Game

solve.py

import base64 as b64


def bintotext(ress, count=7):
    res = ""
    for i in range(0, len(ress), count):
        res += chr(int(ress[i : i + count], 2))
    return res


import cv2 as cv
import numpy as np

img = cv.imread("dct.jpg", cv.IMREAD_GRAYSCALE)
height, width = img.shape
blocks = []
for i in range(0, height, 8):
    for j in range(0, width, 8):
        block = img[i : i + 8, j : j + 8]
        dct = cv.dct(np.float32(block))
        blocks.append(dct)

result = ""
for block in blocks:
    if block[3, 0] > block[1, 2]:
        result += "1"
    else:
        result += "0"

print(bintotext(result, 8))

todct.py

fl = "It's hard to write flag to DCT.flag{64499398}"
fl = "".join([bin(ord(i))[2:].zfill(8) for i in fl])

import cv2 as cv
import numpy as np

r = cv.cvtColor(cv.imread("use2.jpg"), cv.COLOR_BGR2GRAY)
height, width = r.shape
idx = 0
for i in range(0, height, 8):
    for j in range(0, width, 8):
        if idx >= len(fl):
            break
        block = r[i : i + 8, j : j + 8]
        dct = cv.dct(np.float32(block))
        dct[3, 0] = dct[1, 2] + 10 if fl[idx] == "1" else dct[1, 2] - 10
        r[i : i + 8, j : j + 8] = np.uint8(cv.idct(dct))

        idx += 1
# cv.save("dct.jpg", r)

cv.imwrite("dct.jpg", r)