N1CTF 2018 - Lipstick

This challenge was very interesting! It included multiple types of stego and some web scraping.

We're given the image shown above. Looking very innocuous with flat colors, it seems one would try to hide lsb data in it. Let's try zsteg to extract some values:

$ zsteg -a 603fda27-2893-4e60-9d9f-39962b8c8440.png

b1,bgr,lsb,xy       .. text: "flag.txt"
b2,g,lsb,xy         .. file: 5View capture file
b2,g,msb,xy         .. file: VISX image file
b2,b,msb,xy         .. text: "UUuUUWUUUUUuUUUU"
b2,bgr,lsb,xy       .. text: "AAAP@QUDQ"
b3,b,lsb,xy         .. file: GLS_BINARY_MSB_FIRST
b4,r,lsb,xy         .. text: "UDTTUEEDDEDEUEDDTEDETDEDDEDDDDTDUEEETDUDEUDDDDDDDDDDDDUDDDDDDDDDDDDDDDDDDDD\"322\"223##\"\"\"\"\"\"\"\"\"\"\"\"#\"#\"\"\"3\"#"
b4,r,msb,xy         .. text: "333333333;"
b4,g,lsb,xy         .. file: 5View capture file
b4,g,msb,xy         .. file: VISX image file
b4,b,msb,xy         .. text: ["w" repeated 12 times]
b6,g,msb,xy         .. text: "4ES4EQ4MS4M"

Looks like one of the first hits is promising! "flag.txt", maybe we should extract that portion alone, first let's inspect it.

Passing the -v flag into zsteg, we can get the hex dump of that extraction:

zsteg -v 603fda27-2893-4e60-9d9f-39962b8c8440.png b1,bgr,lsb,xy
b1,bgr,lsb,xy       .. text: "flag.txt"
    00000000: 00 00 00 00 00 00 00 e3  50 4b 03 04 14 00 01 08  |........PK......|
    00000010: 08 00 70 42 67 4c 37 20  ff 48 4d 00 00 00 43 00  |..pBgL7 .HM...C.|
    00000020: 00 00 08 00 00 00 66 6c  61 67 2e 74 78 74 ce 98  |......flag.txt..|
    00000030: 5e 65 7a ba 27 91 a6 eb  1b 25 54 f3 b9 6d 0f ca  |^ez.'....%T..m..|
    00000040: 78 7c 20 6a 79 e8 39 99  c8 df ad 3d 9a c8 4b fc  |x| jy.9....=..K.|
    00000050: 53 1d db 6e 95 9d 07 ae  1c 91 eb fe a7 18 33 00  |S..n..........3.|
    00000060: 9e f5 12 8e 7f c6 e6 7e  1a c8 3d cf 94 97 2f b0  |.......~..=.../.|
    00000070: 96 64 f3 a7 d1 94 64 23  98 3f 47 50 4b 01 02 3f  |.d....d#.?GPK..?|
    00000080: 00 14 00 01 08 08 00 70  42 67 4c 37 20 ff 48 4d  |.......pBgL7 .HM|
    00000090: 00 00 00 43 00 00 00 08  00 24 00 00 00 00 00 00  |...C.....$......|
    000000a0: 00 20 00 00 00 00 00 00  00 66 6c 61 67 2e 74 78  |. .......flag.tx|
    000000b0: 74 0a 00 20 00 00 00 00  00 01 00 18 00 5b d9 a1  |t.. .........[..|
    000000c0: f6 a9 b5 d3 01 a5 f5 f6  76 a9 b5 d3 01 a5 f5 f6  |........v.......|
    000000d0: 76 a9 b5 d3 01 50 4b 05  06 00 00 00 00 01 00 01  |v....PK.........|
    000000e0: 00 5a 00 00 00 73 00 00  00 00 00 92 49 24 92 49  |.Z...s......I$.I|
    000000f0: 24 92 49 24 92 49 24 92  49 24 92 49 24 92 49 24  |$.I$.I$.I$.I$.I$|

On the top there's PK, that's the magic byte for zip! A zip hidden in lsb.

