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The Dorabella Cipher

The Dorabella Cipher, said to have been written by Sir Edward Elgar in 1897, is one of the best-known of the uncracked ciphers still around today. A complete solution is given on the pages that follow. 

 

For those readers not familiar with the cipher message, a copy of the message and a brief background are given here

A brief jargon-buster of the terms used in this solution can be viewed here

 

 

Headlines

 

System structure and cryptovariables fully recovered

Plaintext and Running Key text fully recovered

Message-Setting Method, KeyPhrase and Indicator fully explained

Fullstop (or period) after character 65 fully explained

 

 

Main Cryptographic Features:

  • the ciphertext comprises 87 characters, drawn from a set of 1-, 2- or 3-hoop characters in each of the 8 main compass points (24 in all)

                NB - the compass orientation of some of the characters is sometimes difficult to determine; my interpretation of the characters, line-by-line, is shown here

  • the message is doubly enciphered - first by 'Polyalphabetical Encipherment (Vigenère) Applied By Groups '  then by a fixed substitution to convert cipher letters into hoop characters (in practice, the 2 stages can be combined into a single process)

  • the message is in 2 Parts with a separate set of cryptovariables (CVs) for each part

                the change from Part 1 to Part 2 occurs after char 38 - the change point being determined by the message indicator system, as discussed later

  • CV1, CV3, CV4 and CV6 are each derived from a 6-part Running Key (RK), which is used to seed the construction of the CVs as shown

                Running Key is another way of saying 'long, non-repeating key', as opposed to 'short, cyclical key'

  • Parts 1 and 6 of the RK are used to construct Primary Key Mixed Substitution Alphabets CV1 and CV6 which feature in the first stage of encipherment; both CVs employ a classical mix of Transposition systems and associated techniques

  • Parts 3 and 4 of the RK are used to construct Primary Key Mixed Substitution Alphabets CV3 and CV4 which convert the intermediate cipher into the final hoop characters and are linked by construction (again involving Transposition techniques) and by a rotation or flip

  • Parts 2 and 5 of the RK are used directly to encipher the Plaintext - the key being applied ‘By Groups’ and the length of each key group being variable but limited to only certain values; a message-setting Key phrase (CV2 and CV5) controls the Key group lengths (and hence how the message is enciphered and deciphered) and a message indicator conveys the 'By Groups' information for each part of the message to the intended recipient; a full description of the message Key phrase usage is given here

  • the Primary Keys CV1 together with CV3, and CV6 together with CV4, form two complete but separate substitution setups

                some readers will no doubt recognise these as Type IV Quagmires

  • encipherment is subtractive (expressed as Plain-Key=Cipher, usually abbreviated to just P-K=Z, modulo 24) - see examples​

                decipherment is the reverse of the encipherment process (P=Z+K)

 

 

 

Features of the Keytext (click to view)

  • the character set comprises the letters A through Z, omitting J and V, 24 in total; for J use I, for V use U

  • the 1st segment of Running Key, RK1 used in CV1, is a question relating to a mystery News ban (a United Kingdom D-Notice)

  • the 2nd segment, RK2, used to encipher Part 1 of the Plaintext, gives the answer to the question - a specific D-Notice number (6/66)

               [NB The D-Notice system was NOT introduced in the UK until 1912, so the message is NOT 1897 - but more on this later!]

  • the 3rd segment RK3, used in CV3, reveals that naughty things can occur with the letter E  - a feature of Running Key systems

  • RK segments RK4 (used in CV4), RK5 (used to encipher Part 2 of the Plaintext), and RK6 (used in CV6) form a 3-part conundrum using unknowns X and Y (and supporting the NOT 1897)

  • segments 2 and 5 of the RK, used to encipher the Plain text, comprise 48 characters in total (22 and 26 characters respectively); the RK is expanded to 87 characters (38 and 49 respectively) by means of the 'By Groups' variable, controlled by the Message Key Phrase

  • the RK contains several Elgar-like homophones - eg - Mr A (mystery), nuz (news), hearcy (hearsay), siks (six), Qrd (cured), why ("Y")

             There is a further outrageous homophone which presents itself at the end of the solution

  • the RK contains numbers which are spelled out using letters – eg –  "siks”, "sixty" and "cix"

  • the letter ‘Q’ is used to represent both 'Q' and ‘?’ (ie - Q for Query)

 

 

Features of the Plaintext (click to view)

  • the character set comprises the letters A through Z, omitting J and V, 24 in total; for J use I, for V use U (as per the Keytext)

  • it contains almost no punctuation; the one fullstop, after cipher character 65, marks an abbreviation* in the Plaintext

               * the abbreviation is "Brompton OraC."; crossword fans will recognise that C.=Conservative=Tory - hence Brompton Oratory, where the Elgars were wed

  • it contains Elgar-like homophones: Aliss (Alice), tuT (tutee), P (piano), rane (rain), pfog (pea fog), ufeletharge (you feel lethargy)

  • it contains a number which is spelled out using letters, thus –  “eighty six”

 

A Flowchart of the System is shown here and a brief description of the Cryptanalysis is here

 

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