( en, Reserve Hand Procedure) was a German Naval World War II hand- cipher system used as a backup method when no working Enigma machine was available.The Enigma General Procedure Manual, 1940
/ref> The cipher had two stages: a transposition followed by

bigram A bigram or digram is a sequence of two adjacent elements from a string of tokens, which are typically letters, syllables, or words. A bigram is an ''n''-gram for ''n''=2. The frequency distribution of every bigram in a string is commonly used f ...

substitution. In the transposition stage, the cipher clerk would write out the plaintext into a "cage" — a shape on a piece of paper. Pairs of letters were then substituted using a set of bigram tables.

Hugh Sebag-Montefiore Nicholas Hugh Sebag-Montefiore (born 5 March 1955) is a British writer. He trained as a barrister before becoming a journalist and then a non-fiction writer. His second book ''Dunkirk: Fight to the Last Man'' was published in 2006. His previous b ...

, ''Enigma: Battle for the Code'', 2000, pp. 213–214. The cipher was first solved at

Bletchley Park Bletchley Park is an English country house and estate in Bletchley, Milton Keynes ( Buckinghamshire) that became the principal centre of Allied code-breaking during the Second World War. The mansion was constructed during the years following ...

in June 1941 by means of documents captured from

U-boat U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic warfare role ...

the previous month. Thereafter it was solved using cryptanalysis for over three years. Some 1,400 signals were read during that period. The section working on RHV was headed by historian Sir

John H. Plumb Sir John (Jack) Harold Plumb (20 August 1911 – 21 October 2001) was a British historian, known for his books on British 18th-century history. He wrote over thirty books. Biography Plumb was born in Leicester on 20 August 1911. He was educate ...

. The decrypts were sometimes useful in themselves for the intelligence that they contained, but were more important as a source for cribs for solving Naval Enigma. A Mediterranean variant was known as Schlüssel Henno, which was first tackled — unsuccessfully — in May 1943. It was not until after a capture of cipher documents from a raid on

Mykonos Mykonos (, ; el, Μύκονος ) is a Greek island, part of the Cyclades, lying between Tinos, Syros, Paros and Naxos. The island has an area of and rises to an elevation of at its highest point. There are 10,134 inhabitants according to the ...

in April 1944 that the Naval Section was able to read Henno. With over 1,000 signals a month, up to 30 people were assigned to solve the messages. A separate version of RHV existed for U-boats to use, called RHV Offizier. Only six messages in RHV Offizier were broken at

, three by James Hogarth. The work was abandoned in August 1944 after it was found the intelligence value of the decrypts was "rather disappointing".Christopher Morris, "Navy Ultra's Poor Relations", pp. 238–239, in F. H. Hinsley and Alan Stripp, ''The Codebreakers: The Inside Story of Bletchley Park'', 1993.

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A detailed description of the German Reservehandverfahren (R.H.V.) M.Dv.Nr. 929/1Scanned cover of a 1940 Reservehandverfahren manual
{{Cryptography navbox, classical Classical ciphers Enigma machine History of telecommunications in Germany Research and development in Nazi Germany Signals intelligence of World War II

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