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wdormann's profile
Will Dormann
Will Dormann
Will Dormann
@wdormann

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Will Dormann

@wdormann

Vulnerability Analyst at the CERT/CC. My thoughts are my own, not my employer's.

Joined August 2012

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    1. Will Dormann‏ @wdormann Jan 10
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      • Report NetzDG Violation

      Now that Twitter has changed how it handles uploaded images, this unexpected behavior is perhaps more important now than before. Your challenge: Tell me what I've redacted from this image. (Anybody I've talked to about this so far is ineligible to play) It can be done w/o tools.pic.twitter.com/Gg4WXFBljR

      53 replies 520 retweets 1,264 likes
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    2. Will Dormann‏ @wdormann Jan 10
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      Answer: Several apps (e.g. @GIMP_Official, @Apple Preview) do not actually delete content from images with an alpha channel. They simply create an alpha-channel tunnel through the content you think that you're removing. You may think you've removed content, but it's just hidden.pic.twitter.com/vCWi80CH33

      5 replies 48 retweets 150 likes
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    3. Will Dormann‏ @wdormann Jan 10
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      If you remove the alpha channel, you now can see what's behind it. You can do this with ImageMagick, e.g. convert input.png -alpha off output.png You now have an image that doesn't have the alpha channel, so therefore is unredacted. But it's actually even easier than this!pic.twitter.com/wOuA8gJ7g5

      4 replies 27 retweets 191 likes
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    4. Will Dormann‏ @wdormann Jan 10
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      When you open an image in Twitter, there are URI arguments. For example, for the image for this thread, it's: https://pbs.twimg.com/media/EN8zPZeXsAMynMK?format=png&name=360x360 … What if you change "format=png" to "format=jpg" ? https://pbs.twimg.com/media/EN8zPZeXsAMynMK?format=jpg&name=360x360 … You how get the image, but Twitter removes the alpha channel for you!

      6 replies 41 retweets 221 likes
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    5. Will Dormann‏ @wdormann Jan 10
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      Similarly, even if you export directly to something that doesn't support alpha channels in Gimp (e.g. JPEG), by default you'll get a thumbnail in the EXIF data. Any guesses as to what that thumbnail contains?pic.twitter.com/7zAEjpXuHV

      3 replies 9 retweets 94 likes
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    6. Will Dormann‏ @wdormann Jan 11
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      I've found Apple Preview and GIMP to not remove image data properly. If you wish to test your own app, the easiest way is to: 1) Open pic with an alpha channel 2) Cut out content 3) Save as PNG 4) Send it to https://compress-or-die.com/analyze pic.twitter.com/o1OoIBpBqG

      3 replies 8 retweets 53 likes
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    7. Will Dormann‏ @wdormann Jan 11
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      And actually, as I verify this further, I believe that I was mistaken to say that Apple Preview is affected. I cannot seem to reproduce what I thought I originally saw with Preview. So perhaps just GIMP for now. I'm curious if anybody can confirm another misbehaving application.

      3 replies 2 retweets 19 likes
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    8. Will Dormann‏ @wdormann Jan 11
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      And apparently this is "expected behavior" for GIMP users? That is, when a user deletes pixels (e.g. Some are selected and the "Delete" key is pressed on the keyboard), the user should expect that the pixels aren't deleted. They are just made transparent. Neat.pic.twitter.com/s5DDnZ0uPq

      5 replies 8 retweets 40 likes
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      Will Dormann‏ @wdormann Jan 11
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      I can't help but think that this is the developer equivalent of "I reject your reality and substitute my own."pic.twitter.com/BUeVnU0v4z

      10:52 AM - 11 Jan 2020
      • 4 Retweets
      • 60 Likes
      • Stella Jørgen Kann KHAAAAAAAAAAAN! Daniel Isaksen Edmund Craske ~fembot ‏pix ‏Chris Adams‎ Mithgol the Webmaster
      7 replies 4 retweets 60 likes
        1. New conversation
        2. Jon Sneyers 🔻‏ @jonsneyers Jan 12
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          Replying to @wdormann

          If you redact something on paper, do you do it by gluing little black rectangular papers on top of it? That works, if you then make a copy and send that. It doesn't if you send the original and people can just look under the censoring stickers.

          2 replies 0 retweets 1 like
        3. Will Dormann‏ @wdormann Jan 12
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          Replying to @jonsneyers

          No, if you're going for a real-world analogy here, the steps would be: 1) Cut something out of the paper with scissors 2) Make a copy of the paper. 3) Give somebody the copy. In the GIMP world, that person has the part that you cut out with scissors.

          0 replies 0 retweets 1 like
        4. End of conversation
        1. Ashley Bischoff‏ @handcoding Jan 11
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          Replying to @wdormann

          Yeah, that seems to be the GIMP version of “You’re holding it wrong.” 📲 P.S. With a project so obstinate that they won’t even entertain the idea of transitioning to a nonableist name, I can’t say they I’m surprised that they’re patronizing about this too. 🙄

          0 replies 0 retweets 4 likes
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        2. Doxie Dachsie‏ @doxiedachsie Jan 11
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          Replying to @wdormann

          What you call channel is actually a "layer" that can be saved in png images. You can merge the layers in GIMP or photoshop & make a "flat" image where the redaction is permanent. JPEG images don't allow layers & are flat by nature. More than 2 layers are possible.

          1 reply 0 retweets 2 likes
        3. Jmaxxz‏ @jmaxxz Jan 14
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          Replying to @doxiedachsie @wdormann

          What you call a "layer" is actually a channel https://tools.ietf.org/html/rfc2083#section-2.4 …

          0 replies 0 retweets 1 like
        4. End of conversation
        1. Space Cadet‏ @BwompWomp Jan 12
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          Replying to @wdormann

          I disagree. GIMP has pretty well defined goals, and even documentation on how to properly redact. Changing pixels to black bars is the way I would initially approach this.

          0 replies 0 retweets 2 likes
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        1. Adam Reeve‏ @adamreeve Jan 11
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          Replying to @wdormann

          Reason #2,381 why I still pay Adobe $10 a month :)

          0 replies 0 retweets 1 like
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        1. otzenpunk‏ @otzenpunk Jan 12
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          Replying to @wdormann

          Hm, technically this is interesting, but I've never had the idea to redact text images for Twitter upload by creating transparent holes in them. Always black or white boxes for me.

          0 replies 0 retweets 1 like
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        1. Kwadwo Burgee‏ @koburgee Jan 11
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          Replying to @wdormann

          Ooooh I’m going to use that. Sign of the times.

          0 replies 0 retweets 0 likes
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