Extracting it with zsteg we have some trailing bytes, we can extract the zip itself with binwalk and verify using 7z:

$ zsteg 603fda27-2893-4e60-9d9f-39962b8c8440.png -E b1,bgr,lsb,xy > out
$ binwalk -e out
$ cd _out.extracted
$ 7z l 8.zip

   Date      Time    Attr         Size   Compressed  Name
------------------- ----- ------------ ------------  ------------------------
2018-03-06 17:19:31 ....A           67           77  flag.txt
------------------- ----- ------------ ------------  ------------------------
2018-03-06 17:19:31                 67           77  1 files

Next we needed to get the password. At this point we could assume they decided to go with a value from rockyou.txt, but asking the admins it turned out there was another part to the challenge missing.

Reframing with the idea of the challenge named "Lipstick" it seemed to suggest more than just lsb. There are 21 colors, which may map to some character for the zip, but we would need to map the colors to numbers somehow. It is also important for the stego designer to make this somewhat deterministic.

Looking through the color channels again in StegSolve, this turns up:

YSL: Yves Saint Laurent. A recognizable fashion brand which sells lipstick. This may lead us to something predictable!

Looking for lipstick on their online store we can find the full color palette - example.

The other interesting part here is that these colors have clear class names and distinct background colors.  This is perfect for identifying colors in the challenge!

Here's a small script which was used to scrape the colors from the palette:

a = {};

for (let i=0; i<59; ++i) {
  a[$('.swatch_color')[i].style['background-color']] = $('.swatch_color')[i].title;
}

JSON.stringify(a, null, 2);

This gives us a convenient JavaScript object we may query:

