added gronsfeld cipher implementation

Author: Lonercode

ciphers/gronsfeld_cipher.py

@@ -0,0 +1,40 @@
+"""
+Python implementation of the Gronsfeld cipher encryption
+
+The Gronsfeld cipher is similar to the Vigenere cipher,
+with the exception that the key is numeric.
+
+https://www.dcode.fr/gronsfeld-cipher
+"""
+
+def gronsfeld(text: str, key: str) -> str:
+    """
+    Encrypt plaintext with the Gronsfeld cipher
+
+    >>> gronsfeld('hello', '412')
+    'LFNPP'
+    >>> gronsfeld('', '123')
+    ''
+    """
+    alphabets = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+    keys = [int(i) for i in key]
+    encrypted_text = ""
+    upper_case_text = text.upper()
+
+    for i, char in enumerate(upper_case_text):
+        if char in alphabets:
+            new_position = (alphabets.index(char) +
+            keys[i % len(keys)]) % len(alphabets)
+            shifted_letter = alphabets[new_position]
+            encrypted_text += shifted_letter
+        else:
+            encrypted_text += char
+
+    return encrypted_text
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+    encrypted = gronsfeld("hello world", "123")
+    print(f"Encrypted text is {encrypted}")