In the 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.
The Slidex 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 Army’s double 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.
Letter from 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.’
Report of interview with S/Sgt, Communications Section 79 Inf Div, 7th Army. (dated March 1945):
"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.