TLDR;
- Server encrypts either a quadratic residue or a quadratic nonresidue using ElGamal
- Challenges essentially requires us to determine whether or not the plaintext was a quadratic residue or a quadratic nonresidue
- Use quadratic residue properties to determine the answer

[writeup](https://jsur.in/posts/2020-03-30-volgactf-2020-qualifier-writeups#guess)

Original writeup (https://jsur.in/posts/2020-03-30-volgactf-2020-qualifier-writeups#guess).