tldr;- Each bit of the flag is XORed with a random bit derived from 3 other bits- The way the 3 bits are combined to form the XOR has a bias (`0`: `3/8`, `1`:`5/8`)- Take the least frequent bit in each position to recover the flag
[writeup](https://jsur.in/posts/2020-08-09-poseidonctf-2020-writeups#triplet-bits-encryption)