Gronsfeld Cipher Encoder/Decoder

About this calculator

The Gronsfeld cipher is a polyalphabetic substitution cipher, essentially a numeric-key variant of the Vigenère cipher: instead of a repeating keyword of letters, it uses a repeating string of digits (0–9), and each digit shifts the corresponding plaintext letter forward in the alphabet by that many positions, wrapping around from Z back to A. With only 10 possible shift values per position (versus 26 for a full Vigenère key letter), it's slightly weaker than Vigenère but historically easier to memorize and transmit as a number.

Cryptography students and hobbyists use the Gronsfeld cipher to learn the mechanics of polyalphabetic substitution and the Kasiski/index-of-coincidence techniques used to break it, since — like all classical repeating-key ciphers — it's vulnerable to frequency analysis once the key length is discovered. It was historically used by Count Gronsfeld in 17th-century military correspondence. Enter text and a numeric key to encode or decode.