adds a rot with key example

learning_python3.md

@@ -1624,6 +1624,51 @@ The third letter by ROT2 so will become `n`.
 Now, the fourth letter will be offset again by `a` so ROT0 and will become `l`.
 And so on...
 
+<details>
+  <summary>Spoiler warning</summary>
+
+```python
+import string
+
+
+def encrypt_message(msg, rot=13):
+    encoded_message = []
+    for letter in msg:
+        if letter in alphabet:
+            letter_index = alphabet.index(letter)
+            encoded_letter_index = (letter_index + rot) % 26
+            encoded_letter = alphabet[encoded_letter_index]
+            encoded_message.append(encoded_letter)
+        else:
+            encoded_message.append(letter)
+    encoded_message = "".join(encoded_message)
+    return encoded_message
+
+
+def encrypt_with_key(msg, key):
+    encrypted_message = []
+    counter = 0
+    for letter in msg:
+        key_letter = key[counter % len(key)]
+        key_index = alphabet.index(key_letter)
+        encrypted_letter = encrypt_message(letter, key_index)
+        counter += 1
+        encrypted_message.append(encrypted_letter)
+    return "".join(encrypted_message)
+
+
+if __name__ == "__main__":
+    alphabet = list(string.ascii_lowercase)
+    key = "mehdi"
+    print("hello, I'm an encryption program!")
+    result = input("what is your secret? ")
+    secret = encrypt_with_key(result, key)
+    print("your secret message is: {}".format(secret)) 
+
+```
+
+</details>
+
 # Coding challenge - Christmas Tree
 
 Can you make me a program that draws various size Christmas trees?
@@ -1834,6 +1879,12 @@ For some inspiration for a simple question/response system you can have a look [
 
 # Dictionaries as data containers
 
+## Two dimensional lists
+
+TODO - Currency converter with lists.
+
+## Dictionaries
+
 Of the *built-in* types we first say `str`, `int` and `float`.
 Next we saw *sequence* types such as `list` and `tupple`.
 Now we'll dive into a **mapping type** called `dict`.