How to encode a string for saving into cookie

Hi, guys, Do you know how to encode and decode a string with a key, i want to encode a string and save it to the customize cookie, then get it back from this cookie and decode , such as ,

str = ‘myString’ key = ‘234d98’

encode(str, key) #maybe output SX23CVX23VXC422V234S decode(str, key) #myString

The result of encode string is difference when the key is difference . thanks all.