How does the Gronsfeld cipher differ from the Vigenere cipher?

The Gronsfeld cipher is a variation of the Vigenere cipher that specifically utilizes a key number rather than a traditional alphabetic keyword. This key number is typically a sequence of digits, and it governs the shifting of the letters in the plaintext according to the numerical values of the digits in the key. Each digit corresponds to a shift in the alphabet, with '0' representing no shift and '9' representing a shift of nine places back through the alphabet.

This mechanism allows for a straightforward application of the cipher, as each letter of the plaintext is shifted based on the corresponding digit of the key number repeated as necessary. This characteristic sets the Gronsfeld cipher apart from the Vigenere cipher, which employs an alphabetic keyword that leads to a more complex series of shifts determined by the letters in that keyword.

The other options, while they highlight notable distinctions in some ciphers, do not accurately capture the essential difference that defines how the Gronsfeld cipher operates compared to the Vigenere cipher.