construction and use of tactical cryptosystems there are two conflicting
requirements. One is security and the other is ease of use. If a system is
highly secure but hard and time consuming to use then important messages might
be secure from cryptanalysis but they could arrive too late, with disastrous
consequences. On the other hand if a system is extremely easy to use but
insecure then the messages will get through on time but the enemy will also be
able to read them.
code, used by the US and British armies in WWII was easy to use but it
could be solved in a few hours by the German codebreakers.
However the British
transposition cipher and the US Army’s M-209 cipher
machine were basically secure systems, since they could only be solved
through mistakes in encipherment. It seems that contrary to regulations the
Allied troops did not always use these systems in the field since it took too
long to encipher their messages.
the War Office to the Commanders in Chief 21st Army Group, Home Forces, Middle
East, Persia-Iraq (dated February 1945):
I am commanded by the Army Council to
inform you that further consideration has been given to the suitability for
operational purposes of the Low-Grade cipher "Double Transposition"
which was introduced for use throughout the Army by War Office letter
32/Tels/943 dated 5th November, 1943.
2. Experience shows that while this
cipher affords adequate security, unit personnel find it difficult and slow to
operate. There is, therefore, a tendency to avoid the use of cipher with a
consequent possibility of overstrain of other safe means of communication or
the use of wireless in clear to a dangerous extent.
3. It has, therefore, been decided to
adopt a new Low Grade cipher, called LINEX, details of which are given in
appendices A to D, in place of Double Transposition.’
interview with S/Sgt, Communications Section 79 Inf Div, 7th Army. (dated March
"The US Army code machine #209 was found to
be something that hampered operations. It would take at least half hour to get
a message through from the message center by use of this code machine and as a
result the codes of particular importance or speed, for instance mortar messages,
were sent in the clear."
Sources: British national archives WO 193/211
‘Wireless, cable and signal (including cipher) communications: policy and
codes: action from report of Godwin-Austen Committee’, US national archives -
collection RG457 - Entry 9032 - box 1.024 - US COMSEC reports.