Gromark

History

A more secure variation of the Gronsfeld cipher is the Gronsfeld with Mixed Alphabet and Running Key or Gromark cipher. It was developed by the American Cryptogram Association (ACA) member DUMBO and first appeared in the Mar-Apr 1969 edition of the ACA's magazine The Cryptogram. The original Gromark cipher was invented by the Bavarian Field Marshal Count Jobst Maximilian von Gronsfeld (1598-1662) and is a shortened variation of the Vigenère cipher using only 10 of the 26 alphabets with the key using characters in the range A-J or 0-9.

Description

To encipher text using the Gromark cipher choose a keyword and from it create a keyword mixed alphabet. This is produced by creating a keyed alphabet from the keyword with repeated letters being omitted and followed by unused letters of the alphabet in alphabetic order. This is then written into a transposition block with the number of letters in the keyword determining the number of columns in the block. For example appending unused alphabet letters to the key word GRONSFELD produces the keyed alphabet GRONSFELDABCHIJKMPQTUVWXYZ. Writing this into a transposition block gives:

4 8 7 6 9 3 2 5 1

G R O N S F E L D

A B C H I J K M P

Q T U V W X Y Z

Finally the letters in each column are taken off the block in alphabetical order to produce the mixed keyed alphabet. Using the above table this produces the key DP EKY FJX GAQ LMZ NHV OCU RBT SIW or DPEKYFJXGAQLMZNHVOCURBTSIW.

Straight alphabet: abcdefghijklmnopqrstuvwxyz

Keyed alphabet: DPEKYFJXGAQLMZNHVOCURBTSIW

Next a 5-digit primer and create a running digital key by adding successive pairs of digits, subtracting 10 if the total is greater than 9, and appending the result to the end of the running key. This is repeated until its length is the same as the plaintext. So if the primer is 32941 adding the 1st number, 3 to the 2nd number, 2 gives 3+2=5 and the running key becomes 329415. Then the 2nd number, 2 is added to the 3rd number, 9 to give 2+9=11. Subtracting 10 gives 1 and the running key becomes 3294151. This continues to the length of the plaintext producing the running key 329415135664812029322125433797606636629928181099919880076807348707257797246696.

Running key: 329415135664812029322125433797606636629928181099919880076807348707257797246696

Plaintext: onlytwothingsareinfinitetheuniverseandhumanstupidityandimnotsureabouttheformer

Encryption is carried out by taking the 1st plaintext letter o and running key number above it, 3. Locate the plaintext letter in the straight alphabet and count 3 places to the right giving r. Below this in the keyed alphabet is the 1st ciphertext letter O. Next the 2nd plaintext letter n and counting right 2 places, the 2nd running key number, is plaintext p. Below this in the cipher row is the 2nd ciphertext letter H and so on.

Ciphertext: OHRERPHTMNUQDPUYQTGQHABASQXPTHPYSIXJUFVKNGNDRRIOMAEJGZKHCBNDBIWLDGVWDDVLXCSCZS

Example

Keyword: GRONSFELD (487693251)

Primer: 32941

Encipher the Albert Einstein quote "Only two things are infinite, the universe and human stupidity, and I'm not sure about the former.”

Key: 329415135664812029322125433797606636629928181099919880076807348707257797246696

Plaintext: onlytwothingsareinfinitetheuniverseandhumanstupidityandimnotsureabouttheformer

Ciphertext: OHRERPHTMNUQDPUYQTGQHABASQXPTHPYSIXJUFVKNGNDRRIOMAEJGZKHCBNDBIWLDGVWDDVLXCSCZS

Solving

Solving method: Hill Climbing search.

After selecting the Gromark cipher, the Key primer must be entered in the Cipher Settings ribbon group.