Gronsfeld Cipher Encoder/Decoder

How it is calculated

Every letter is shifted along the alphabet by a digit from the key, and the key digits repeat in order. Encoding is C = (P + d) mod 26; decoding is P = (C − d) mod 26, where P and C are letter positions 0 to 25 and d is the current key digit (0 to 9). A digit 0 leaves the letter unchanged.

Details of this tool: only the 26 letters A–Z are shifted, and upper and lower case are kept. Spaces, digits, punctuation and accented letters are copied through as they are and do not use up a key digit, so the key moves on only at letters. Anything in the key that is not a digit is dropped, so 3-1-4 is read as 314. An empty text or a key without digits gives no result. Mode decides encode or decode, and decoding needs the same key.

Common mistakes

• Using a keyword such as LEMON. That is the Vigenère cipher; here the key must be digits, and letters in it are simply ignored, so a word key gives no result.
• Decoding with the wrong mode. Pasting ciphertext while the mode is still on encode shifts it forward again and produces a different scrambled text.
• Expecting the key to advance over spaces and punctuation. It advances only on letters, so a message encoded by hand with a key that counts every character will not match.
• Expecting accented letters, such as Turkish ş or ğ, to be encrypted. They are passed through unchanged.
• Treating the result as secure. With only ten possible shifts per position and a short repeating key, this is a puzzle cipher, not protection for private data.