c0rrupt

Problem

We found this file. Recover the flag. You can also find the file in /problems/c0rrupt_0_1fcad1344c25a122a00721e4af86de13.

The File

Solution

Checking the file mystery indicates nothing. However, viewing the head of the mystery file shows it looks like a PNG image:

 $ xxd -g 1 mystery | head
 00000000: 89 65 4e 34 0d 0a b0 aa 00 00 00 0d 43 22 44 52  .eN4........C"DR
 00000010: 00 00 06 6a 00 00 04 47 08 02 00 00 00 7c 8b ab  ...j...G.....|..
 00000020: 78 00 00 00 01 73 52 47 42 00 ae ce 1c e9 00 00  x....sRGB.......
 00000030: 00 04 67 41 4d 41 00 00 b1 8f 0b fc 61 05 00 00  ..gAMA......a...
 00000040: 00 09 70 48 59 73 aa 00 16 25 00 00 16 25 01 49  ..pHYs...%...%.I
 00000050: 52 24 f0 aa aa ff a5 ab 44 45 54 78 5e ec bd 3f  R$......DETx^..?
 00000060: 8e 64 cd 71 bd 2d 8b 20 20 80 90 41 83 02 08 d0  .d.q.-.  ..A....
 00000070: f9 ed 40 a0 f3 6e 40 7b 90 23 8f 1e d7 20 8b 3e  [email protected]@{.#... .>
 00000080: b7 c1 0d 70 03 74 b5 03 ae 41 6b f8 be a8 fb dc  ...p.t...Ak.....
 00000090: 3e 7d 2a 22 33 6f de 5b 55 dd 3d 3d f9 20 91 88  >}*"3o.[U.==. ..

The PNG Specification tells that the PNG signature (first eight bytes of a PNG datastream) must be 137 80 78 71 13 10 26 10 (decimal) or 89 50 4E 47 0D 0A 1A 0A (hex).

So lets replace the signature using a hex editor:

 $ xxd -g 1 mystery.png | head
 00000000: 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 43 22 44 52  .PNG........C"DR
 00000010: 00 00 06 6a 00 00 04 47 08 02 00 00 00 7c 8b ab  ...j...G.....|..
 00000020: 78 00 00 00 01 73 52 47 42 00 ae ce 1c e9 00 00  x....sRGB.......
 00000030: 00 04 67 41 4d 41 00 00 b1 8f 0b fc 61 05 00 00  ..gAMA......a...
 00000040: 00 09 70 48 59 73 aa 00 16 25 00 00 16 25 01 49  ..pHYs...%...%.I
 00000050: 52 24 f0 aa aa ff a5 ab 44 45 54 78 5e ec bd 3f  R$......DETx^..?
 00000060: 8e 64 cd 71 bd 2d 8b 20 20 80 90 41 83 02 08 d0  .d.q.-.  ..A....
 00000070: f9 ed 40 a0 f3 6e 40 7b 90 23 8f 1e d7 20 8b 3e  [email protected]@{.#... .>
 00000080: b7 c1 0d 70 03 74 b5 03 ae 41 6b f8 be a8 fb dc  ...p.t...Ak.....
 00000090: 3e 7d 2a 22 33 6f de 5b 55 dd 3d 3d f9 20 91 88  >}*"3o.[U.==. ..

After the header come a series of chunks. Each chunk starts with 4 bytes for the length of the chunk, 4 bytes for the type, then the chunk content itself (with the length declared earlier) and 4 bytes of a checksum. See the "5.3 Chunk layout" section of this page for more information.

The first chunk is called IHDR and has the length of 0xD, so we know that the next 8 bytes are 00 00 00 0D 49 48 44 52 (hex):

 $ xxd -g 1 mystery.png | head
 00000000: 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52  .PNG........IHDR
 00000010: 00 00 06 6a 00 00 04 47 08 02 00 00 00 7c 8b ab  ...j...G.....|..
 00000020: 78 00 00 00 01 73 52 47 42 00 ae ce 1c e9 00 00  x....sRGB.......
 00000030: 00 04 67 41 4d 41 00 00 b1 8f 0b fc 61 05 00 00  ..gAMA......a...
 00000040: 00 09 70 48 59 73 aa 00 16 25 00 00 16 25 01 49  ..pHYs...%...%.I
 00000050: 52 24 f0 aa aa ff a5 ab 44 45 54 78 5e ec bd 3f  R$......DETx^..?
 00000060: 8e 64 cd 71 bd 2d 8b 20 20 80 90 41 83 02 08 d0  .d.q.-.  ..A....
 00000070: f9 ed 40 a0 f3 6e 40 7b 90 23 8f 1e d7 20 8b 3e  [email protected]@{.#... .>
 00000080: b7 c1 0d 70 03 74 b5 03 ae 41 6b f8 be a8 fb dc  ...p.t...Ak.....
 00000090: 3e 7d 2a 22 33 6f de 5b 55 dd 3d 3d f9 20 91 88  >}*"3o.[U.==. ..

