Check is done via $m = s^e \mod{n}$.  It's obvious that $m = 0$ and $s = 0$ is a feasible solution for this equation.
```from pwn import *
r = remote('rsasign1.2017.teamrois.cn', 3000)print r.recvuntil('\n')r.send('\x00'*125)  # message with paddingprint r.recvuntil('\n')print r.recvuntil('\n')r.send('00')  # signatureprint r.recv(1024)```
The flag is RCTF{your_fir5t_sig_test}