Add running key cipher

ciphers/running_key_cipher.py

@@ -0,0 +1,69 @@
+def running_key_encrypt(key: str, plaintext: str) -> str:
+    """
+    Encrypts the plaintext using the Running Key Cipher.
+
+    :param key: The running key (long piece of text).
+    :param plaintext: The plaintext to be encrypted.
+    :return: The ciphertext.
+    """
+    plaintext = plaintext.replace(" ", "").upper()
+    key = key.replace(" ", "").upper()
+    key_length = len(key)
+    ciphertext = []
+
+    for i in range(len(plaintext)):
+        p = ord(plaintext[i]) - ord("A")
+        k = ord(key[i % key_length]) - ord("A")
+        c = (p + k) % 26
+        ciphertext.append(chr(c + ord("A")))
+
+    return "".join(ciphertext)
+
+
+def running_key_decrypt(key: str, ciphertext: str) -> str:
+    """
+    Decrypts the ciphertext using the Running Key Cipher.
+
+    :param key: The running key (long piece of text).
+    :param ciphertext: The ciphertext to be decrypted.
+    :return: The plaintext.
+    """
+    ciphertext = ciphertext.replace(" ", "").upper()
+    key = key.replace(" ", "").upper()
+    key_length = len(key)
+    plaintext = []
+
+    for i in range(len(ciphertext)):
+        c = ord(ciphertext[i]) - ord("A")
+        k = ord(key[i % key_length]) - ord("A")
+        p = (c - k) % 26
+        plaintext.append(chr(p + ord("A")))
+
+    return "".join(plaintext)
+
+
+def test_running_key_encrypt():
+    """
+    >>> key = "How does the duck know that? said Victor"
+    >>> plaintext = "DEFEND THIS"
+    >>> ciphertext = running_key_encrypt(key, plaintext)
+    >>> decrypted_text = running_key_decrypt(key, ciphertext)
+    >>> decrypted_text == "DEFENDTHIS"
+    True
+    """
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+    test_running_key_encrypt()
+
+    key = "How does the duck know that? said Victor"
+    plaintext = input("Enter the plaintext: ").upper()
+    encrypted_text = running_key_encrypt(key, plaintext)
+    decrypted_text = running_key_decrypt(key, encrypted_text)
+
+    print("\nPlaintext:", plaintext)
+    print("Encrypted:", encrypted_text)
+    print("Decrypted:", decrypted_text)