Basic factor example with snarkjs from js

.gitignore

@@ -3,3 +3,4 @@ sync_pb2.pyc
 .data*
 *.log
 build
+node_modules

zksnarks/factor/snark/circuit.circom

@@ -0,0 +1,8 @@
+template Multiplier() {
+   signal private input a;
+   signal private input b;
+   signal output c;
+   c <== a*b;
+}
+
+component main = Multiplier();

zksnarks/factor/src/hello.js

@@ -0,0 +1,68 @@
+var fs = require('fs');
+const zkSnark = require('snarkjs');
+
+// Utils
+
+// Dealing with JSON BigInt serialization/deserialization
+function stringifyReplacer(key, value) {
+    if (typeof value === 'bigint') {
+        return value.toString() + 'n';
+    } else {
+        return value;
+    }
+}
+
+function parseReviver(key, value) {
+    if (typeof value === 'string' && /^\d+n$/.test(value)) {
+        return BigInt(value.slice(0, -1));
+    }
+    return value;
+}
+
+function serialize(value) {
+    return JSON.stringify(value, stringifyReplacer);
+}
+
+function deserialize(json_str) {
+    return JSON.parse(json_str, parseReviver);
+}
+
+
+// Using Groth
+
+// 1. Compile circuit with circom:
+// circom snark/circuit.circom -o build/circuit.json
+const circuitDef = deserialize(fs.readFileSync("build/circuit.json", "utf-8"));
+const circuit = new zkSnark.Circuit(circuitDef);
+
+// 2. Inspect circuit with e.g.:
+// circuits.nConstraints
+
+// 3. Perform setup - only done once
+function performSetup(circuit) {
+    const setup = zkSnark.groth.setup(circuit);
+    fs.writeFileSync("build/myCircuit.vk_proof", serialize(setup.vk_proof), "utf8");
+    fs.writeFileSync("build/myCircuit.vk_verifier", serialize(setup.vk_verifier), "utf8");
+    console.log("Toxic waste:", setup.toxic);  // Must be discarded.
+}
+// TODO: Conditionally run if file exists
+// performSetup(circuit);
+
+// 4. Generate proof
+const input = {
+    "a": "3",
+    "b": "11"
+}
+const witness = circuit.calculateWitness(input);
+const vk_proof = deserialize(fs.readFileSync("build/myCircuit.vk_proof", "utf8"));
+
+const {proof, publicSignals} = zkSnark.groth.genProof(vk_proof, witness);
+
+// 5. Verify proof
+const vk_verifier = deserialize(fs.readFileSync("build/myCircuit.vk_verifier", "utf8"));
+
+if (zkSnark.groth.isValid(vk_verifier, proof, publicSignals)) {
+    console.log("The proof is valid");
+} else {
+    console.log("The proof is not valid");
+}