TLDR;
- Encryption oracle where the message is encrypted by evaluating it on some polynomial in $\mathbb{F}_P[x]$ with prime $P$
- Constant term of the polynomial is the flag
- We have as many encryptions as we like
- Use polynomial interpolation to recover the coefficients and get the flag
- (note) Asking for the encryption of $P$ itself is not allowed. If it were, the encryption would be the flag

[writeup](https://jsur.in/posts/2020-04-14-hexionctf-2020-writeups)

Original writeup (https://jsur.in/posts/2020-04-14-hexionctf-2020-writeups).