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Reservehandverfahren (RHV) (German: 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^[1].

The cipher had two stages: a transposition followed by bigram 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^[2].

The Reservehandverfahren cipher was first solved at Bletchley Park in June 1941 by means of documents captured from U-boat U-110 the previous month. Thereafter it was solved using cryptanalysis for over three years. Some 1,400 signals were read during that period, an average of 12 per day. The section working on RHV was headed by historian Sir John H. Plumb. 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^[2].

A Mediterranean variant was known as Schlüssel Henno, which was first tackled — unsuccessfully — in May 1943. It wasn't until after a capture of cipher documents from a raid on Mykonos in April 1944 that the Naval Section was able to read Henno. With over 1,000 signals a month, up to thirty people were assigned to solve the messages. The work was abandoned in August 1944 after it was found the intelligence value of the decrypts was "rather disappointing"^[3].

• Werftschlüssel

1. ^ The Enigma General Procedure Manual, 1940
2. ^ ^{a b} Hugh Sebag-Montefiore, Enigma: Battle for the Code, 2000, pp. 213–214.
3. ^ Christoper 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.

• Scanned cover of a 1940 Reservehandverfahren manual
• The Archives of german technical Manuals 1900-1945 (includes the Reservehandverfahren manual)