diff --git a/ciphers/mono_alphabetic_ciphers.py b/ciphers/mono_alphabetic_ciphers.py
new file mode 100644
index 0000000000000000000000000000000000000000..0a29d64428969615e9ded5b27fa263b1199d7bd6
--- /dev/null
+++ b/ciphers/mono_alphabetic_ciphers.py
@@ -0,0 +1,59 @@
+LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+
+def translate_message(key, message, mode):
+    """
+    >>> translate_message("QWERTYUIOPASDFGHJKLZXCVBNM","Hello World","encrypt")
+    'Pcssi Bidsm'
+    """
+    chars_a = LETTERS if mode == "decrypt" else key
+    chars_b = key if mode == "decrypt" else LETTERS
+    translated = ""
+    # loop through each symbol in the message
+    for symbol in message:
+        if symbol.upper() in chars_a:
+            # encrypt/decrypt the symbol
+            sym_index = chars_a.find(symbol.upper())
+            if symbol.isupper():
+                translated += chars_b[sym_index].upper()
+            else:
+                translated += chars_b[sym_index].lower()
+        else:
+            # symbol is not in LETTERS, just add it
+            translated += symbol
+    return translated
+
+
+def encrypt_message(key: str, message: str) -> str:
+    """
+    >>> encrypt_message("QWERTYUIOPASDFGHJKLZXCVBNM", "Hello World")
+    'Pcssi Bidsm'
+    """
+    return translate_message(key, message, "encrypt")
+
+
+def decrypt_message(key: str, message: str) -> str:
+    """
+    >>> decrypt_message("QWERTYUIOPASDFGHJKLZXCVBNM", "Hello World")
+    'Itssg Vgksr'
+    """
+    return translate_message(key, message, "decrypt")
+
+
+def main():
+    message = "Hello World"
+    key = "QWERTYUIOPASDFGHJKLZXCVBNM"
+    mode = "decrypt"  # set to 'encrypt' or 'decrypt'
+
+    if mode == "encrypt":
+        translated = encrypt_message(key, message)
+    elif mode == "decrypt":
+        translated = decrypt_message(key, message)
+    print(f"Using the key {key}, the {mode}ed message is: {translated}")
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+    main()