can you help me come up with a secret code? and explain why you think it's a mathematical exercise.

Here's an example:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z E F G H I J K L M N O P Q R S T U V W X Y Z A B C D

If I want to encode my friend's dog's name, BANGU, I find the letters in the top row and replace them with the letters in the bottom row:

BANGU FERKY is the coded name

I can uncode this way

IXLER which is my coded friend's son's name

ETHAN I started on the bottom row to decode and replaced the letters with the top row