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Corrupted File

  1. As the problem states, the solution lies in the header of the given file:

    bash head unopenable.gif | xxd

  2. Some googling told that GIF files start with GIF87a or GIF89a, but the given file's header begins with 9a. Edit the header accordingly, Voila!, now the GIF is openable.

  3. Use some online tool to get the frames of the GIF, which give us the encoded flag.

  4. Decode the flag:

    bash echo "<encoded-flag>" | base64 -d