The most basic encryption method is to map a char to another char by a certain math rule.
Because every char has an ASCII value, we can manipulate this value with a simple math expression. 
For example 'a' + 1 would give us 'b', because 'a' value is 97 and 'b' value is 98.
You will need to write a method which does exactly that - 
get a string as text and an int as the rule of manipulation, and should return encrypted text.
for example:
encrypt("a",1) = "b"
 *Full ascii table is used on our question (256 chars) - so 0-255 are the valid values.*
Good luck.
def encrypt(text, key):
    return "".join(chr((ord(ch) + key) & 255) for ch in text)
def encrypt(text, key):
    return "".join(chr((ord(ch) + key) & 255) for ch in text)