added gronsfeld cipher implementation

ciphers/gronsfeld_cipher.py

@@ -0,0 +1,45 @@
+from string import ascii_uppercase
+
+
+def gronsfeld(text: str, key: str) -> str:
+    """
+    Encrypt plaintext with the Gronsfeld cipher
+
+    >>> gronsfeld('hello', '412')
+    'LFNPP'
+    >>> gronsfeld('hello', '123')
+    'IGOMQ'
+    >>> gronsfeld('', '123')
+    ''
+    >>> gronsfeld('yes, ¥€$ - _!@#%?', '0')
+    'YES, ¥€$ - _!@#%?'
+    >>> gronsfeld('yes, ¥€$ - _!@#%?', '01')
+    'YFS, ¥€$ - _!@#%?'
+    >>> gronsfeld('yes, ¥€$ - _!@#%?', '012')
+    'YFU, ¥€$ - _!@#%?'
+    >>> gronsfeld('yes, ¥€$ - _!@#%?', '')
+    Traceback (most recent call last):
+      ...
+    ZeroDivisionError: integer modulo by zero
+    """
+    ascii_len = len(ascii_uppercase)
+    key_len = len(key)
+    encrypted_text = ""
+    keys = [int(char) for char in key]
+    upper_case_text = text.upper()
+
+    for i, char in enumerate(upper_case_text):
+        if char in ascii_uppercase:
+            new_position = (ascii_uppercase.index(char) + keys[i % key_len]) % ascii_len
+            shifted_letter = ascii_uppercase[new_position]
+            encrypted_text += shifted_letter
+        else:
+            encrypted_text += char
+
+    return encrypted_text
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()