The program (source given), unpacks a custom archive file. If we run strings on the archive file, however, it appears that not all the files get unpacked, specifically a hidden "flag.png" file.
000db100: 3db9 bff7 28f0 e20b bb76 0a00 00e7 1f66 =...(....v.....f
000db110: 6c61 672e 706e 6700 4f6e 58a9 15ea 15fd lag.png.OnX.....
000db120: 1fe7 12e7 57ae 4da3 1fe7 90e6 1fe7 01e7 ....W.M.........