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was a German Naval
World War II hand-cipher
system used as a backup method when no working Enigma machine
was available.
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.
The cipher was first solved at Bletchley Park
in June 1941 by means of documents captured from U-boat
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. 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 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 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 Bletchley, three by James Hogarth. The work was abandoned in August 1944 after it was found the intelligence value of the decrypts was "rather disappointing".
Kriegsmarine
The Kriegsmarine was the name of the German Navy during the Nazi regime . It superseded the Kaiserliche Marine of World War I and the post-war Reichsmarine. The Kriegsmarine was one of three official branches of the Wehrmacht, the unified armed forces of Nazi Germany.The Kriegsmarine grew rapidly...
World War II hand-cipher
Cipher
In cryptography, a cipher is an algorithm for performing encryption or decryption — a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts...
system used as a backup method when no working Enigma machine
Enigma machine
An Enigma machine is any of a family of related electro-mechanical rotor cipher machines used for the encryption and decryption of secret messages. Enigma was invented by German engineer Arthur Scherbius at the end of World War I...
was available.
The cipher had two stages: a transposition
Transposition cipher
In cryptography, a transposition cipher is a method of encryption by which the positions held by units of plaintext are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext. That is, the order of the units is changed...
followed by bigram
Bigram
Bigrams or digrams are groups of two written letters, two syllables, or two words, and are very commonly used as the basis for simple statistical analysis of text. They are used in one of the most successful language models for speech recognition...
substitution
Substitution cipher
In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system; the "units" may be single letters , pairs of letters, triplets of letters, mixtures of the above, and so forth...
. 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.
The cipher was first solved at Bletchley Park
Bletchley Park
Bletchley Park is an estate located in the town of Bletchley, in Buckinghamshire, England, which currently houses the National Museum of Computing...
in June 1941 by means of documents captured from U-boat
U-boat
U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...
the previous month. Thereafter it was solved using cryptanalysis
Cryptanalysis
Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key...
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. 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 wasn't until after a capture of cipher documents from a raid on Mykonos
Mykonos
Mykonos is a Greek island, part of the Cyclades, lying between Tinos, Syros, Paros and Naxos. The island spans an area of and rises to an elevation of at its highest point. There are 9,320 inhabitants most of whom live in the largest town, Mykonos, which lies on the west coast. The town is also...
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 Bletchley, three by James Hogarth. The work was abandoned in August 1944 after it was found the intelligence value of the decrypts was "rather disappointing".