Now the file is identified as a PNG file:

 $ file fixed.png
 fixed.png: PNG image data, 1642 x 1095, 8-bit/color RGB, non-interlaced

pngcheck lists two more errors to solve:

 pngcheck -vf mystery.png
 File: mystery.png (202940 bytes)
 chunk IHDR at offset 0x0000c, length 13
     1642 x 1095 image, 24-bit RGB, non-interlaced
 chunk sRGB at offset 0x00025, length 1
     rendering intent = perceptual
 chunk gAMA at offset 0x00032, length 4: 0.45455
 chunk pHYs at offset 0x00042, length 9: 2852132389x5669 pixels/meter
 CRC error in chunk pHYs (computed 38d82c82, expected 495224f0)
 :  invalid chunk length (too large)

We need to fix the CRC (checksum) error in chunk pHYs and find a chunk with an invalid length.

The CRC error means that either the checksum (CRC value) is corrupted, or the data is. To solve this error in chunk pHYs we can simply replace the CRC (expected) value with the computed value. The purpose of the CRC section is to check for corruption of the data. It essentially is a checksum of that chunk. While replacing the CRC will work, the more "correct" method is to correct the content of pHYs in order to get the same CRC, thus fixing the corruption that occurred. The below table shows the layout of the pHYs chunk:

Name

Length

Current Value

Pixels per unit, X axis

4 bytes (PNG unsigned integer)

aa 00 16 25

Pixels per unit, Y axis

4 bytes (PNG unsigned integer)

00 00 16 25

Unit specifier

1 byte

Since the pixels per unit differ in just one byte, and the 0xaa for the X axis makes the value very large, it makes sense to place a zero instead. This fixes the checksum.

 $ xxd -g 1 mystery.png | head
 00000000: 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52  .PNG........IHDR
 00000010: 00 00 06 6a 00 00 04 47 08 02 00 00 00 7c 8b ab  ...j...G.....|..
 00000020: 78 00 00 00 01 73 52 47 42 00 ae ce 1c e9 00 00  x....sRGB.......
 00000030: 00 04 67 41 4d 41 00 00 b1 8f 0b fc 61 05 00 00  ..gAMA......a...
 00000040: 00 09 70 48 59 73 00 00 16 25 00 00 16 25 01 49  ..pHYs...%...%.I
 00000050: 52 24 f0 aa aa ff a5 ab 44 45 54 78 5e ec bd 3f  R$......DETx^..?
 00000060: 8e 64 cd 71 bd 2d 8b 20 20 80 90 41 83 02 08 d0  .d.q.-.  ..A....
 00000070: f9 ed 40 a0 f3 6e 40 7b 90 23 8f 1e d7 20 8b 3e  [email protected]@{.#... .>
 00000080: b7 c1 0d 70 03 74 b5 03 ae 41 6b f8 be a8 fb dc  ...p.t...Ak.....
 00000090: 3e 7d 2a 22 33 6f de 5b 55 dd 3d 3d f9 20 91 88  >}*"3o.[U.==. ..

 $ pngcheck -v -f mystery.png
 File: fixed.png (202940 bytes)
 chunk IHDR at offset 0x0000c, length 13
     1642 x 1095 image, 24-bit RGB, non-interlaced
 chunk sRGB at offset 0x00025, length 1
     rendering intent = perceptual
 chunk gAMA at offset 0x00032, length 4: 0.45455
 chunk pHYs at offset 0x00042, length 9: 5669x5669 pixels/meter (144 dpi)
 :  invalid chunk length (too large)
 ERRORS DETECTED in fixed.png

The invalid chunk length (too large) error does not specify a chunk, so we must begin at the start and check each chunk, with the knowledge of the format of chunks and each field’s length: 4 bytes (length) - 4 bytes (chunk type) - lengthbytes (data) - 4 bytes (CRC).
The chunk after pHYs has a size of 0xaaaaffa5, which is very large, and a type of \xabDET which doesn't exist. The closest chunk type is IDAT. Let's fix that by replaying the chunk name with 49 44 41 54 (hex).
To solve the large chunk length we need to calculate the chunk length and update its value. IDAT chunks must be consecutive so lets find the next one. We find the next IDAT at offset 0x10008. The first IDAT was at offset 0x57. The difference is FFB1. We must subtract 4 bytes for the length field of the second IDAT, subtract 4 bytes for the CRC of the first IDAT, and subtract 4 bytes again for the chunktype of the first IDAT. Subtracting 12 in total, we get FFA5. Replace the length field with 00 00 FF A5 (hex). The original value was 0xAAAAFFA5 so we just needed to overwrite the AAAA with 0000.
Running pngcheck mystery.png shows mystery.png (1642x1095, 24-bit RGB, non-interlaced, 96.3%). which means there are no errors and we can open the image:

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picoCTF{c0rrupt10n_1847995}

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