From b24d03152415d9e58e02973d9dd4c324416b79d5 Mon Sep 17 00:00:00 2001
From: jude <jude@jellywx.com>
Date: Fri, 21 Apr 2023 11:08:49 +0100
Subject: [PATCH] fix shared random to use resampling

---
 static/js/modules/interface/proofs.js |  2 ++
 static/js/modules/interface/random.js | 19 +++++++++++++------
 2 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/static/js/modules/interface/proofs.js b/static/js/modules/interface/proofs.js
index 7346c4d..7c4604a 100644
--- a/static/js/modules/interface/proofs.js
+++ b/static/js/modules/interface/proofs.js
@@ -163,3 +163,5 @@ export function verifyRegions(obj, key) {
 window.verifyRegions = verifyRegions;
 
 // verifyRegions(proveRegions({A:paillier.pubKey.encrypt(0n),B:paillier.pubKey.encrypt(1n),C:paillier.pubKey.encrypt(0n),D:paillier.pubKey.encrypt(0n),E:paillier.pubKey.encrypt(0n)}), paillier.pubKey)
+
+function proveRange() {}
diff --git a/static/js/modules/interface/random.js b/static/js/modules/interface/random.js
index 6b11331..2ed81e1 100644
--- a/static/js/modules/interface/random.js
+++ b/static/js/modules/interface/random.js
@@ -7,8 +7,7 @@ class RandomSession {
         this.cipherTexts = {};
         this.cipherKeys = {};
         this.ourKey = CryptoJS.lib.WordArray.random(32).toString();
-        // 32-bit as JavaScript does funny stuff at 53-bit levels.
-        this.ourNoise = CryptoJS.lib.WordArray.random(4);
+        this.ourNoise = CryptoJS.lib.WordArray.random(8);
         this.finalValue = null;
         this.resolvers = [];
     }
@@ -102,7 +101,7 @@ export class Random {
             ) {
                 // Lock out wait calls as they may resolve to never-ending promises.
                 await navigator.locks.request(`random-${data.session}`, () => {
-                    let total = parseInt(session.ourNoise, 16);
+                    let total = BigInt("0x" + session.ourNoise.toString());
 
                     for (let participant of Object.keys(session.cipherKeys)) {
                         let decrypted = CryptoJS.AES.decrypt(
@@ -110,10 +109,18 @@ export class Random {
                             session.cipherKeys[participant]
                         ).toString();
 
-                        total += parseInt(decrypted, 16);
+                        total += BigInt("0x" + decrypted);
                     }
 
-                    session.finalValue = total % session.range;
+                    // Find first good block of bits to avoid modular bias
+                    let blockSize = BigInt(Math.ceil(Math.log2(session.range)));
+                    let blockMask = 2n ** blockSize - 1n;
+
+                    while ((total & blockMask) >= BigInt(session.range)) {
+                        total >>= blockSize;
+                    }
+
+                    session.finalValue = total & blockMask;
 
                     this.resolve(data.session);
                 });
@@ -122,7 +129,7 @@ export class Random {
     }
 
     /**
-     * Resolve a session by calling any callbacks associated with the session and then deleting it.
+     * Resolve a session by calling any callbacks associated with the session.
      *
      * @param sessionId
      */