Kristian Glass - Do I Smell Burning?

Mostly technical things

Image Metadata, GPS Location, Exif Tags and Privacy Leaks

Images can contain metadata. This is useful for things like “when was this photo taken?” or information about the camera, or a text description of what the image contains.

Many cameras (e.g. your iPhone) will include GPS coordinates of where the photo was taken. This is useful for things like “where were we when we had that amazing pizza?” or “where was that beautiful sunset??”.

This is much more problematic when the selfie you just shared includes the precise location of your home.

The metadata format you’re most likely to encounter is called Exif (Exchangeable image file format).

There are a number of tools that will let you view them: your OS probably provides a way; I like Eric M. Johnston’s exiftags:

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$ exiftags example-photo-from-my-iphone.jpg
exiftags: maker note not supported
Camera-Specific Properties:

Equipment Make: Apple
Camera Model: iPhone 6
Camera Software: 11.2.6
Sensing Method: One-Chip Color Area
Focal Length (35mm Equiv): 29 mm

Image-Specific Properties:

Horizontal Resolution: 72 dpi
Vertical Resolution: 72 dpi
Image Created: 2018:03:25 14:26:07
Exposure Time: 1/33 sec
F-Number: f/2.2
Exposure Program: Normal Program
ISO Speed Rating: 250
Lens Aperture: f/2.2
Brightness: 1.9 EV
Exposure Bias: 0 EV
Metering Mode: Spot
Flash: No Flash, Compulsory
Focal Length: 4.15 mm
Color Space Information: sRGB
Image Width: 2044
Image Height: 1492
Exposure Mode: Auto
White Balance: Auto
Scene Capture Type: Standard
Latitude: (Redacted)
Longitude: (Redacted)
Altitude: (Redacted)
Time (UTC): 13:25:57
Date (UTC): 2018:03:25

Many popular image hosting/sharing services will strip much of this metadata on upload – either all of it, or the most sensitive things (like location): Imgur apparently does, WhatsApp apparently does, Facebook apparently does, et cetera.

Jekyll – the software that runs this blog – doesn’t. It’s easy to not think about this when building software.

I like Phil Harvey’s exiftool for working with Exif data.

You can strip out particularly-sensitive GPS info with:

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exiftool -gps:all= -overwrite_original_in_place example.jpg

or just wipe the whole lot:

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exiftool -all -overwrite_original_in_place example.jpg

So

It’s easy to get used to trusting tools to do the right thing.

It’s easy for a convenient feature to become a significant problem.

I’ve known about Exif tags and the potential privacy issues for a long time and I still nearly managed to blog photos tagged with a pretty exact home location. I caught myself in time, and fortunately I have no specific need to fear doing so, but it’s still very much something I’d rather not share with the world!

You can usually turn location tagging off, but it’s far from unreasonable to have it switched on: I often find it useful to have a “geo-log”, and figured I was sufficiently aware of the privacy concerns that I’d be ok.

“Be mindful of the data you might be sharing” doesn’t feel like particularly useful or actionable advice, but it’s the best I’ve got.

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