Johann Franz Graf von Gronsfeld-Bronkhorst (1640 – 1719) allegedly suggested to the writer and Jesuit priest Caspar Schott (1608 – 1666) on a journey from Mainz to Frankfurt the polyalphabetic monographic substitution cipher named after him today.
Schott reported on this in 1665 in his Schola stenographia.
This is a further development of the Trithemius cipher, but only the first ten lines of the tabula recta are used.
These are numbered from 0 to 9 and now a decimal number is chosen as the key, for example “1954”. Then the first letter of the plaintext is encrypted after the line with the number “1”, i.e. the first digit of this number, the second letter after the line with the number “9”, and so on. The fifth letter is encrypted again after the line with the number “1”, and so on. The first line, indexed by 0, thus contains the plaintext alphabet.
The Gronsfeld cipher is only a weaker variant of the method already proposed by Vigenère in 1586. It was therefore already obsolete at the time of its invention, because it works with only 10 different ciphertext alphabets, while the Vigenère cipher uses 26 alphabets.