Announcing my Purple simulator
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:date: 2014-02-20 19:59
:tags: Python, m209, Enigma, Purple, simulator
:slug: announcing-my-purple-simulator
:author: Brian Neal
:Summary: I recently created a simulation of the Japanese PURPLE cipher machine
          in Python.

After working on a `M-209 simulator`_ last summer and an `Enigma simulator`_ the
year before, I got the World War 2 era cipher machine bug again. It all started
by reading about the `Japanese PURPLE machine`_ at Frode Weierud's excellent
CryptoCellar_ website. I read with great interest a paper available at that
site titled *PURPLE Revealed: Simulation and Computer-aided Cryptanalysis of
Angooki Taipu B* by Wes Freeman, Geof Sullivan, and Frode Weierud. This paper
is a fascinating read, with detailed descriptions of how the machine worked,
the complicated keying system employed by the Japanese, and information on
using a technique known as *hill-climbing* to mount a ciphertext only attack on
a messages. Highly recommended!

After reading the paper I knew I had to try to write a simulator for the PUPRLE
machine in Python_. Writing these simulators is great fun. You have to
understand how the machine works, get a bunch of tiny details correct, have
a bit of patience, and do a lot of detective work and debugging. Once you have
them built, they aren't all that complicated when coded on a modern computer.
But you get an amazing appreciation for these machines as originally
implemented as electro-mechanical devices. The PURPLE machine, for example,
required almost 2,000 wired connections! It is even more amazing when you know
the U.S. code breakers built replicas without even seeing an original Japanese
machine. When the war ended, only a few parts of one Japanese machine were ever
recovered!

I had a lot of fun writing this and playing with it. I hope someone else finds
it interesting or educational. Please contact me and let me know if you use it
for anything. Please see these links for documentation, the code, and licensing
details:

* `Purple simulator on PyPI`_
* `Purple simulator on Bitbucket`_

Once again, many thanks to Frode Weierud, Geoff Sullivan, and Wes Sullivan for
the very interesting paper. I'm also grateful to Messrs. Weierud and Sullivan
who provided me with valuable plaintext, advice and encouragement while
I worked on this project.

I also recommend the book *Battle of Wits: The Complete Story of Codebreaking
in World War II* by Stephen Budiansky, which details the efforts and the people
behind the cracking of the PURPLE cipher.

.. _M-209 simulator: https://m209.readthedocs.org/en/latest/
.. _Enigma simulator: http://py-enigma.readthedocs.org/en/latest/
.. _Japanese PURPLE machine: http://cryptocellar.web.cern.ch/cryptocellar/simula/purple/
.. _CryptoCellar: http://cryptocellar.web.cern.ch/cryptocellar/
.. _Python: http://www.python.org/
.. _Purple simulator on PyPI: https://pypi.python.org/pypi/purple
.. _Purple simulator on Bitbucket: https://bitbucket.org/bgneal/purple/