Input From Multiple Parties

One of the most compelling applications of co-SNARKs is enabling distrusting parties to jointly compute a ZK proof for private shared state. This example illustrates generating a proof for a simple multiplication circuit involving two parties, each providing inputs.

We reuse the circuit introduced in our Quick Start guide ¹.

In this scenario, two input files are needed, one from each party. Both parties locally split (secret share) their respective inputs and submit them to computing nodes, which may or may not be part of the input nodes. These nodes then generate the witness and compute the proof.

To view the necessary steps, execute:

sh run_full_with_merge.sh

To understand the process involved in generating the proof, inspect the relevant part of the script:

EXAMPLE_NAME=multiplier2

# split inputs into shares
cargo run --release --bin co-circom -- split-input --circuit test_vectors/$EXAMPLE_NAME/circuit.circom --input test_vectors/$EXAMPLE_NAME/input0.json --protocol REP3 --curve BN254 --out-dir test_vectors/$EXAMPLE_NAME --config test_vectors/$EXAMPLE_NAME/config.toml
cargo run --release --bin co-circom -- split-input --circuit test_vectors/$EXAMPLE_NAME/circuit.circom  --input test_vectors/$EXAMPLE_NAME/input1.json --protocol REP3 --curve BN254 --out-dir test_vectors/$EXAMPLE_NAME --config test_vectors/$EXAMPLE_NAME/config.toml
# merge inputs into single input file
cargo run --release --bin co-circom -- merge-input-shares --inputs test_vectors/$EXAMPLE_NAME/input0.json.0.shared --inputs test_vectors/$EXAMPLE_NAME/input1.json.0.shared --protocol REP3 --curve BN254 --out test_vectors/$EXAMPLE_NAME/input.json.0.shared
cargo run --release --bin co-circom -- merge-input-shares --inputs test_vectors/$EXAMPLE_NAME/input0.json.1.shared --inputs test_vectors/$EXAMPLE_NAME/input1.json.1.shared --protocol REP3 --curve BN254 --out test_vectors/$EXAMPLE_NAME/input.json.1.shared
cargo run --release --bin co-circom -- merge-input-shares --inputs test_vectors/$EXAMPLE_NAME/input0.json.2.shared --inputs test_vectors/$EXAMPLE_NAME/input1.json.2.shared --protocol REP3 --curve BN254 --out test_vectors/$EXAMPLE_NAME/input.json.2.shared

This sequence illustrates the secret-sharing of inputs by both parties and the subsequent merging of these inputs into a single file, executed by the computing nodes.

The multiplier circuit designates one of the inputs as public, bypassing the need for MPC. Nevertheless, this example is for demonstration purposes only, to show the necessary steps for generating a proof in a multi-party scenario. ↩

Footnotes​

Footnotes