The Delilah Secure Speech System (fwd)

The Delilah Secure Speech System
--------------------------------

by John Graham-Cumming on 3/6/12

http://blog.jgc.org/2012/03/delilah-secure-speech-system.html


Part of the new exhibit being unveiled at Bletchley Park is the Delilah
Secure Speech system that Alan Turing developed during the Second World
War. Details of the system are in Andrew Hodges' excellent biography of
Turing and have recently been placed in the National Archives ( references
FO 850/256 and HW 25/36 for people who want to go an see them for
themselves ).

Delilah was intended to be fairly portable ( unlike SIGSALY ) and usable
in the field (such as in a tank) and allow secure speech communication
between people over radio or telephone.

A team at Bletchley Park has been working to rebuild Delilah from the
report ( with some assistance from GCHQ ) and I was able to see and
photograph the reconstructed machine. Here are some pictures :

http://1.bp.blogspot.com/-wGQGRzPW7rQ/T1Xzac6YpxI/AAAAAAAABLc/hTl0T9S73bU/s1600/IMG_3183.JPG
http://2.bp.blogspot.com/-3HYhStEPj_8/T1Xzaw6_92I/AAAAAAAABLo/WcGw0Fa1Zog/s1600/IMG_3184.JPG
http://3.bp.blogspot.com/-_Vt6hEiy_IE/T1XzbRkCm3I/AAAAAAAABL0/CHTs1zb91tw/s1600/IMG_3185.JPG
http://3.bp.blogspot.com/-NjgxmP33OuI/T1XzcO7PDYI/AAAAAAAABMA/f2MuAaBVz2A/s1600/IMG_3186.JPG
http://4.bp.blogspot.com/-T-VVUIrpUXk/T1Xzcl3aTzI/AAAAAAAABMM/Ddyv0zDD8DY/s1600/IMG_3187.JPG
http://3.bp.blogspot.com/-0qy-LAeMGlc/T1Xz-sfZVnI/AAAAAAAABMY/mh1zua_nsPQ/s1600/IMG_3188.JPG
http://1.bp.blogspot.com/-E-poo-PSyOQ/T1Xz_QtEKxI/AAAAAAAABMk/oDyvoSqW3us/s400/IMG_3189.JPG

And here are pictures of the original machine taken from recently
declassified documents :

http://1.bp.blogspot.com/-RcoT_LJP2lI/T1X15EoBxNI/AAAAAAAABMw/cHfha3KElf4/s1600/Delilah001.png

Briefly, Delilah worked as follows. The incoming speech was limited a
channel of 2kHz which was then sampled at 4kHz to produce 4,000 samples
per second of the incoming waveform. These samples were normalized to a
range of 0 to 1 and added using modulo arithmetic to a key stream
consisting of values in the range 0 to 1.

The resulting waveform was then transmitted and at the opposite end the
original waveform could be constructed by adding back (again using modulo
arithmetic) the same key stream. Both ends had to be synchronized for this
scheme to work ( and use the same key ).

The key was set on wheels visible in the photograph above that generated a
stream of pseudo-random numbers which when added to the incoming signal
would result in something close to pure noise being transmitted.


http://blog.jgc.org/2012/03/delilah-secure-speech-system.html