| Technical Specification of the Enigma | Much of the German cipher traffic was encrypted on the Enigma machine, and the term "Ultra" has often been used almost synonymously with "" |
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| "The Commercial Enigma: Beginnings of Machine Cryptography", Cryptologia, 26 1 , pp | Geneva, Illinois, USA: Riverbank Laboratories |
, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two, edited and translated by Christopher Kasparek, Frederick, MD, University Publications of America, 1984,.
26| "Recovering the Wiring of Enigma's Umkehrwalze A", Cryptologia 24 1 , pp55—66 | Thomas Adamson May 2, 2012 |
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| The index of coincidence and its applications in cryptology | Ultra also encompassed decrypts of the German that were used by the German High Command, and decrypts of and other Italian ciphers and codes, as well as of Japanese ciphers and codes such as and |
Philip Marks, "Umkehrwalze D: Enigma's Rewirable Reflector — Part I", Cryptologia 25 2 , April 2001, pp.
| 00 , Radio Times, Issue 3957, 16 December 1999, p | Ralph Erskine: The Poles Reveal their Secrets — Alastair Dennistons's Account of the July 1939 Meeting at Pyry |
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| BBC Radio 4 FM, 25 December 1999 14 | ; Sullivan, Geoff; Weierud, Frode July 1998 |