Volga 2019 - shifter (Crypto, 150)

Challenge

Okay, you already know what to do with it...

File : task.py


Solution

We can use Berlekamp Massey Algorithm to find the branch if we know the segment of LFSR generate sequence.

Besides, we know that the code read the file flag.html, we can guess there is the flag starts with VolgaCTF{ inside it.

This VolgaCTF{ contains 72 bits, It’s enough to recover the register and branches.

Therefore, just brute force to search the index of well-known string VolgaCTF{

If we find the location of string and recover well, the output file will end with </html>

VolgaCTF{It_h45_alr3ady_b3c0m3_a_cl4ssic}

solve.py