• Sailor’s Odyssey (Cunningham) 1
• Sandwith, Commander Humphrey 1
• Sardinia 1
• Sarmiento, Commander Antonio 1
• Sayers, Dorothy L. 1
• Scheer, Admiral Reinhard 1
• Schellenberg, SS General Walter 1
• Scherbius, Arthur 1, 2
• Schlüsselanleitung für die Chiffriermaschine Enigma 1, 2, 3
• Schmidt, Hans Thilo (‘Asché’) 1, 2, 3
• Schmidt, Wulf (‘Tate’) 1
• Seahorse 1
• Secret Intelligence Service (SIS)
  • air intelligence 1
  • early refusal to pay for Enigma manuals 1, 2, 3
  • and GC&CS 1
  • obtains Enigma manuals 1
  • outbreak of WWII 1
  • Radio Security Service (RSS) 1, 2
  • secrecy and ‘need to know’ 1, 2
  • Section V counter-espionage 1, 2
  • Section VIII 1
  • Venlo incident 1, 2
• Security Service (MI5) see MI5
• Sergueiev, Natalie ‘Lily’ (‘Treasure’) 1
• Serocold, Claude 1, 2, 3
• Shakespeare, William 1
• Sicherheitsdienst (SD) 1, 2, 3, 4, 5g
• Sicily 1
• Signalbuch der Kaiserlichen Marine (SKM) 1, 2, 3, 4
• Sinclair, Admiral Hugh (‘C’) 1, 2, 3, 4, 5, 6
• SIS see Secret Intelligence Service (SIS)
• Soviet Union 1, 2, 3
• Spain 1, 2
• Spanish Civil War 1, 2, 3
• Spencer, Gilbert 1
• starfish 1, 2g
• Stecker board 1, 2, 3, 4, 5g
• Stevens, Major Richard 1, 2
• Strachey, Lytton 1
• Strachey, Oliver 1, 2, 3
  • ISOS (Illicit Services Oliver Strachey) 1, 2, 3, 4