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)