Broken Signals

Task description

Our signals intelligence team intercepted a file during a routine sweep, but it appears corrupted. The file cannot be opened by any standard application. Something is wrong with the file itself — can you repair it and recover the hidden message?

Analysis

Inspecting the first and last bytes of the provided broken_signal.dat (15,916 bytes):

Head: 00000000 0d0a1a0a 0000000d 49484452 00000190 0000012c 10060000 00bd2739 81...
Tail: ...00000000 49454e44 ae426082

We immediately recognize the structure of a PNG image:

• Bytes 0x04..0x07 are 0d 0a 1a 0a, which is the second half of the 8-byte PNG file signature (89 50 4e 47 0d 0a 1a 0a).
• Immediately following is the IHDR chunk (0000000d 49484452) specifying a 400x300 16-bit RGBA image, with a valid CRC-32 (0xbd273981).
• All subsequent chunks (cHRM, bKGD, tIME, tEXt, IDAT, and IEND) are intact and pass CRC-32 verification.
• Only the first 4 bytes of the PNG magic header were zeroed out (00 00 00 00 instead of 89 50 4e 47 / \x89PNG).

Solution

We restore the first 4 bytes of broken_signal.dat to \x89PNG (89 50 4e 47):

with open("broken_signal.dat", "rb") as f:
    data = bytearray(f.read())

data[:4] = b"\x89PNG"

with open("repaired.png", "wb") as f:
    f.write(data)

Opening the repaired repaired.png image reveals the text:

SIGNAL INTERCEPTED
hiCTF{h34d3r_r3p41r_m4st3r}