## The Frobenius endomorphism with finite fields

The Frobenius endomorphism is defined as:

$\Phi(x)=x^p$

where p is the characteristic of the ring you’re working with. Simple, right?

If you’re working with a field with prime order, then Frobenius is actually the identity map. Since the order of the multiplicative subgroup is p, when you raise to the power of p you get back to x due to Fermat’s little theorem. Things get more interesting when you’re working with a extension field (i.e. a field which order is a prime power).

I’m studying pairings for my master’s degree and the Frobenius endomorphism appears all the time in their computation. For example, you need to do a “final exponentation” which can be split in multiple exponentiations, and some of them are to the power of p. This is good because powering to p is “easy” due to Frobenius, or at least all the papers I read said so. But for a while I couldn’t see why, and that’s the reason I’m posting this. It’s really easy; it’s just not that obvious to see why.