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Comment by eps | Hacker News Reader

Are EXIF tags added by the iOS when taking a picture? Or by the app calling the camera API after the photo is taken? I would guess it's latter.

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Yeah, EXIF tags have to be explicitly added to an image by the app (using CGImageDestinationSetProperties()), regardless of how a photo is taken.

As an aside, I'm kind of surprised that there's no easy way to add EXIF data with AssetsLibrary or UIImageWriteToSavedPhotosAlbum(). I would have sworn that there was one before I went and looked just now.

What is interesting is, if a few person in the graph start identifying themselves on their profiles. Then there might be a way to identify the others in a picture based on his nearby graph, other attributes(like gender + common friends).

Others have already shown how easy is to de-anonymize a social graph from public datasets[couldn't find the relevant paper]

Relevant research is at http://www.cs.utexas.edu/~shmat/shmat_oak09.pdf (warning: PDF).

UIImagePickerController, which is used to take photos, does not return EXIF data. Apparently, it is possible to hack around this limitation, but Apple doesn't take too kindly to this sort of behavior.

Does it matter though? The iOS app already asks to get your location information. It's that same location info that would go into the putative EXIF tag, so Color already has the info. Or is there something else in EXIF that would be relevant in this case?

The user may have agreed to let the Camera app access their GPS data, but refused to let your app access their GPS data. For the case of Color, no it's not relevant. For the question posed by the (G?)GP, I think it is.

UIImagePickerController does indeed return EXIF data. In the delegate method:

  - (void)imagePickerController:(UIImagePickerController *)c
  didFinishPickingMediaWithInfo:(NSDictionary *)info

you can retrieve a metadata dictionary from the 'info' dictionary. One of the keys in that dictionary is "{Exif}" which points to another dictionary that has all the EXIF data.

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