DEV Community

Eoin Butler
Eoin Butler

Posted on Originally published at signunder.com AI-assisted

Open-source C2PA signing: SignUnder's code is on GitHub

#opensource #rust #dotnet #ai

SignUnder's code is public on GitHub at AivaStack/signunder, under the Apache-2.0 licence, since 20 October 2026. It is the code the hosted service runs: the API, the C2PA signer, the watermark and fingerprint worker, the rules engine with its rule packs, and the .NET and Python SDKs.

SignUnder marks AI-generated images so the company that publishes them can show where they came from, under its own name, and meet labelling rules such as Article 50 of the EU AI Act.

What is in the repository?

  • The API (.NET 10): signing, verification, the registry of signed outputs, accounts and the public verify routes.
  • The signer (Rust), a thin gRPC service around c2pa-rs, the C2PA reference implementation, pinned to an exact version so the manifests it writes do not drift. It signs through a cloud KMS, and can add a CAWG identity assertion naming the company that publishes the content.
  • The watermark and fingerprint worker (Python): an invisible TrustMark watermark bound into each manifest, and a PDQ perceptual hash, so an output can still be recognised after a platform strips its metadata.
  • The rules engine and eight rule packs: EU Article 50, the Code of Practice, California, mainland China, South Korea, India, New York and the platforms' labelling rules. Every rule cites its primary source and the date it was read; the free checker runs on them.
  • The architecture and every technical decision, as decision records, and the evidence prepared for the C2PA conformance program.

Why open it?

A signature on AI-generated content is a claim about where the content came from, and the rules that decide which marks a company needs are a reading of the law. Both are easier to trust when anyone can read how they are made: what the manifest says, what the watermark carries, which source each rule rests on. Opening the code also lets a company check exactly what runs on its behalf, or run it itself.

Can I run it myself?

Yes:

git clone https://github.com/AivaStack/signunder
cd signunder
make up
Enter fullscreen mode Exit fullscreen mode

That starts the whole service in Docker, signing with a development key and certificate that are committed for the purpose; the README has the details. Content signed that way is valid but shows as unrecognised in Content Credentials viewers, because the development certificate is not on the C2PA trust list. Recognised signing in your own name needs two things the code cannot supply: your key in a KMS or HSM, and a certificate from a certificate authority on that list, issued once your product passes the C2PA conformance program. The own-name guide explains why that name cannot be delegated.

What it does not do yet: images only (JPEG, PNG, WebP), and the watermark, while it survives recompression and resizing in our tests, was lost to a 25% crop on two of four photographs.

What does the hosted service add?

The keys held in an HSM, the conformance listing and certificate, the public verify page for each company, the identity checks behind the "verified by SignUnder" line, and rule packs kept current against their sources every week. The code is the same; the hosted service is the operation around it. How the pieces fit together: under the hood.

I'm the developer of SignUnder. Much of the code was written with Claude Code; the decision records in docs/adr/ show how each choice was made. Originally published at signunder.com.

Top comments (0)