The messages had been encrypted with a small hand-operated device recently invented by the British army to make their field communications more secure. The device was a kind of cipher disc, with two alphabets printed on rings that rotated with respect to each other, but with a twist: while the outer ring had the usual 26 letters, the inner ring had 27. The extra letter introduced a degree of irregularity, making it harder for a codebreaker to visualize the alphabets sliding against each other. The device also allowed the cryptographer to change the alphabets quickly and easily. The British had
