Editing Leo Marks (section)

===Developments of cryptographic practice===
One of Marks's first challenges<ref name ="HCN1">{{Cite book |last=Marks |first=Leo |title=Between Silk and Cyanide: A Codemaker's Story 1941–1945 |year=1998 |publisher=Harper Collins |location=London |isbn=0-00-255944-7 |page=5}}</ref> was to phase out double [[transposition cipher]]s using [[key (cryptography)|keys]] based on poems. These [[poem code|poem ciphers]] had the limited advantage of being easy to memorise, but significant disadvantages,<ref name ="HCN1"/> including limited cryptographic security, substantial minimum message sizes (short ones were easy to crack), and the fact that the method's complexity caused encoding errors. Cryptographic security was enhanced by Marks's innovations, especially "worked-out keys." He was credited with inventing the letter [[one-time pad]], but while he did independently discover the method, he later found it already in use at Bletchley.<ref>{{Cite book |title=Between Silk and Cyanide: A Codemaker's War, 1941-1945 |last=Marks |first=Leo |publisher=Simon & Schuster |year=2001 |page=250}}</ref>