Reservehandverfahren

Reservehandverfahren (RHV) (English: 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.

In the transposition stage, the cipher clerk would write out the plaintext into a "cage" — a shape on a piece of paper.

[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.

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.

It was not until after a capture of cipher documents from a raid on Mykonos in April 1944 that the Naval Section was able to read Henno.