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Minimal C2PA Rust example application

This repository is an example of a minimal application using the C2PA Rust library that illustrates how to use an external signing application and does not require OpenSSL.

The application uses a separate signer app to show how to keep the private keys secure. It signs assets using the Ed25519 digital signature scheme, but does not have the ability to validate manifests, since validation currently requires using OpenSSL.

NOTE: The example was built and tested on macOS, and these instructions assume you are using macOS.

Overview​

The app uses the new C2PA Rust library API which has Builder and Reader data structures. All the code for the main program is in src/main.rs, which calls out to the standalone singer app defined in src/bin/signer.rs.

The main program sign_external function calls the signer app. The signer app reads bytes from stdin, signs them using a private key, then writes the signed bytes to stdout. The signer app uses a test/development private key and certificate in the src/fixtures directory.

Building the application​

Prerequisites: Install Rust.

To build the application, first build the signer binary and then run the release:

cargo build --release --bin signer

You'll see a number of Compiling messages in the terminal, and finally:

Finished release [optimized] target(s) in 47.08s

This commands builds signer binary, target/release/signer.

The application executable is saved to target/release/c2pa_min and on an ARM-based Mac is about 3.6 MB in size. With OpenSSL, this example is about 2MB larger and can perform validation.

Running the application​

Once you've built the app, run it by entering this command:

cargo run --release

You'll see messages in the terminal like this:

   Compiling c2pa_min v0.1.0 (/path_to_repo/c2pa-min)
    Finished release [optimized] target(s) in 30.45s
     Running `target/release/c2pa_min`
Output written to target/output.jpg

You can confirm that Content Credentials were added to target/output.jpg by using C2PA Tool (with c2patool target/output.jpg) or by uploading the image to Verify.