How is the Gronsfeld cipher different from Vigenère?

Vigenère uses a keyword of letters, so each shift ranges 0–25. Gronsfeld uses a key of digits, so each shift only ranges 0–9. The mechanics are otherwise identical: add the key value to each letter modulo 26.

What does each key digit do?

Each digit is the shift amount for the corresponding letter of the message. The digits repeat cyclically, so a five-digit key shifts the sixth letter by the first digit again.

Why might Gronsfeld be weaker than Vigenère?

Because digits only span 0 to 9, the cipher uses at most ten of the 26 possible Caesar shifts. This smaller alphabet of shifts gives a cryptanalyst less to search through, making frequency analysis a little easier.

What happens to spaces, digits and punctuation in the text?

Only letters are enciphered and advance the key. Spaces, numerals and punctuation pass through unchanged, and letter case is preserved throughout.

Is the Gronsfeld cipher secure for real use?

No. Like all classical polyalphabetic ciphers it falls to Kasiski examination and frequency analysis. Use it for puzzles, education and historical curiosity rather than protecting genuine secrets.

Gronsfeld Cipher Encoder & Decoder

The Gronsfeld cipher is a polyalphabetic substitution cipher that works exactly like the Vigenère cipher except that its key is a sequence of digits rather than a keyword of letters. It is named after the 17th-century Count Gronsfeld and was once a popular hand cipher because a short numeric key is easy to remember and to apply.

How it works

Each letter of the alphabet is numbered 0 (A) through 25 (Z). The numeric key is repeated cyclically beneath the message, and each key digit gives the shift for the letter above it. Encryption uses the standard Vigenère addition:

C = (P + d) mod 26

where d is the current key digit. Decryption simply subtracts instead:

P = (C - d) mod 26

Because digits only run from 0 to 9, the Gronsfeld cipher chooses from just ten of the 26 possible Caesar shifts. This is its only real difference from Vigenère, and it is what makes Gronsfeld a touch easier to break.

Example

With the key 31415 and plaintext HELLO:

P:  H  E  L  L  O
d:  3  1  4  1  5
C:  K  F  P  M  T

H (7) plus 3 gives K, E (4) plus 1 gives F, and so on. Decrypting KFPMT with the same key subtracts each digit to recover HELLO.

Notes

Only letters are shifted; spaces and punctuation pass through and do not consume a key digit, keeping the digits aligned with the letters.
A longer, non-repeating-looking key resists Kasiski examination better, but the small 0–9 shift range remains the cipher’s weakness.
For a letter-keyed version with a full 0–25 shift range, use the Vigenère or Autokey cipher instead.