{
  "rgb(188, 11, 40)": "1 Le Rouge - Blood Red (Satin)",
  "rgb(184, 52, 75)": "04 Rouge Vermillon - Raspberry Red (Satin)",
  "rgb(221, 136, 133)": "06 Rose Bergamasque - Delicate Nude Pink (Satin)",
  "rgb(207, 26, 119)": "7 Le Fuchsia - Pure Saturated Fuschia (Satin)",
  "rgb(206, 135, 133)": "10 Beige Tribute - Dark Nude (Satin)",
  "rgb(194, 105, 111)": "11 Rose Carnation - Soft Peony Rose (Satin)",
  "rgb(213, 29, 55)": "13 Le Orange - Bright Orange (Satin)",
  "rgb(234, 94, 107)": "17 Rose Dahlia - Coral Orange (Satin)",
  "rgb(161, 24, 96)": "19 Fuchsia Pink - Bright Fuschia Pink (Satin)",
  "rgb(238, 133, 199)": "22 Pink Celebration - Bubble Gum Pink (Satin)",
  "rgb(235, 130, 98)": "23 Coral Poetique - Pink Coral (Satin)",
  "rgb(233, 152, 131)": "24 Blond Ingenu - Frosted Coral (Satin)",
  "rgb(218, 116, 155)": "26 Rose Libertin - Bright Purple Pink (Satin)",
  "rgb(208, 65, 121)": "27 Fuchsia Innocent - Hot Pink (Satin)",
  "rgb(235, 105, 92)": "36 Corail Legende - Orange Coral (Satin)",
  "rgb(202, 102, 174)": "49 Tropical Pink - Hot Pink (Satin)",
  "rgb(212, 18, 29)": "50 Rouge Neon - Bright Red (Satin)",
  "rgb(235, 85, 71)": "51 Corail Urbain - Medium Peach Orange (Satin)",
  "rgb(246, 98, 96)": "52 Rosy Coral - Rosy Coral (Satin)",
  "rgb(126, 69, 58)": "53 Beige Promenade - Browd Red (Satin)",
  "rgb(94, 35, 41)": "54 Prune Avenue - Dark Maroon (Satin)",
  "rgb(193, 9, 43)": "56 Orange Indie - Mauve (Satin)",
  "rgb(192, 8, 62)": "57 Luminous Pink - Magenta (Satin)",
  "rgb(165, 63, 103)": "58 Luminous Mauve - Bubblegum Pink (Satin)",
  "rgb(212, 122, 111)": "59 Golden Melon - Golden Orange (Satin)",
  "rgb(170, 64, 64)": "66 Rosewood - Soft Pink Nude (Satin)",
  "rgb(220, 53, 77)": "67 Rouge Heroine - Gold Metallic (Satin)",
  "rgb(203, 132, 123)": "70 Le Nu - Basic Nude (Satin)",
  "rgb(148, 23, 41)": "72 Rouge Vinyle - Deep Raspberry (Satin)",
  "rgb(215, 11, 22)": "73 Rhythm Red - Magenta Pink (Satin)",
  "rgb(229, 35, 23)": "74 Orange Electro - Electric Orange (Satin)",
  "rgb(183, 48, 62)": "75 Rose Mix - Deep Rose (Satin)",
  "rgb(209, 50, 116)": "76 Explicit Pink - Spring Pink (Satin)",
  "rgb(206, 10, 74)": "77 Fuschia Live - Blush Rose (Satin)",
  "rgb(161, 0, 6)": "201 Orange Imagine - Orange Red (Matte)",
  "rgb(161, 0, 52)": "202 Rose Crazy - Cranberry Red (Matte)",
  "rgb(167, 11, 35)": "203 Rouge Rock - Rich Red (Matte)",
  "rgb(131, 35, 37)": "204 Rouge Scandal - Maroon Red (Matte)",
  "rgb(97, 37, 29)": "206 Grenat Satisfaction - Brown Red (Matte)",
  "rgb(184, 20, 70)": "208 Fuchsia Fetiche - Raspberry Red (Matte)",
  "rgb(231, 51, 37)": "213 Orange Seventies - Orange with Brown undertones (Matte)",
  "rgb(215, 91, 89)": "214 Wood On Fire - Pinky Nude (Matte)",
  "rgb(184, 38, 59)": "216 Red Clash - Mauvey Red (Matte)",
  "rgb(201, 88, 108)": "217 Nude Trouble - Mauvey Beige (Matte)",
  "rgb(203, 93, 70)": "218 Coral Remix - Coral Nude (Matte)",
  "rgb(195, 1, 9)": "219 Rouge Tatouage – Vibrant Pink Red (Matte)",
  "rgb(230, 26, 1)": "220 Crazy Tangerine – Electric Orange (Matte)",
  "rgb(106, 19, 25)": "222 Black Red Code – Rust Red (Matte)",
  "rgb(51, 26, 19)": "225 - Minimal Black - Matte Finish - Black (Matte)",
  "rgb(165, 106, 92)": "340 Golden Copper - Reddish Pink (Satin)",
  "rgb(163, 77, 86)": "09 Rose Stiletto - Rich Berry Rose (Satin)",
  "rgb(92, 47, 44)": "205 Prune Virgin - Burgundy (Matte)",
  "rgb(174, 67, 95)": "207 Rose Perfecto - Blush Pink (Matte)",
  "rgb(226, 0, 74)": "211 Decadent Pink - Bright Pink (Matte)",
  "rgb(126, 32, 50)": "212 Alternative Plum - Burgundy Wine (Matte)",
  "rgb(207, 44, 125)": "215 Lust For Pink - Purple Bubblegum Pink (Matte)",
  "rgb(214, 0, 112)": "221 Rose Ink – Deep Mauve Pink (Matte)",
  "rgb(232, 11, 44)": "223 Corail Anti-Mainstream – Neon Coral (Matte)",
  "rgb(219, 90, 121)": "224 Rose Illicite – Creamy Dusty Pink (Matte)"
}

We can now query this based on values found inside the image. But how do we get those values in the image? At first trying macOS's digital color picker didn't work too well, instead let's use Python!

from PIL import Image

im = Image.open('603fda27-2893-4e60-9d9f-39962b8c8440.png')
rgb_im = im.convert('RGB')

for x in range(0, 21):
  r, g, b = rgb_im.getpixel((50 + (150 * x), 1))
  print(r, g, b)

This yields:

