The address 9bca65c9376209ede04b5df3b02cb832f8997ff978069d171dc9cbfca657f91a signed multiple messages using a one-time signature (merkle root: 878c9f44e5284cc3b0bd9ade03e5ad023f323f830b2588756ec5ed84ee124fb4) using this one could calculate the secret key of his, and sign any message.
More info: [https://github.com/NicolaiSoeborg/ctf-writeups/tree/master/2019/Facebook%20CTF/postquantumsig](https://github.com/NicolaiSoeborg/ctf-writeups/tree/master/2019/Facebook%20CTF/postquantumsig)