Virtual Dragon

A virtual simulation of the American cribbing machine used at Bletchley Park during World War 2

Find Out More Run Virtual Dragon

What is the Dragon?

During World War 2, Bletchley Park created a number of machines to assist in breaking the German Lorenz SZ cipher machine. The American Signal Security Agency (SSA) in Arlington Hall, Virginia also designed and built two specialized Rapid Analytic Machines or RAM for this purpose, one of which was a crib dragging machine called the Dragon. It was delivered to Bletchley Park in October 1944.

A crib is a guess at a section of plaintext which may appear within the secret enciphered message and the Dragon was able to search through an enciphered message trying to find a point at which the plaintext matched the crib.

This website includes an online simulation of the Dragon to show how it worked and allow you to run it.

What do I need to know to run it!
Instructions for use


Use your browser zoom to see the big picture or to look a little closer.

PC: Use Ctrl with +/- or with 0 for 100% or use Ctrl and mouse wheel
Mac: Use Command and +/- or with 0 for 100%
Tablet/Phone: Just pinch zoom as normal

Click or swipe dials and switches

Click on any of the knobs to set the PSI wheels. Click on the crib board (center) to fit or remove the pins and start and stop the search with the switches on the far right panel.

Look for Info Boxes

Look for the helpful blue info boxes. These give extra information about panels you can interact with and some options and selections.


Download a basic run-through of the available controls and what they do.

These will give you an idea on how the Dragon was used.

Get Tutorial

What does it do?

Credits and links

A number of people I need to say thanks and tip my hat to... .. The National Museum of Computing where my obsession was sparked!

National Cryptologic Museum .. The National Cryptologic Museum was an invaluable source of info & photos. .. The National Archives and Records Administration (NARA) .. The UK's National Archives.

Jim Reeds - American Dragon .. for his fantastic article on "American Dragon" in Cryptologia Vol 35, 2010

The Bill Tutte Memorial Fund .. "Bill Tutte achieved one of the greatest intellectual feats of World War Two by breaking the extremely complex Lorenz code without ever seeing the machine that generated it."

Captain Jerry Roberts and all the codebreakers in the Testery. Without these people's code breaking skills and their daily breaking of the Lorenz wheels, Dragon and Colossus would have been unable to function.

Let me know what you think!

Have I got something wrong, it doesn't work quite correctly on your computer or maybe you just want to say hi - find me on Twitter or check my Facebook page

Martin Gillow