DEFCON 18 CTF quals - Forensic 200 writeup

Posted on July 01, 2010 in Blog • 6 min read

Second forensic challange of the DEFCON 18 CTF qualifications: the suggestion was “find the key” and the related file is here. (Mirrors: #1)

Trying to identify the file.

$ file f200_02b7b50f575759cff7.tar.lzma
f200_02b7b50f575759cff7.tar.lzma: data

So we can try to trust the file extension.

$ unlzma -d f200_02b7b50f575759cff7.tar.lzma

$ tar xvf f200_02b7b50f575759cff7.tar
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IMG_0896.png
IMG_0897.png
IMG_0898.png
IMG_0899.png
IMG_0900.png
IMG_0901.png
IMG_0902.png
IMG_0903.png
IMG_0904.png
IMG_0905.png
IMG_0906.png
IMG_0907.png
IMG_0908.png
IMG_0909.png
IMG_0910.png
IMG_0911.png
IMG_0912.png
IMG_0913.png
IMG_0914.png
IMG_0915.png
IMG_0916.png
IMG_0917.png
IMG_0918.png
IMG_0919.png
IMG_0920.png
IMG_0921.png
IMG_0922.png
IMG_0923.png
IMG_0924.png
IMG_0925.png
IMG_0926.png
IMG_0927.png
IMG_0928.png
IMG_0929.png
IMG_0930.png
IMG_0931.png
IMG_0932.png
IMG_0933.png
IMG_0934.png
IMG_0935.png
IMG_0936.png
IMG_0937.png
IMG_0938.png
IMG_0939.png
IMG_0940.png
IMG_0941.png
IMG_0942.png
IMG_0943.png
IMG_0944.png
IMG_0945.png
IMG_0946.png
IMG_0947.png
IMG_0948.png
IMG_0949.png
IMG_0950.png
IMG_0951.png
IMG_0952.png
IMG_0953.png
IMG_0954.png
IMG_0955.png
IMG_0956.png
IMG_0957.png
IMG_0958.png
IMG_0959.png
IMG_0960.png
IMG_0961.png
IMG_0962.png
IMG_0963.png
IMG_0964.png
IMG_0965.png
IMG_0966.png
IMG_0967.png
IMG_0968.png
IMG_0969.png
IMG_0970.png
IMG_0971.png
IMG_0972.png
IMG_0973.png
IMG_0974.png
IMG_0975.png
IMG_0976.png
IMG_0977.png
IMG_0978.png
IMG_0979.png
IMG_0980.png
IMG_0981.png
IMG_0982.png
IMG_0983.png
IMG_0984.png
IMG_0985.png
IMG_0986.png
IMG_0987.png
IMG_0988.png
IMG_0989.png
IMG_0990.png
IMG_0991.png
IMG_0992.png
IMG_0993.png
IMG_0994.png
IMG_0995.png
IMG_0996.png
IMG_0997.png
IMG_0998.png
IMG_0999.png
IMG_1000.png
IMG_1001.png
IMG_1002.png
IMG_1003.png
IMG_1004.png
IMG_1005.png
IMG_1006.png
IMG_1007.png
IMG_1008.png
IMG_1009.png
IMG_1010.png
IMG_1011.png
IMG_1012.png
IMG_1013.png
IMG_1014.png
IMG_1015.png
IMG_1016.png
IMG_1017.png
IMG_1018.png
IMG_1019.png
IMG_1020.png
IMG_1021.png
IMG_1022.png
IMG_1023.png
IMG_1024.png
IMG_1025.png
IMG_1026.png
IMG_1027.png
IMG_1028.png
IMG_1029.png
IMG_1030.png
IMG_1031.png
IMG_1032.png
IMG_1033.png
IMG_1034.png
IMG_1035.png
IMG_1036.png
IMG_1037.png
IMG_1038.png
IMG_1039.png
IMG_1040.png
IMG_1041.png
IMG_1042.png
IMG_1043.png
IMG_1044.png
IMG_1045.png
IMG_1046.png
IMG_1047.png
IMG_1048.png
IMG_1049.png
IMG_1050.png
IMG_1051.png
IMG_1052.png
IMG_1053.png
IMG_1054.png
IMG_1055.png
IMG_1056.png
IMG_1057.png
IMG_1058.png
IMG_1059.png
IMG_1060.png
IMG_1061.png
IMG_1062.png
IMG_1063.png
IMG_1064.png
IMG_1065.png
IMG_1066.png
IMG_1067.png
IMG_1068.png
IMG_1069.png
IMG_1070.png
IMG_1071.png
IMG_1072.png
IMG_1073.png
IMG_1074.png
IMG_1075.png
IMG_1076.png
IMG_1077.png
IMG_1078.png
IMG_1079.png
IMG_1080.png
IMG_1081.png
IMG_1082.png
IMG_1083.png
IMG_1084.png
IMG_1085.png
IMG_1086.png
IMG_1087.png
IMG_1088.png
IMG_1089.png
IMG_1090.png
IMG_1091.png
IMG_1092.png
IMG_1093.png
IMG_1094.png
IMG_1095.png
IMG_1096.png
IMG_1097.png
IMG_1098.png
IMG_1099.png
IMG_1100.png
IMG_1101.png
IMG_1102.png
IMG_1103.png
IMG_1104.png
IMG_1105.png
IMG_1106.png
IMG_1107.png
IMG_1108.png
IMG_1109.png
IMG_1110.png
IMG_1111.png
IMG_1112.png
IMG_1113.png
IMG_1114.png
IMG_1115.png
IMG_1116.png
IMG_1117.png
IMG_1118.png
IMG_1119.png
IMG_1120.png
IMG_1121.png

There are a lot of images, apparently all of the same size. We can try to use pngtools the examine the respect of image attributes and the presence of metadata.

$ pnginfo IMG_1116.png
IMG_1116.png...
Image Width: 118 Image Length: 810
Bitdepth (Bits/Sample): 8
Channels (Samples/Pixel): 4
Pixel depth (Pixel Depth): 32
Colour Type (Photometric Interpretation): RGB with alpha channel
Image filter: Single row per byte filter
Interlacing: No interlacing
Compression Scheme: Deflate method 8, 32k window
Resolution: 0, 0 (unit unknown)
FillOrder: msb-to-lsb
Byte Order: Network (Big Endian)
Number of text strings: 0 of 0

There are no interesting metadata. But it seems that each image contains some pixels, we can try to compose all the images.

$ convert -background skyblue *.png -layers flatten +repage
Flatten.png

And inside there is the key: http://is.gd/ced7F