Can a PNG be encrypted without losing the exif data?

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