No Two Digital Cameras Are the Same: Fingerprinting Via Sensor Noise

September 19, 2011 at 9:25 am 5 comments

The previous article looked at how pieces of blank paper can be uniquely identified. This article continues the fingerprinting theme to another domain, digital cameras, and ends by speculating on the possibility of applying the technique on an Internet-wide scale.

For various kinds of devices like digital cameras and RFID chips, even supposedly identical units that come out of a manufacturing plant behave slightly differently in characteristic ways, and can therefore be distinguished based on their output or behavior. How could this be? The unifying principle is this:

Microscopic physical irregularities due to natural structure and/or manufacturing defects cause observable, albeit tiny, behavioral differences.

Digital camera identification belongs to a class of techniques that exploits ‘pattern noise’ in the ‘sensor arrays’ that capture images. The same techniques can be used to fingerprint a scanner by analyzing pixel-level patterns in the images scanned by it, but that’ll be the focus of a later article.

A long-exposure dark frame [source]. Click image to see full size. Three ‘hot pixels’ and some other sensor noise can be seen.

A photo taken in the absence of any light doesn’t look completely black; a variety of factors introduce noise. There is random noise that varies in every image, but there is also ‘pattern noise’ due to inherent structural defects or irregularities in the physical sensor array. The key property of the latter kind of noise is that it manifests the same way every image taken by the camera.[1] Thus, the total noise vector produced by a camera is not identical between images, nor is it completely independent.

The pixel-level noise components in images taken by the same camera are correlated with each other.

Nevertheless, separating the pattern noise from random noise and the image itself — after all, a good camera will seek to minimize the strength or ‘power’ of the noise in relation to the image — is a very difficult task, and is the primary technical challenge that camera fingerprinting techniques must address.

Security vs. privacy. A quick note about the applications of camera fingerprinting. We saw in the previous article that there are security-enhancing and privacy-infringing applications of document fingerprinting. In fact, this is almost always the case with fingerprinting techniques. [2]

Camera fingerprinting can be used on the one hand for detecting forgeries (e.g., photoshopped images), and to aid criminal investigations by determining who (or rather, which camera) might have taken a picture. On the other hand, it could potentially also be used for unmasking individuals who wish to disseminate photos anonymously online.

Sadly, most papers studying fingerprinting study only the former type of application, which is why we’ll have to speculate a bit on the privacy impact, even though the underlying math of fingerprinting is the same.

Most fingerprinting techniques have both security-enhancing and privacy-infringing applications. The underlying principles are the same but they are applied slightly differently.

Another point to note is that because of the focus on forensics, most of the work in this area so far has studied distinguishing different camera models. But there are some preliminary results on distinguishing ‘identical’ cameras, and it appears that the same techniques will work.

In more detail. Let’s look at what I think is the most well-known paper on sensor pattern noise fingerprinting, by Binghamton University researchers Jan Lukáš, Jessica Fridrich, and Miroslav Golja. [3] Here’s how it works: the first step is to build a reference pattern of a camera from multiple known images taken from it, so that later an unsourced image can be compared against these reference patterns. The authors suggest using at least 50, but for good measure, they use 320 in their experiments. In the forensics context, the investigator probably has physical possession of the camera and therefore can generate an unlimited number of images. We’ll discuss what this requirement means in the privacy-breach context later.

There are two steps to build the reference pattern. First, for each image, a denoising filter is applied, and the denoised image is subtracted from the original to leave only the noise. Next, the noise is averaged across all the reference images — this way the random noise cancels out and leaves the pattern noise.

Comparing a new image to a reference pattern, to test if it came from that camera, is easy: extract the noise from the test image, and compare this noise pixel-by-pixel with the reference noise. The noise from the test image includes random noise, so the match won’t be close to perfect, but nevertheless the correlation between the two noise patterns will be roughly equal to the contribution of pattern noise towards the total noise in the test image. On the other hand, if the test image didn’t come from the same camera, the correlation will be close to zero.

The authors experimented with nine cameras, of which two were from the same brand and model (Olympus Camedia C765). In addition, two other cameras had the same type of sensor. There was not a single error in their 2,700 tests, including those involving the two ‘identical’ cameras — in each case, the algorithm correctly identified which of the nine cameras a given image came from. By extrapolating the correlation curves, they conservatively estimate that for a False Accept Rate of 10-3, their method achieves a False Reject Rate of anywhere between 10-2 to 10-10 or even less depending on the camera model and camera settings.

The takeaway from this seems to be that distinguishing between cameras of different models can be performed with essentially perfect accuracy. Distinguishing between cameras of the same model also seems to have very high accuracy, but it is hard to generalize because of the small sample size.

Improvements. Impressive as the above numbers are, there are at least two major ways in which this result can, and has been improved. First, the Binghamton paper is focused on a specific signal, sensor noise. But there are several stages in image acquisition and processing pipeline in the camera, each of which could leave idiosyncratic effects on the image. This paper out of Turkey incorporates many such effects by considering all patterns of certain types that occur in the lower order (least significant) bits of the image, which seems like a rather powerful technique.

The effects other than sensor noise seem to help more with identifying the camera model than the specific device, but to the extent that the former is a component of the latter, it is useful. They achieve a 97.5% accuracy among 16 test cameras — but with cellphone cameras with pictures at a resolution of just 640×480.

