Ciphers
Gronsfeld cipher
Vigenère with a repeating string of digits for the Key.
Also found as Gronsfeld · numeric Vigenère
How it works
Gronsfeld is Vigenère with digits for the Key. Repeat the digit string and add each digit as a Caesar shift. Each Key position is limited to 0–9. In Decipher, only ASCII letters advance the Key.ACA GronsfeldMilitary Cryptanalysis, Part II
C[i] = (P[i] + D[i mod m]) mod 26D[j] ∈ {0, 1, …, 9}Worked example
- Plaintext
CODE- Keystream
3143- Ciphertext
FPHH
What to look for
Period tests work much as they do for Vigenère. Once the text is split by phase, each position has only ten possible shifts. That narrows the search, although other periodic ciphers can produce similar clues.