Tags: coppersmith 

Rating: 0

tl;dr:
1. Notice that only a handful of `a/b` values meet the bitlength requirement
2. For each `a/b` brute-force `r` and use Coppersmith to recover unknown high bits

Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/primordial