Dorabella & D'Agapeyeff Ciphers - Solutions

Jargon Buster - some basic definitions

 

Cryptology embraces the twin arts of cryptography and cryptanalysis: cryptography being the study of codes and ciphers with a view to devising a system to be used in encrypting a message or messages, whereas cryptanalysis is the study of codes and ciphers with a view to decrypting a message or messages (NOT intended to be read by you)

To encrypt is to encode OR to encipher (a plaintext message eg); to decrypt is to decode OR decipher an encrypted message and recover the original plaintext message. Leaving aside codes and those ciphers which are derived from some electro/mechanical device, hand ciphers generally fall into 2 categories:

Transposition and Substitution. In a Transposition system, the letters or characters that make up the Plain message are retained but their order is changed according to some rule (the system), hopefully known only to the sender and the intended recipient. Various types of Transposition can be seen in Dorabella. [But to make life more complicated, the Transposition is applied not to the Plaintext but in the construction of component parts of the Cipher System.] A Transposition will often involve a rectangular cage or box - where (eg) the plaintext is written in by row and the ciphertext is then extracted by column. The possibilities are endless (and hence, as HFG argues, there is no need for a system of a quasi-random nature). In a Substitution cipher, as the name implies, the Plaintext letters (or whatever) are substituted for something different. Sometimes the 2 types can be combined – making life harder for the person attempting to decrypt the message, as happens in Dorabella. [This of course can apply also to the intended recipient!]

Cryptovariable A cryptovariable (CV) is a component part of a cipher system and can be fixed or changed: periodically (eg – every hour, day etc), by message, by user etc.. A Substitution system will often have 3 CVs - a Plain Alphabet, Cipher Alphabet and a Message Key. According to HFG, the alphabets are usually referred to as Primary Keys and the message key as a Secondary Key. The Dorabella cipher has 2 sets of CVs – 1 set for each of the 2 message parts. Since the Secondary Key (the message key) is shared across both parts of the message, that makes a total of 6 CVs (2*2 + 2).

Running Key is another way of saying long, non-repeating key (as opposed to a short, cyclic key, which will usually be formed from a keyword). Pages from novels were often used as a source of running key. Unusually, in Dorabella the running key is used to form the alphabets as well as to encipher the Plaintext - hence part of the difficulty in decrypting the message. This counts as yet another CV, making a grand total of 7 (12 if you count it in segments).

In a Polyalphabetic substitution (poly meaning 'many'), the user has at his/her disposal a number of alphabets, the number usually being equal to the length of the alphabets. In the case of Dorabella, there are 24 characters – hence, there are 24 alphabets to choose from. These are often displayed in table form, the table being called a Vigenère table (after Blaise de Vigenère), with each table row or alphabet being an offset by one of the previous row. The first letter of each row will usually act as the Key or Indicator letter for that particular row (as happens with Dorabella).

Index of Coincidence - a statistic used by cryptanalysts in examining both plain-text and cipher-text

Steganography - the art of concealing a message, image etc. within another message etc

Concealment Cipher - goes by various names - eg - null cipher, open-letter cipher. Can be combined with conventional means of encipherment - ie - Transposition, Substitution.