LoadSomeBits

LoadSomeBits

PROBLEM

Can you find the flag encoded inside this image? You can also find the file in /problems/loadsomebits_2_c5bba4da53a839fcdda89e5203ac44d0 on the shell server.

HINT

Look through the Least Significant Bits for the image

If you interpret a binary sequence (seq) as ascii and then try interpreting the same binary sequence from an offset of 1 (seq[1:]) as ascii do you get something similar or completely different?

SOLUTION

First it is important to know what is LSB. If you'll know what it is you'll be able to play with.

I wrote some code in python for this problem:

bits = ""
for seq in range(16):
    with open("pico2018-special-logo.bmp", "rb") as f:
        data = f.read()
        data = data[seq:]

        for c in data:
            lsb = str(c & 0x1)
            bits += lsb

        bytess = [chr(int(bits[i:i+8], 2)) for i in range(0, len(bits), 8)]
        lsbstr = "".join(bytess)

        if "picoCTF" in lsbstr:
            print(lsbstr)
            break

This will break when it encounters our flag.

FLAG - picoCTF{st0r3d_iN_tH3_l345t_s1gn1f1c4nT_b1t5_2705826400}