added writeups exported from ctfnote

Quick-Response-misc.md

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+# Quick Response - misc
+
+![](https://notes.c3h2.de/pad/uploads/9a90b18f-69a4-48ea-b747-21651fbe0d41.png)
+
+
+fridgebuyer
+
+- The QR is 29x29 == version 3
+- Each module = 32x32 px
+- What should be solid is alternated. The 7x7 finder patterns should be solid but are not visible in the 3 corners (top left, bottom left, top right)
+- What should be alternated are solid lines (timing strips)
+- So the original QR was likely XORed with a Checkerboard pattern
+- https://www.pclviewer.com/rs2/qrmasking.htm Mask 000: (i + j) mod 2 = 0 is this
+- But this was applied to entire qr code, not only to the data region
+- So need to XOR every module with (row + col) % 2 again.
+
+### sol
+
+```
+from PIL import Image
+import numpy as np
+from pyzbar.pyzbar import decode
+
+img = Image.open("quick-response.png").convert("RGB")
+arr = np.array(img)
+
+# extract module grid 
+module_size = 32
+n = 29
+dark = np.array([20, 22, 27])
+
+grid = np.zeros((n, n), dtype=int)
+for r in range(n):
+    for c in range(n):
+        cy = r * module_size + module_size // 2
+        cx = c * module_size + module_size // 2
+        pixel = arr[cy, cx]
+        if np.sum((pixel.astype(int) - dark.astype(int)) ** 2) < 100:
+            grid[r, c] = 1
+
+# XOR (i + j) mod 2 = 0
+for r in range(n):
+    for c in range(n):
+        grid[r, c] ^= (r + c) % 2
+
+# write fixed QR code
+scale = 10
+border = 4
+total = (n + 2 * border) * scale
+out = Image.new("L", (total, total), 255)
+px = out.load()
+for r in range(n):
+    for c in range(n):
+        if grid[r, c] == 1:
+            for dy in range(scale):
+                for dx in range(scale):
+                    px[(c + border) * scale + dx, (r + border) * scale + dy] = 0
+
+result = decode(out)
+for r in result:
+    print(r.data.decode())
+
+```
+
+![](https://notes.c3h2.de/pad/uploads/1251408d-9160-4c39-9e94-e2897518cad9.png)
+
+**gigem{d1d_y0u_n0t1c3_th3_t1m1n9_b175}**