What do you see when you look at this image?

apple puzzle

Sample screenshots of the appearance of the four variations for different users.
Picture: David Buchanan

We all remember the “What color is this dress?” phenomenon followed by “Yanny vs. Laurel.” Now the world has a new puzzle to obsess over, and it will pit Apple users against their longtime rivals.

Here’s how it works: watch this image on a PC or Android phone (or any other non-Apple product). Then view the same image on an Apple device. If you were already using a Mac, iPhone, or iPad, switch to a different platform.

Let’s try again with a different picture.

Pretty neat, eh? When viewing the first image on a PC or Android phone, you should have seen the message “HELLO WORLD”. On an Apple device, that same image file reads “HELLO APPLE”. (There is one caveat which we’ll discuss in more detail below.)

Hello Apple Hello everyone

Picture: David Buchanan

The second image is even more trippy. On a non-Apple device, the image shows an older IBM PC, but switch to an i-device and you will see an older Mac.

IBM and Apple

Picture: David Buchanan

OK, so what’s going on here? Let engineer and cryptographer David Buchanan – the mastermind behind this puzzle – do the explanation:

“I found this while writing my own multi-threaded PNG decoder. Thinking about my design, I realized I had an exploitable implementation bug, ”Buchanan wrote in an blog post. “After learning that Apple has its own implementation of parallel decodable PNGs, I realized that they had made the exact same mistake! “

In simplified terms, there are different ways of decoding an image. You can do this in a single thread or in parallel threads split into blocks (a method used to speed up program execution), as Buchanan attempted. He discovered a bug in his own project, then found out that Apple had made the same mistake. He determined that it was possible to render a PNG file where: unzip (a + b)! = Unzip (a) + unzip (b) (where! = Means not equal). That is, two parts decoded separately and then assembled did not always create the same results as these same two parts decoded together.

“It could happen if a ends halfway through an uncompressed block. So it’s possible that an image has two possible interpretations, depending on whether a parallel or non-parallel decoder decodes it, ”Buchanan continued.

For you skeptics, when I showed these images to my brother, a researcher at the University of Virginia, he hashed the files (converted them to numbers) and confirmed that they are indeed the same, proving that Buchanan does not cut corners by serving you different files depending on the browser or operating system you are using.

If you are having difficulty reproducing this puzzle, it may be due to your browser as the trick relies on Apple software, not hardware. After viewing the image on multiple devices and browsers, we learned that Apple products running Safari will always display HELLO APPLE while non-Apple devices using other browsers will display HELLO WORLD.

But things get confusing once you start to mix and match.

When using browsers other than Safari on Apple mobile devices (iPad, iPhone), HELLO APPLE message appeared. Strangely enough, using these same browsers (Chrome, Opera, Edge, for example) on a Mac would show the message HELLO WORLD. While we can’t be sure, this suggests that browser apps written for the App Store use Apple’s PNG render image library, while browsers downloaded as programs on macOS use their own. PNG parser.

Anyway, if you want to have fun Buchanan released a tool called ‘Ambiguous PNG Packer’ which allows anyone to create images that look different when executed through Apple software. We must warn, however, that these discrepancies in scanning image files could be a security concern, and something, Buchanan said, “should be approached with caution.”

Update 1:30 p.m. ET: We originally published this article with embedded photos. Unfortunately, our CMS modified the files enough to shatter the illusion. We’ve updated the article with links that will take you to photos that should look different on Apple compared to other platforms.

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