to describe the network that used the same Enigma setting sheets. Initially these were recorded using coloured pencils and were given the names red, light blue etc., and later the names of birds such as kestrel. During World War II the settings for most networks lasted for 24 hours, although towards the end of the war, some were changed more frequently. The sheets had columns specifying, for each day of the month, the rotors to be used and their positions, the ring positions and the plugboard connections. For security, the dates were in reverse chronological order down the page, so that each row could be cut off and destroyed when it was finished with. Up until 15 September 1938, the transmitting operator indicated to the receiving operator(s) how to set their rotors, by choosing a three letter message key (the key specific to that message) and enciphering it twice using the specified initial ring positions (the Grundstellung). The resultant 6-letter indicator, was then transmitted before the enciphered text of the message. Suppose that the specified Grundstellung was RAO, and the chosen 3-letter message key was IHL, the operator would set the rotors to RAO and encipher IHL twice. The resultant ciphertext, DQYQQT, would be transmitted, at which point the rotors would be changed to the message key (IHL) and then the message itself enciphered. The receiving operator would use the specified Grundstellung RAO to decipher the first six letters, yielding IHLIHL. The receiving operator, seeing the repeated message key would know that there had been no corruption and use IHL to decipher the message. The weakness in this indicator procedure came from two factors. First, use of a global Grundstellung —this was changed in September 1938 so that the operator selected his initial position to encrypt the message key, and sent the initial position in clear followed by the enciphered message key. The second problem was the repetition of message key within the indicator, which was a serious security flaw. The message setting was encoded twice, resulting in a relation between first and fourth, second and fifth, and third and sixth character. This security problem enabled the Polish Cipher Bureau to break into the pre-war Enigma system as early as 1932. On 1 May 1940 the Germans changed the procedures to encipher the message key only once. British efforts In 1927, the UK openly purchased a commercial Enigma. Its operation