the most complicated part of the system. It needs to be located in a place where both the spy and his handler may have a legitimate (ie: not spy-related)  reason to be. For instance: the roof of a building is NOT a good location for a DLB because no ordinary person has a legitimate reason to go there. Some better choices of DLB would be places where the public comes and goes all the time. Cafes, phone booths (though we find less of these nowadays), train stations on the other hand are good choices for setting up the box.

The box doesn't need to be a real box. It could simply be a place where you leave an envelope, a piece of paper, or a sticker with a message. It must be established so that nobody will disturb it. And it must also be set up so that both spy and handler can access it without raising the suspicion of the people around. For example, sticking your piece of paper under the table in a cafe is NOT a good idea because it has a high chance of being detected by the cafe's employees who do the cleaning or by customers. Leaving your piece of paper under the foot of the cafe's heavy sofa is also not a good idea since you'll make a lot of noise attracting the attention of everybody around. If the cafe has heavy sofas with little space between them and the ground, then it can be a good choice for sticking your piece of paper there while pretending to lace your shoes or adjust the hem of your pants.

The signal itself is much easier to set up. It only needs to be a visible sign that the other person will see easily while walking around. Several movies have used the classic chalk mark on a mailbox, or if you're a fan of X-Files, you might remember the time when Scully contacts Mulder's informant by putting tape in the shape of a cross on the window. Sadly, it is a VERY BAD way of doing it since it is directly related to Mulder's apartment. A better way to do it, would be a simple line drawn with a marker in a phone booth and visible by people walking by. It needs to be drawn while pretending to give a phone call (or better, actually making a phone call) when nobody's looking. The chalk mark on a mailbox was also an actual method used by Aldrich Ames using the following mailbox.

Photo license: CC-BY dbking

The disadvantage of DLBs is speed. This aspect is constrained by the ability of people to check frequently if the signal is present. If the signal can be set up on a person's route from home to work, then it can be checked twice per day. If it is set up on a person's route to their weekly sport activity, then it will be checked once per week.


I hope that was interesting though there is little reason for most of us to use it in our lives. But at least it can make things more understandable next time you watch a spy movie.

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