Leo Mark's first encounter with Vera preceded his entry into SOE. As chief cryptographer, he dabbled in psychology and wrote one move script so grim that it was banned by censors. He would ask each girl who wanted to work on codes, “Do you do crosswords?” If they did, they were in. Leo’s explanations came later. He was the Jewish genius who invented a safer option to SOE’s old coding system. He produced one-time coding pads of finest silk, inserted in the lining of an agent’s clothes. Random numbers were printed on the silk; each line of numbers was used for coding one message only, and wa then cut off. If the Gestapo closed in, the silk vanished at the touch of a match. He silks gave an agent the chance to live a little longer, rather than swallow a cyanide pill to cheat the tortures. Later, Vera said that the agents worked “between silk and cyanide.”
Spymistress: The Life of Vera Atkins, The Greatest Female Secret Agent of World War II by William Stevenson, page 150.