(188, 11, 40)
(208, 65, 121)
(212, 122, 111)
(194, 105, 111)
(235, 130, 98)
(207, 26, 119)
(192, 8, 62)
(188, 11, 40)
(188, 11, 40)
(209, 50, 116)
(106, 19, 25)
(188, 11, 40)
(188, 11, 40)
(212, 18, 29)
(215, 91, 89)
(221, 136, 133)
(206, 10, 74)
(212, 18, 29)
(126, 69, 58)
(215, 91, 89)
(221, 136, 133)

Now we can grep for each color in the JSON result, and produce integer values for each color's associated edition number:

$ IFS=$'\n'; for i in `cat image-colors` ; do grep $i ./lipsticks.json >> found; done
$ cat found | sed 's/.*: "//g;s/ .*//;s/^0*//'
1
27
59
11
23
7
57
1
1
76
222
1
1
50
214
6
77
50
53
214
6
$ cat found | sed 's/.*: "//g;s/ .*//;s/^0*//' | tr -d '\n'
1275911237571176222115021467750532146

Attempting this number didn't work for the password. Another hint came out about using binary, let's try with that:

$ cat found | cut -d: -f1 | xargs python -c 'import sys; print "".join([bin(int(x)).lstrip("0b") for x in sys.argv[1:]])'
1011111101011111001011100010101000101100011010100101010111001000111001010101101011100

Still doesn't work. Maybe we need to convert this to the character representation:

$ cat found | sed 's/.*: "//g;s/ .*//;s/^0*//' | xargs python -c 'import sys; print "".join([bin(int(x)).lstrip("0b") for x in sys.argv[1:]])' | perl -lpe '$_=pack"B*",$_'
白学家

Then extracting the archive we scraped earlier using this password, we get the flag:

$ cat flag.txt
flag{White_Album_is_Really_worth_watching_on_White_Valentine's_Day}

Pragyan CTF 2018 - Pictorial mess (300)

This was an annoying but very rewarding challenge in the end once the idea came together.
We start off with a file called files.zip which contains seven png's from 0-6.png:

Source files: https://github.com/vitapluvia/Pragyan-CTF-2018/tree/master/stego/pictoral-mess

Looking at these we have the following images filled with color:

First we could see if any of these images have hidden values in the RGB values, they seem like a good candidate for this. Opening them in stegsolve and cycling through the channel filters, we start to see letters appear:



This seems to spell out 'MakeMeTall', attempting to submit this as the flag failed though.  It was just a clue towards the next step.

After some procrastination, it was time to try what must have been the right path. Edit the PNG in a hex editor and change the height value.

For this we'll source the amazing corkami binary references - https://github.com/corkami/pics, in particular the PNG reference:

There are many great hex editors out there, feel free to use your favorite if you're following along.

We want to look for IHDR and then skip past the next 4 bytes for the width to get to the height.
For each image, this value is set to 0x0000017C.

To make the image taller, let's increase that height value to 0x0000047C.
After we save the new copy of this image, we'll notice a CRC error when checking it using pngcheck.

zlib warning:  different version (expected 1.2.5, using 1.2.11)

0.png  CRC error in chunk IHDR (computed b301c292, expected e139ed35)
ERROR: 0.png

If we run pngcheck again, we get no errors, and if we look at the image it seems to have some extra information on the bottom!

Now we can repeat this process for the other images and check out the collection we get back:



Now this looks a lot like the bottom strips may be bits which could decode to character values. There are 20 'bits' per image and 7 images total. If we stack these bits, we get the header of this writeup:

If we attempt to concatenate these rows of bits together, we don't get anything too useful at all.
If we go from top to bottom, left to right, we're able to find a message. Trying the first column we can see what it decodes to:

$ python
>>> chr(0b1110000)
'p'

This is the beginning of the expected flag format 'pctf{', looks good!

Let's throw this in a small python script to decode it all:

#!/usr/bin/env python
from pwn import unbits

b = [
  '11111101111110101111',
  '11111011111111111111',
  '10101011000101110011',
  '00001001110010000101',
  '00110000111100001011',
  '01011110110001101000',
  '01001011100001111001',
]

flag = ''

for x in range(0, 20):
  bits = [0]
  for y in range(0, 7):
    bits += b[y][x]
  flag += unbits(bits)

print flag

This results in:

pctf{B3yondth3s1ght}