Can a PNG be encrypted without losing the exif data? - image

Question

I've been working on this Reddit puzzle:
http://www.reddit.com/r/playitforward/comments/1v6jfh/contest_first_one_to_solve_this_riddle_gets_my/
and most of users in the thread are stumped. Full disclosure, I'd love to win the prize, but by bringing attention to it and asking for assistance, I understand that I lessen my chances, but at this point I want to know what the image says more than anything.
We narrowed down the cyphers to a URL to a text file with PNG exif data, but when opened as a PNG, it turns out corrupted. Could this PNG be encrypted or purposely corrupted in a way to preserve the exif data and what would be the best way to unravel it? Note that the string of numbers and "AK" were explicitly linked to this clue, so I can only assume there is maybe an Asynchronous Key involved or some standard pioneered by Arjen Kampf Lenstra or some Angry Kid behind it all.

Answer 1

Sure, using ImageMagick like this:
# Look at rose image before we start, and its header
identify -verbose rose.jpg
Image: rose.jpg
Format: JPEG (Joint Photographic Experts Group JFIF format)
Mime type: image/jpeg
Class: DirectClass
Geometry: 70x46+0+0
Units: Undefined
Type: TrueColor
Endianess: Undefined
Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 8-bit
Channel statistics:
Pixels: 3220
Red:
min: 31 (0.121569)
max: 255 (1)
mean: 145.56 (0.570825)
standard deviation: 69.1755 (0.271277)
kurtosis: -1.38839
skewness: 0.139004
entropy: 0.97057
Green:
min: 27 (0.105882)
max: 255 (1)
mean: 89.2475 (0.34999)
standard deviation: 52.4516 (0.205693)
kurtosis: 2.60505
skewness: 1.80798
entropy: 0.869705
Blue:
min: 21 (0.0823529)
max: 255 (1)
mean: 80.4214 (0.315378)
standard deviation: 54.9267 (0.215399)
kurtosis: 2.93861
skewness: 1.9566
entropy: 0.85334
Image statistics:
Overall:
min: 21 (0.0823529)
max: 255 (1)
mean: 105.076 (0.412064)
standard deviation: 59.3109 (0.232592)
kurtosis: 1.24657
skewness: 1.44732
entropy: 0.897872
Rendering intent: Perceptual
Gamma: 0.454545
Chromaticity:
red primary: (0.64,0.33)
green primary: (0.3,0.6)
blue primary: (0.15,0.06)
white point: (0.3127,0.329)
Background color: white
Border color: srgb(223,223,223)
Matte color: grey74
Transparent color: black
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 70x46+0+0
Dispose: Undefined
Iterations: 0
Compression: JPEG
Quality: 92
Orientation: Undefined
Properties:
date:create: 2015-10-04T18:46:03+01:00
date:modify: 2015-10-04T18:46:03+01:00
jpeg:colorspace: 2
jpeg:sampling-factor: 1x1,1x1,1x1
signature: 38a8912b601557d5a377bff360f03804c383c3298b48d9917504b488e8f4152b
Artifacts:
filename: rose.jpg
verbose: true
Tainted: False
Filesize: 2.65KB
Number pixels: 3.22K
Pixels per second: 3.22EB
User time: 0.000u
Elapsed time: 0:01.000
Version: ImageMagick 6.9.1-10 Q32 x86_64 2015-10-02 http://www.imagemagick.org
Now add a comment into the image and encrypt it as encrypted.png:
convert -comment "Freddy frog" rose.jpg -encipher passphrase.txt encrypted.png
Check the header of encrypted image to see if EXIF data and comment and other data are visible within it - yes, they are:
identify -verbose encrypted.png
Image: encrypted.png
Format: PNG (Portable Network Graphics)
Mime type: image/png
Class: DirectClass
Geometry: 70x46+0+0
Units: Undefined
Type: TrueColor
Endianess: Undefined
Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 8-bit
Channel statistics:
Pixels: 3220
Red:
min: 0 (0)
max: 255 (1)
mean: 126.755 (0.497077)
standard deviation: 73.7824 (0.289343)
kurtosis: -1.18047
skewness: 0.0142557
entropy: 0.99254
Green:
min: 0 (0)
max: 255 (1)
mean: 127.937 (0.501712)
standard deviation: 75.0501 (0.294314)
kurtosis: -1.23185
skewness: -0.0233363
entropy: 0.992485
Blue:
min: 0 (0)
max: 255 (1)
mean: 127.594 (0.500368)
standard deviation: 74.64 (0.292706)
kurtosis: -1.22352
skewness: -0.0177342
entropy: 0.992544
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 127.428 (0.499719)
standard deviation: 74.4927 (0.292128)
kurtosis: -1.21239
skewness: -0.00900116
entropy: 0.992523
Rendering intent: Perceptual
Gamma: 0.45455
Chromaticity:
red primary: (0.64,0.33)
green primary: (0.3,0.6)
blue primary: (0.15,0.06)
white point: (0.3127,0.329)
Background color: white
Border color: srgb(223,223,223)
Matte color: grey74
Transparent color: black
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 70x46+0+0
Dispose: Undefined
Iterations: 0
Compression: Zip
Orientation: Undefined
Properties:
cipher:mode: CTR
cipher:nonce: d3d57ca43eacb27a9d72b65ef976923e5b761c7aaaee1d1914d1769ca4834488
cipher:type: AES
comment: Freddy frog <--- comment is visible
date:create: 2015-10-04T18:48:43+01:00
date:modify: 2015-10-04T18:48:43+01:00
png:bKGD: chunk was found (see Background color, above)
png:cHRM: chunk was found (see Chromaticity, above)
png:gAMA: gamma=0.45454544 (See Gamma, above)
png:IHDR.bit-depth-orig: 8
png:IHDR.bit_depth: 8
png:IHDR.color-type-orig: 2
png:IHDR.color_type: 2 (Truecolor)
png:IHDR.interlace_method: 0 (Not interlaced)
png:IHDR.width,height: 70, 46
png:sRGB: intent=0 (Perceptual Intent)
png:text: 6 tEXt/zTXt/iTXt chunks were found
signature: 273e3934027f6ffbcf00b3eca7eb0c576d8fd180e87133112ecacd59225986ee
Artifacts:
filename: encrypted.png
verbose: true
Tainted: False
Filesize: 10.1KB
Number pixels: 3.22K
Pixels per second: 3.22EB
User time: 0.000u
Elapsed time: 0:01.000
Version: ImageMagick 6.9.1-10 Q32 x86_64 2015-10-02 http://www.imagemagick.org
Now look at the encrypted image - junk
Decrypt image as decrypted.jpg - looks like a rose to me :-)
convert encrypted.png -decipher passphrase.txt decrypted.jpg