Gronsfeld Cipher

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The Gronsfeld Cipher is a variant of the Vigenère cipher but uses only numeric keys, making it simpler. Each digit in the key determines how many positions to shift the corresponding letter in the plaintext.

Steps to Encrypt and Decrypt Using the Gronsfeld Cipher

1. Choose a numeric key: The key is a sequence of digits (e.g., 31415).

2. Prepare the plaintext: Write down the message you want to encrypt, e.g., "HELLO".

3. Repeat the key: If the key is shorter than the plaintext, repeat it to match the length of the plaintext.

4. Encrypt the plaintext: For each letter in the plaintext:

• Convert the letter to its position in the alphabet (A=0, B=1, ..., Z=25).

• Add the corresponding digit from the key.

• Wrap around using modulo 26 if the result exceeds 25.

• Convert the result back to a letter.

1. Decrypt the ciphertext: Reverse the process by subtracting the key digits instead of adding.

Practical Example: Encryption

Inputs:

• Plaintext: HELLO

• Key: 31415

Encryption Process:

Convert the letters of the plaintext to their numeric positions:

H = 7, E = 4, L = 11, L = 11, O = 14

Repeat the key to match the length of the plaintext:

Key:  31415

Add each key digit to the corresponding plaintext letter (modulo 26):

7 + 3 = 10  → K
4 + 1 = 5   → F
11 + 4 = 15 → P
11 + 1 = 12 → M
14 + 5 = 19 → T

Resulting ciphertext:

Ciphertext: KFPMT

Practical Example: Decryption

Inputs:

• Ciphertext: KFPMT

• Key: 31415

Decryption Process:

Convert the letters of the ciphertext to their numeric positions:

K = 10, F = 5, P = 15, M = 12, T = 19

Subtract each key digit from the corresponding ciphertext letter (modulo 26):

10 - 3 = 7   → H
5 - 1 = 4    → E
15 - 4 = 11  → L
12 - 1 = 11  → L
19 - 5 = 14  → O

Resulting plaintext:

Plaintext: HELLO

Python Code

The Python code for the Gronsfeld Cipher has been implemented. It includes both encryption and decryption functions, along with an example usage.

def gronsfeld_encrypt(plaintext, key):
    # Convert key to a list of integers
    key_digits = [int(d) for d in str(key)]
    key_length = len(key_digits)

    ciphertext = ""

    for i, char in enumerate(plaintext):
        if char.isalpha():
            shift = key_digits[i % key_length]
            base = ord('A') if char.isupper() else ord('a')
            encrypted_char = chr((ord(char) - base + shift) % 26 + base)
            ciphertext += encrypted_char
        else:
            ciphertext += char  # Preserve non-alphabet characters

    return ciphertext

def gronsfeld_decrypt(ciphertext, key):
    # Convert key to a list of integers
    key_digits = [int(d) for d in str(key)]
    key_length = len(key_digits)

    plaintext = ""

    for i, char in enumerate(ciphertext):
        if char.isalpha():
            shift = key_digits[i % key_length]
            base = ord('A') if char.isupper() else ord('a')
            decrypted_char = chr((ord(char) - base - shift) % 26 + base)
            plaintext += decrypted_char
        else:
            plaintext += char  # Preserve non-alphabet characters

    return plaintext

# Example Usage
plaintext = "HELLO"
key = 31415

ciphertext = gronsfeld_encrypt(plaintext, key)
decrypted_text = gronsfeld_decrypt(ciphertext, key)

print("Plaintext:", plaintext)
print("Key:", key)
print("Ciphertext:", ciphertext)
print("Decrypted Text:", decrypted_text)

Output

Plaintext: HELLO
Key: 31415
Ciphertext: KFPMT
Decrypted Text: HELLO