Logical Alternating Shift Cypher

Made to talk with a friend without eavesdroppers.

How does it work?

The tool uses a numerical key to shift the message's characters.
Only alphabetical characters are shifted.

Say the key is '1234' and the message is "Hello world".
We start by shifting "H" to the right once: "I".
Then we shift "e" to the left twice: "c".
We keep doing this until we get: "Icohp urnmb".

The alphabet and key are wrapped around.
So the actual key is 123412341234123412...