diff --git a/lib/utils/gcs.js b/lib/utils/gcs.js
index c1fd9972..93a9baef 100644
--- a/lib/utils/gcs.js
+++ b/lib/utils/gcs.js
@@ -21,8 +21,7 @@ var DUMMY = Buffer.allocUnsafe(0);
 function GCSFilter() {
   this.n = 0;
   this.p = 0;
-  this.modp = Int64(0);
-  this.modnp = Int64(0);
+  this.m = Int64(0);
   this.data = DUMMY;
 }
 
@@ -41,7 +40,7 @@ GCSFilter.prototype.header = function header(prev) {
 
 GCSFilter.prototype.match = function match(key, data) {
   var br = new BitReader(this.data);
-  var term = siphash(data, key).imod(this.modnp);
+  var term = siphash(data, key).imod(this.m);
   var last = Int64(0);
   var value;
 
@@ -75,7 +74,7 @@ GCSFilter.prototype.matchAny = function matchAny(key, items) {
 
   for (i = 0; i < items.length; i++) {
     item = items[i];
-    hash = siphash(item, key).imod(this.modnp);
+    hash = siphash(item, key).imod(this.m);
     values.push(hash);
   }
 
@@ -125,7 +124,7 @@ GCSFilter.prototype.readU64 = function readU64(br) {
 
   rem = br.readBits64(this.p);
 
-  return num.imul(this.modp).iadd(rem);
+  return num.ishln(this.p).iadd(rem);
 };
 
 GCSFilter.prototype.toBytes = function toBytes() {
@@ -171,15 +170,14 @@ GCSFilter.prototype.fromData = function fromData(P, key, items) {
 
   this.n = items.length;
   this.p = P;
-  this.modp = Int64(1).ishln(this.p);
-  this.modnp = Int64(this.n).imul(this.modp);
+  this.m = Int64(this.n).ishln(this.p);
 
   bw = new BitWriter();
 
   for (i = 0; i < items.length; i++) {
     item = items[i];
     assert(Buffer.isBuffer(item));
-    hash = siphash(item, key).imod(this.modnp);
+    hash = siphash(item, key).imod(this.m);
     values.push(hash);
   }
 
@@ -187,7 +185,7 @@ GCSFilter.prototype.fromData = function fromData(P, key, items) {
 
   for (i = 0; i < values.length; i++) {
     hash = values[i];
-    rem = hash.sub(last).iand(this.modp.subn(1));
+    rem = hash.sub(last).imaskn(this.p);
     value = hash.sub(last).isub(rem).ishrn(this.p);
     last = hash;
 
@@ -214,8 +212,7 @@ GCSFilter.prototype.fromBytes = function fromBytes(N, P, data) {
 
   this.n = N;
   this.p = P;
-  this.modp = Int64(1).ishln(this.p);
-  this.modnp = Int64(this.n).imul(this.modp);
+  this.m = Int64(this.n).ishln(this.p);
   this.data = data;
 
   return this;