Given a natural number $n$ and an element $k \in \mathbb{Z}_n$, how many solutions are there in $\mathbb{Z}_n$ to the equation  $x^2+y^2 =k$? That is, I'm wondering whether there is a mod-$n$ version of the sum of squares function discussed in [this post](https://mathoverflow.net/questions/29644/enumerating-ways-to-decompose-an-integer-into-the-sum-of-two-squares).

As an example, to write 1 as a sum of squares modulo 7, one has permutations of $0^2+1^2$ but also $2^2+2^2$ and $2^2+5^2$ and $5^2+5^2$.