GramJS : Fix password randomly not working (#73)

src/lib/gramjs/Helpers.js

@@ -87,6 +87,15 @@ function mod(n, m) {
     return ((n % m) + m) % m
 }
 
+/**
+ * returns a positive bigInt
+ * @param n {BigInt}
+ * @param m {BigInt}
+ * @returns {BigInt}
+ */
+function bigIntMod(n,m) {
+    return ((n.remainder(m)).add(m)).remainder(m)
+}
 /**
  * Generates a random bytes array
  * @param count
@@ -252,6 +261,7 @@ module.exports = {
     generateKeyDataFromNonce,
     sha1,
     sha256,
+    bigIntMod,
     modExp,
     getRandomInt,
     sleep,

src/lib/gramjs/Password.js

@@ -1,6 +1,7 @@
 const Factorizator = require('./crypto/Factorizator')
 const { types } = require('./tl')
-const { readBigIntFromBuffer, readBufferFromBigInt, sha256, modExp, generateRandomBytes } = require('./Helpers')
+const { readBigIntFromBuffer, readBufferFromBigInt, sha256, bigIntMod, modExp,
+    generateRandomBytes } = require('./Helpers')
 const crypto = require('crypto')
 const SIZE_FOR_HASH = 256
 
@@ -211,7 +212,7 @@ function computeCheck(request, password) {
     const bForHash = numBytesForHash(request.srp_B)
     const gX = modExp(BigInt(g), x, p)
     const k = readBigIntFromBuffer(sha256(Buffer.concat([ pForHash, gForHash ])), false)
-    const kgX = (k.multiply(gX)).remainder(p)
+    const kgX = bigIntMod(k.multiply(gX),p)
     const generateAndCheckRandom = () => {
         const randomSize = 256
         // eslint-disable-next-line no-constant-condition
@@ -229,7 +230,7 @@ function computeCheck(request, password) {
         }
     }
     const [ a, aForHash, u ] = generateAndCheckRandom()
-    const gB = (B.subtract(kgX)).remainder(p)
+    const gB = bigIntMod(B.subtract(kgX),p)
     if (!isGoodModExpFirst(gB, p)) {
         throw new Error('bad gB')
     }