Add Gronsfeld cipher implementation

ciphers/gronsfeld.py

@@ -0,0 +1,63 @@
+def gronsfeld_encrypt(plaintext, key):
+    """
+    Encrypts a plaintext using the Gronsfeld cipher.
+
+    Parameters:
+    plaintext (str): The message to be encrypted.
+    key (str): A numeric key used for encryption (e.g., "31415").
+
+    Returns:
+    str: The encrypted message.
+    """
+    ciphertext = ""
+    key = [int(k) for k in key]
+    key_length = len(key)
+
+    for i, letter in enumerate(plaintext):
+        if letter.isalpha():
+            shift = key[i % key_length]
+            base = ord("A") if letter.isupper() else ord("a")
+            # Shift and wrap around the alphabet
+            ciphertext += chr((ord(letter) - base + shift) % 26 + base)
+        else:
+            ciphertext += letter
+
+    return ciphertext
+
+
+def gronsfeld_decrypt(ciphertext, key):
+    """
+    Decrypts a ciphertext using the Gronsfeld cipher.
+
+    Parameters:
+    ciphertext (str): The message to be decrypted.
+    key (str): A numeric key used for decryption (e.g., "31415").
+
+    Returns:
+    str: The decrypted message.
+    """
+    plaintext = ""
+    key = [int(k) for k in key]
+    key_length = len(key)
+
+    for i, letter in enumerate(ciphertext):
+        if letter.isalpha():
+            shift = key[i % key_length]
+            base = ord("A") if letter.isupper() else ord("a")
+            # Reverse the shift to decrypt
+            plaintext += chr((ord(letter) - base - shift) % 26 + base)
+        else:
+            plaintext += letter
+
+    return plaintext
+
+
+# Example usage:
+plaintext = input("Enter a message to encrypt: ")
+key = input("Enter a key (e.g., 31415): ")
+
+encrypted_message = gronsfeld_encrypt(plaintext, key)
+print(f"Encrypted: {encrypted_message}")
+
+decrypted_message = gronsfeld_decrypt(encrypted_message, key)
+print(f"Decrypted: {decrypted_message}")