Pedersen Hash Base Points Calculation

calcpedersenbases/calcpedersenbases.js

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+const bn128 = require("snarkjs").bn128;
+const bigInt = require("snarkjs").bigInt;
+const createBlakeHash = require("blake-hash");
+const assert = require("assert");
+
+function getPoint(S) {
+    const F = bn128.Fr;
+    const h = createBlakeHash("blake256").update(S).digest();
+
+    assert(h.length == 32);
+
+    let sign = false;
+    if (h[31] & 0x80) {
+        h[31] = h[31] & 0x7F;
+        sign = true;
+    }
+
+    let x = bigInt(0);
+    for (let i=0; i<32; i++) {
+        x = x.shl(8);
+        x = x.add(bigInt(h[i]));
+    }
+
+    const a = bigInt("168700");
+    const d = bigInt("168696");
+
+    const x2 = F.square(x);
+
+    let y = F.sqrt(F.div(
+        F.sub(F.one, F.mul(a, x2)),
+        F.sub(F.one, F.mul(d, x2))));
+
+    if (y == null) return null;
+
+    if (sign) y = F.neg(y);
+
+    return [bn128.Fr.affine(x), bn128.Fr.affine(y)];
+}
+
+
+function generatePoint(S) {
+    let p= null;
+    let idx = 0;
+    while (p==null) {
+        let sidx = "" + idx;
+        while (sidx.length<16) sidx = "0"+sidx;
+        p = getPoint(S+"_"+sidx);
+        idx++;
+    }
+    assert(inCurve(p));
+    return p;
+}
+
+function inCurve(p) {
+    const F = bn128.Fr;
+
+    const a = bigInt("168700");
+    const d = bigInt("168696");
+
+    const x2 = F.square(p[0]);
+    const y2 = F.square(p[1]);
+
+    return F.equals(
+        F.add(F.mul(a, x2), y2),
+        F.add(F.one, F.mul(F.mul(x2, y2), d)));
+}
+
+const g = [
+    bigInt("17777552123799933955779906779655732241715742912184938656739573121738514868268"),
+    bigInt("2626589144620713026669568689430873010625803728049924121243784502389097019475")];
+
+if (!inCurve(g)) {
+    throw new Error("Generator not In curve -> Some thing goes wrong...");
+}
+
+for (let i=0; i<25; i++) {
+    let S = "" +i;
+    while (S.length<16) S = "0"+S;
+    const P = generatePoint("Iden3_PedersenGenerator_"+S);
+    console.log(`[${P[0].toString()}, ${P[1].toString()}]`);
+}
+
+
+