Caesar Cipher Lite

You recently read about the Caesar Cipher, one of the first and the easiest to break cryptographic algorithms known. Since it was very easy to break, you decided to improve it by building a lite version of it. Here is your modified algorithm. Suppose you want to encrypt a plaintext $P$ of length $N$. First, you generate a secret cryptographic key $K$ which is an array of length $N$. Then, for