Second is the effect of the scene itself on the noise. Denoising transformations are not perfect — sharp boundaries look like noise. The Binghamton researchers picked their denoising filter (a wavelet transform) to minimize this problem, but a recent paper by Chang-Tsun Li claims to do it better, and shows even better numerical results: with 6 cameras (all different models), accurate (over 99%) identification for image fragments cropped to just 256 x 512.

What does this mean for privacy? I said earlier that there is a duality between security and privacy, but let’s examine the relationship in more detail. In privacy-infringing applications like mass surveillance, the algorithm need not always produce an answer, and it can occasionally be wrong when it does. The penalty for errors is much lower. On the other hand, the matching algorithm in surveillance-like applications needs to handle a far larger number of candidate cameras. The key point is:

The parameters of fingerprinting algorithms can usually be tweaked to handle a larger number of classes (i.e., devices) at the expense of accuracy.

My intuition is that state-of-the-art techniques, configured slightly differently, should allow probabilistic deanonymization from among tens of thousands of different cameras. A Flickr or Picasa profile with a few dozen images should suffice to fingerprint a camera.[4] Combined with metadata such as location, this puts us within striking distance of Internet-scale source-camera identification from anonymous images. I really hope there will be some serious research on this question.

Finally, a word defenses. If you find yourself in a position where you wish to anonymously publicize a sensitive photograph you took, but your camera is publicly tied to your identity because you’ve previously shared pictures on social networks (and who hasn’t), how do you protect yourself?

Compressing the image is one possibility, because that destroys the ‘lower-order’ bits that fingerprinting crucially depends on. However, it would have to be way more aggressive than most camera defaults (JPEG quality factor ~60% according to one of the studies, whereas defaults are ~95%). A different strategy is rotating the image slightly in order to ‘desynchronize’ it, throwing off the fingerprint matching. An attack that defeats this will have to be much more sophisticated and will have a far higher error rate.

The deanonymization threat here is analogous to writing-style fingerprinting: there are simple defenses, albeit not foolproof, but sadly most users are unaware of the problem, let alone solutions.

[1] That was a bit simplified; mathematically, there is an additive component (dark signal nonuniformity) and a multiplicative component (photoresponse nonuniformity). The former is easy to correct for, and higher-end cameras do, but the latter isn’t.

[2] Much has been said about the tension between security and privacy at a social/legal/political level, but I’m making a relatively uncontroversial technical statement here.

[3] Fridrich is incidentally one of the pioneers of speedcubing i.e., speed-solving the Rubik’s cube.

[4] The Binghamton paper uses 320 images per camera for building a fingerprint (and recommends at least 50); the Turkey paper uses 100, and Li’s paper 50. I suspect that if more than one image taken from the unknown camera is available, then the number of reference images can be brought down by a corresponding factor.

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Entry filed under: Uncategorized. Tags: , , , .

Everything Has a Fingerprint: The Case of Blank Paper Fingerprinting of RFID Tags and High-Tech Stalking

5 Comments Add your own

  • 1. A Gould  |  September 19, 2011 at 12:58 pm

    Would it not be possible to alter the “fingerprint” in post-processing? Either by using “denoise” yourself before uploading, or replacing the noise pattern with a randomized set.

    Reply
    • 2. Arvind Narayanan  |  September 19, 2011 at 1:12 pm

      Good question. Reliably removing all the noise is hard — after all, that’s why it’s a research topic — and is discussed in Section V of the Binghamton paper. As for adding more noise, my guess is that JPEG compression will give you more bang for the buck, i.e., a lower image degradation penalty. It is also much easier to carry out using standard image-editing tools.

      Reply
    • 3. Daniel Neely  |  September 19, 2011 at 2:14 pm

      Subtracting noise is a standard part of post-processing in taking astrophotos. I’m not sure how suitable the process would be for anonymizing ordinary camera images though. It involves taking dark (aperture covered) and flat (scope pointed at a uniformly bright source) frames and then using them to subtract out the noise in the image.

      The main reason I’m not sure if would be suitable for de-anonymizing images is that due to very faint targets and long exposure times the base noise levels tend to be much higher and are readily visible in the un-processed images. The other potential issue is that because dark current is a factor of thermal noise and vignetting a function of aperture that a single set of images pre-loaded at the factory for automatic cleanup would be sub-optimal and until image sensors/processors get much faster taking 3 images from the sensor for every picture taken isn’t likely to be acceptable from a performance standpoint. OTOH with flash memory having gotten as cheap as it is, large libraries of raw dark/flat files could be stored in the firmware to use a lookup tables. I don’t know how many would be needed to fully cover normal operating proceedures though.

      The very bottom of this (long) page has a set of before/after images to show the noise levels: http://astrosurf.com/re/process.html

      Reply
      • 4. Arvind Narayanan  |  September 19, 2011 at 2:30 pm

        Thanks for the comment. Dark frame subtraction is indeed performed by many digital cameras, but not flat fielding. From the paper.

        “While it is commonly done for astronomical imaging, consumer digital cameras do not flat-field their images because it is difficult to achieve a uniform sensor illumination inside the camera.”

        Reply
  • 5. Publius  |  September 20, 2011 at 4:16 am

    Perhaps one or more of the camera manufacturers could add a new feature to their camera.

    Software that randomizes the noise within the digital image ?

    Reply

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About 33bits.org

I'm an assistant professor of computer science at Princeton. I research (and teach) information privacy and security, and moonlight in technology policy.

This is a blog about my research on breaking data anonymization, and more broadly about information privacy, law and policy.

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