Devised to commemorate the work of Hugh Foss, along with Alan Turing and others, at Bletchley Park during the 2nd world war.
The Enigma Machine had three coding wheels which were used to scramble messages into a ciphered form or to decipher encoded messages. These are represented here by hands across, circles and turning with both hands.
Cryptographic literature often uses a few stock characters to illustrate problems. Alice and Bob are the main actors who hope to exchange secret messages without them being intercepted or decrypted by Eve, the eavesdropper. Here 1st woman and 2nd man are Alice and Bob while 3rd woman is Eve. Bars 1-8 and 9-16 represent encryption and decryption with Alice and Bob exchanging places as Eve tries to get close to Bob. The final progression is suitably scrambled with all changing partners. Alice and Bob end up together for the repeat.