tldr;- this is based on biased nonce attacks on ECDSA https://eprint.iacr.org/2019/023.pdf- the signature scheme is essentially ECDSA, except slightly different- there is a bias in both the LSB and the MSB- find the hidden number problem instance and solve with LLL- recover the private key, sign the auth message, and win!
[DUCTF GitHub](https://github.com/DownUnderCTF/Challenges_2020_public/tree/master/crypto/impeccable)
[writeup](https://jsur.in/posts/2020-09-20-downunderctf-2020-writeups#impeccable)