This image changes depending on whether you open it from an Apple device (Mac, iPhone or iPad), Android or PC. It is statistically rarer to see “Hello Apple” than “Hello World”.
Click on this link and take a good look at this image.
What do you read there?
If you see “Hello Apple”, it means that you opened this link inserted in a Numerama article from an Apple device (iPhone, iPad or Mac), on the Safari browser.
If you read “Hello World” on the other hand, you are on another device or on another browser.
As noted by The Bleeping Computer, this PNG is special because it does not display the same way depending on the browsers used or the device on which it was saved.
It is the engineer David Buchanan who is at the origin of this particularly interesting creation, which upsets certain principles that one thinks acquired when one uses devices or that one strolls on a Web site. It would seem to make sense: if you send an image in PNG format to your friend, they are supposed to receive the same as what you see on your device.
And yet, this is not always the case.
It is not, as our colleagues from Frandroid recalled, two different images. That David Buchanan has shown is that it is possible to take advantage of the fact that Apple software does not decode PNGs in the same way as others. He explains that he discovered that ” Apple has its own implementation of parallel decoding »And that this makes it possible to carry out tests as funny as they are curious.
Apple software decodes PNG differently
To put it roughly, he realized that it was possible for a single image to have not one, but two renderings, depending on how it was decoded by the device or browser being used.
To bring it to light, you need to be able to completely create a specific PNG that contains two separate rendering information, as can be seen in a piece of code Buchanan made public:
Therefore, for example, Safari does not decode PNGs in the same way as Google Chrome or Firefox, an image may therefore render one or another depending on whether you use Apple’s browser or not.
Same thing if you save / view the image directly on your Apple device: the software will show you a different rendering than on a PC which will decode the image in a different way.
To show that it is possible to exploit what can be compared to a flaw, the engineer Buchanan published on Github a tool which he developed, which allows anyone to create a “double image”. interpretation ”like the one he invented. ” Design PNGs that will look different on Apple software », Can we read on the presentation page.
He actually implemented his proposal by creating a second image, this time of a computer, which is a Mac or an IBM depending on where you open it.
In summary :
This is what you will see based on where you view the image. There are several cases taken into account: viewing the image online or when the image is saved directly on the device you are using.
| “Hello Apple” | ” Bonjour Monde “ | |
| On Safari | ✅ | ❌ |
| On other browsers | ❌ | ✅ |
| Recorded on an iPhone / iPad | ✅ | ❌ |
| Recorded on Mac | ✅ | ❌ |
| Recorded on a PC | ❌ | ✅ |
| Recorded on an Android smartphone | ❌ | ✅ |
We have ruled out the ability to view the image on Safari from a PC, since updates for Windows were discontinued years ago, in update 5.1.7.