Elliptigo

BuckeyeCTF

We are given an implementation of the 25519 curve and are allowed to give a base point use to generate a private key for AES. Since we can choose the base point (The generator we should try to generate an anomalous curve).

By looking here, we can see there already is some point that have a low order. We can take any and then we can decrypt the message with either (0, 1 the point or the other 5 possibilities).

This gives us the flag buckeye{p01nt5_0f_l0w_0rd3r}

Original writeup (https://github.com/Y-CTF/writeups/blob/main/BuckeyeCTF/crypto/Elliptigo/Elliptigo.md).