Poem code
Encyclopedia
The poem code is a simple, and insecure, cryptographic method
.
The method works by the sender and receiver pre-arranging a poem to use. The sender chooses a set number of words at random from the poem and gives each letter in the chosen words a number. The numbers are then used as a key
for some cipher
to conceal the plaintext
of the message. The cipher used was often double transposition
. To indicate to the receiver which words had been chosen an indicator group is sent at the start of the message.
The method is insecure for, if one message is broken by any means (including threat, torture, or even cryptanalysis
), future messages will be readable if the source poem has been identified. Since the poems used must be memorable for ease of use by an agent
, there is a temptation to use well known poems or poems from well known poets. (e.g. SOE
agents often used verses by Shakespeare, Racine
, Tennyson
, Molière
, Keats
, etc.).
Every poem code message commenced with an indicator-group of five letters, which showed which five words of an agent's poem had been used to encrypt the message. If the agent used the same poem code words to send a number of messages, these words could be discovered easily by enemy cryptographers. If the words could be identified as coming from a famous poem or quotation, then all of the future traffic submitted in that poem code could be read.
When Leo Marks
was appointed codes officer of the Special Operations Executive
(SOE) in London during World War II
, he very quickly recognized the weakness of the technique, and the consequent damage to agents and to their organizations on the Continent, and began to press for changes. Eventually, the SOE began using original compositions (thus not in any published collection of poems from any poet) to give added protection (see The Life That I Have
, an example), but also adopted other more secure methods such as Worked-out Keys (WOKs) and the one-time pad
.
A project of Marks', named by him "Operation Gift-Horse", was a deception scheme aimed to disguise the more secure WOK code traffic as poem code traffic, so that German cryptographers would think "Gift-Horsed" messages were easier to break than they actually were. This was done by adding false duplicate indicator groups to WOK-keys, to give the appearance that an agent had repeated the use of certain words of their code poem. The aim of Gift Horse was to waste the enemy's time, and was deployed prior to D-Day
, when code traffic increased dramatically.
Cryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...
.
The method works by the sender and receiver pre-arranging a poem to use. The sender chooses a set number of words at random from the poem and gives each letter in the chosen words a number. The numbers are then used as a key
Key (cryptography)
In cryptography, a key is a piece of information that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa...
for some 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...
to conceal the plaintext
Plaintext
In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties....
of the message. The cipher used was often double 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...
. To indicate to the receiver which words had been chosen an indicator group is sent at the start of the message.
The method is insecure for, if one message is broken by any means (including threat, torture, or even 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...
), future messages will be readable if the source poem has been identified. Since the poems used must be memorable for ease of use by an agent
Secret Agent
Secret Agent is a British film directed by Alfred Hitchcock, loosely based on two stories in Ashenden: Or the British Agent by W. Somerset Maugham. The film starred John Gielgud, Peter Lorre, Madeleine Carroll, and Robert Young...
, there is a temptation to use well known poems or poems from well known poets. (e.g. SOE
Special Operations Executive
The Special Operations Executive was a World War II organisation of the United Kingdom. It was officially formed by Prime Minister Winston Churchill and Minister of Economic Warfare Hugh Dalton on 22 July 1940, to conduct guerrilla warfare against the Axis powers and to instruct and aid local...
agents often used verses by Shakespeare, Racine
Jean Racine
Jean Racine , baptismal name Jean-Baptiste Racine , was a French dramatist, one of the "Big Three" of 17th-century France , and one of the most important literary figures in the Western tradition...
, Tennyson
Alfred Tennyson, 1st Baron Tennyson
Alfred Tennyson, 1st Baron Tennyson, FRS was Poet Laureate of the United Kingdom during much of Queen Victoria's reign and remains one of the most popular poets in the English language....
, Molière
Molière
Jean-Baptiste Poquelin, known by his stage name Molière, was a French playwright and actor who is considered to be one of the greatest masters of comedy in Western literature...
, Keats
John Keats
John Keats was an English Romantic poet. Along with Lord Byron and Percy Bysshe Shelley, he was one of the key figures in the second generation of the Romantic movement, despite the fact that his work had been in publication for only four years before his death.Although his poems were not...
, etc.).
Every poem code message commenced with an indicator-group of five letters, which showed which five words of an agent's poem had been used to encrypt the message. If the agent used the same poem code words to send a number of messages, these words could be discovered easily by enemy cryptographers. If the words could be identified as coming from a famous poem or quotation, then all of the future traffic submitted in that poem code could be read.
When Leo Marks
Leo Marks
Leopold Samuel Marks was an English cryptographer, screenwriter and playwright.-Early life:Born the son of an antiquarian bookseller in London, he was first introduced to cryptography when his father showed him a copy of Edgar Allan Poe's story, "The Gold-Bug"...
was appointed codes officer of the Special Operations Executive
Special Operations Executive
The Special Operations Executive was a World War II organisation of the United Kingdom. It was officially formed by Prime Minister Winston Churchill and Minister of Economic Warfare Hugh Dalton on 22 July 1940, to conduct guerrilla warfare against the Axis powers and to instruct and aid local...
(SOE) in London during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, he very quickly recognized the weakness of the technique, and the consequent damage to agents and to their organizations on the Continent, and began to press for changes. Eventually, the SOE began using original compositions (thus not in any published collection of poems from any poet) to give added protection (see The Life That I Have
The Life That I Have
The Life That I Have is a short poem written by Leo Marks and used as a poem code in the Second World War....
, an example), but also adopted other more secure methods such as Worked-out Keys (WOKs) and the one-time pad
One-time pad
In cryptography, the one-time pad is a type of encryption, which has been proven to be impossible to crack if used correctly. Each bit or character from the plaintext is encrypted by a modular addition with a bit or character from a secret random key of the same length as the plaintext, resulting...
.
A project of Marks', named by him "Operation Gift-Horse", was a deception scheme aimed to disguise the more secure WOK code traffic as poem code traffic, so that German cryptographers would think "Gift-Horsed" messages were easier to break than they actually were. This was done by adding false duplicate indicator groups to WOK-keys, to give the appearance that an agent had repeated the use of certain words of their code poem. The aim of Gift Horse was to waste the enemy's time, and was deployed prior to D-Day
D-Day
D-Day is a term often used in military parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours for similar...
, when code traffic increased dramatically.
See also
- Book cipherBook cipherA book cipher is a cipher in which the key is some aspect of a book or other piece of text; books being common and widely available in modern times, users of book ciphers take the position that the details of the key is sufficiently well hidden from attackers in practice. This is in some ways an...
- The Life That I HaveThe Life That I HaveThe Life That I Have is a short poem written by Leo Marks and used as a poem code in the Second World War....
(also known as Yours, arguably the most famous code poem)