code: def findpk(self, message, rnum, snum, flag):
          
          m = self.GFn.value(message)
          r = self.GFn.value(rnum)
          s = self.GFn.value(snum)
  
          R = self.ec.decompress(r, flag)
  
          # return (R*s - self.G * m)*(1/r)
          return R * (s / r) - self.G * (m / r)
docs: find pubkey Y from message m, signature (r,s)
          Y = (R*s-G*m)/r
          note that there are 2 pubkeys related to a signature