zk-node-server-c creates a server for zk-SNARK witness generation and proof creation. This project builds heavily on the zk-node-server built by the Stealthdrop team; big thanks and shoutout to them! Their server works great for most applications, so if you're just looking for a server to handle zk-SNARK witness generation and proof creation on moderately sized circuits, I recommend first checking out their work to see if it fits your needs.
The zk-node-server built by the Stealthdrop team runs witness generation in WASM. However, the WASM witness generator will not work for circuits above a certain constraint size (around 10 million constraints in Circom) due to memory limits, so for large circuits it is instead recommended to use C++ for witness generation, and this requires a different workflow for handling requests.
First, handling large zk-SNARKs requires a specialized environment using rapidsnark as well as a patched version of Node.js with no garbage collection. See this document on best practices for large circuits for more details.
After performing the necessary installations in the linked document, set the $path_to_rapidsnark_prover environmental variable. Then clone this repo, run npm install, and create an inputs subdirectory.
Set the $circuit_name environmental variable, and then build the C++ binary for your circuit's witness generation, following the steps in the best practices document. This should create a "$circuit_name"_cpp subdirectory with the executable inside.
Next, follow the steps in the best practices document to create a zkey for your circuit. This should create a "$circuit_name".zkey file in the root directory.
Finally, run npm start to start the server.
Big thanks again to the Stealthdrop team, who supplied most of the server code. Thanks to Jonathan Wang and Yi Sun as well, for figuring out the best practices for large circuits. This project was part of the 0xPARC effort to write an implementation of elliptic curve pairings in Circom; the resulting circuit had over 19 million constraints, hence the need for this server.