What is the Gromark cipher?

Gromark stands for gronsfeld with mixed alphabet and running key. A keyword is written above the alphabet in rows, and the columns are read off in the keyword's alphabetical order to make a mixed alphabet. The key is a string of digits that grows from a primer: each new digit is the sum of two earlier ones, keeping only the last digit, like a Fibonacci sequence. Each message letter is shifted by its key digit and then swapped through the mixed alphabet.

ACA puzzles often write the primer as the first five digits of the ciphertext.

Example

NFYCKBTIJCNWZYCACJNAYNLQPWWSTWPJQFL with the keyword ENIGMA and primer 23452 decodes to THEREAREUPTOTENSUBSTITUTESPERLETTER.