Even if this particular approach likely provides medium to low assurance, which approach is more trustworthy:

  • Suppose I take a set of photos that I own, and crop all of them after I magnified by 25%. How reasonable is it to assume I "own" the picture if I share some or all of that cropping?

  • Is it more or less reasonable to prove ownership by sharing a higher quality image, can that prove ownership if I share a portion of that?

Both of these techniques are used in-practice by various digital media companies. I would assume the degree of cropping / de-resoultion has usability and security tradeoffs and would love to hear about the optimal approach.

Secondarily, I'm thinking this can be used as a UX metaphor in a ZKP/Zero knowledge proof system. If this is actually a viable security scheme, I will hide the technical details (bitcoin) from the user.

  • I'm trying to work out how this applies to security. What's the application?
    – Polynomial
    Sep 6, 2017 at 15:14
  • Copyright, authentication in a sense, stenography if a value is embedded in the photo. The outcome of this question will determine how I create a UX metaphor for non-security inclined people. Sep 6, 2017 at 15:15
  • You could prove ownership by providing the original with his metadata, cropping a picture don't remove you as the autor.
    – Tridam
    Sep 6, 2017 at 15:17
  • Since I want to prevent person 2 from pretending to be owner, I'm thinking only share a portion of said image. Like a One time password. Sep 6, 2017 at 15:19
  • 1
    I don't think the question as it stands is particularly well-defined. It reads like an XY problem, where you're asking about the details of potential solutions (providing higher resolutions, source files, demonstrating JPEG recompression, etc.) rather than the core problem itself of proving ownership of an image.
    – Polynomial
    Sep 6, 2017 at 15:26

1 Answer 1


There are a few ways to achieve proof of ownership.

For image content where there are higher resolution source files that show content from outside the frame (e.g. a RAW file from a camera, or a Photoshop / Illustrator file) you could digitally sign the content using a personal certificate (with a proper trusted root CA) and use a Timestamp Authority (TSA) to prove that your content and signature was not created after a particular date. You can create this signed file before publication and it will provide strong evidence that you did create it.

Additionally, you could research a concept called robust digital watermarking. This area of research focuses on embedded watermarking features into common media formats (JPEG, MPEG, MP3, etc.) which survive transformation such as cropping, brightness/contrast adjustment, time stretching, re-encapsulation (e.g. moving an X.264 video stream from an MPEG-4 container to a Matroska container) and various other attempts to obfuscate the source. This sort of thing has been implemented in various copyright enforcement systems and, as I understand it, underpins a lot of the copyright detection mechanisms on sites like YouTube.

  • This is awesome for the implementor and I'll likely implement this behind the scenes. But as far as "end users getting it", is the specific UX metaphor of using de-resolution and cropping viable or laughable from a pro perspective? Sep 6, 2017 at 15:39
  • I don't even understand the metaphor you're trying to make there, so I'm going to say it's probably useless for the layman.
    – Polynomial
    Sep 6, 2017 at 15:42
  • I also don't understand the need for the metaphor - why not just say something like "your images are digitally signed with information about the author and the time of creation in order to provide strong indication of ownership in the unfortunate event of a dispute", and then go on to explain how that works and how it resists things like cropping.
    – Polynomial
    Sep 6, 2017 at 15:46
  • I want to create a system analogous to Bitcoin public/private keys for authentication. Since "a photo I have" is unique like a private key, and a zoom/filter is "derived output" like a public key, I think there is a natural symmetry there... that if done correctly, can help users get a real feel for zero knowledge proofs. Sep 6, 2017 at 15:49
  • I honestly have no input then. It sounds to me like a solution looking for a problem but I don't know (or care) enough about Bitcoin internals to really understand what you're trying to do.
    – Polynomial
    Sep 6, 2017 at 16:21

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