diff --git a/ciphers/running_key_cipher.py b/ciphers/running_key_cipher.py
new file mode 100644
index 000000000..6bda417be
--- /dev/null
+++ b/ciphers/running_key_cipher.py
@@ -0,0 +1,75 @@
+"""
+https://en.wikipedia.org/wiki/Running_key_cipher
+"""
+
+
+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 = []
+    ord_a = ord("A")
+
+    for i, char in enumerate(plaintext):
+        p = ord(char) - 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 = []
+    ord_a = ord("A")
+
+    for i, char in enumerate(ciphertext):
+        c = ord(char) - 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() -> None:
+    """
+    >>> key = "How does the duck know that? said Victor"
+    >>> ciphertext = running_key_encrypt(key, "DEFEND THIS")
+    >>> running_key_decrypt(key, ciphertext) == "DEFENDTHIS"
+    True
+    """
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+    test_running_key_encrypt()
+
+    plaintext = input("Enter the plaintext: ").upper()
+    print(f"\n{plaintext = }")
+
+    key = "How does the duck know that? said Victor"
+    encrypted_text = running_key_encrypt(key, plaintext)
+    print(f"{encrypted_text = }")
+
+    decrypted_text = running_key_decrypt(key, encrypted_text)
+    print(f"{decrypted_text = }")