If $p-1$ is a power of $2$ where $p$ is prime how to prove that $p=2^{2^s}+1$ for some $s \in \Bbb N$.

Surely, $p=2^k+1$ then what will we use next?

P.S: I got it finally. We have to use $(a-b)|(a^n -b^n)$ then we have to use that $(2^r+1)|(2^k+1)$ if $k=rs$ with $s$ odd; leading to a contradiction.

So I am using a tag of proof verification.