TetCTF is the first CTF I have played in 2021. I recalled from last year that they have cool challenges. This year, there are three crypto challenges. In particular, unevaluated is the hardest among them. Although I did not solve them, I dug into rabbit holes and had a lot of struggle, uh, fun.
Challenge Summary There is a 128-bit prime $p$. Define $\cdot: \mathbb{Z}_{p^2}^2\times\mathbb{Z}_{p^2}^2\rightarrow\mathbb{Z}_{p^2}^2$ by
\[(x_1, y_1)\cdot(x_2, y_2) := \left(\left(x_1x_2-y_1y_2\right)\ \text{mod}\ p^2, \left(x_1y_2+y_1x_2\right)\ \text{mod}\ p^2\right),\]...