diff --git a/lib/bcoin/utils.js b/lib/bcoin/utils.js
index 92841158..319b00f8 100644
--- a/lib/bcoin/utils.js
+++ b/lib/bcoin/utils.js
@@ -180,19 +180,9 @@ utils.writeU64 = function writeU64(dst, num, off) {
   if (!off)
     off = 0;
 
-  var num = new bn(num);
-  var left = +num.shrn(32).toString(10);
-  var right = +num.maskn(32).toString(10);
-
-  dst[off] = right & 0xff;
-  dst[off + 1] = (right >>> 8) & 0xff;
-  dst[off + 2] = (right >>> 16) & 0xff;
-  dst[off + 3] = (right >>> 24) & 0xff;
-
-  dst[off + 4] = left & 0xff;
-  dst[off + 5] = (left >>> 8) & 0xff;
-  dst[off + 6] = (left >>> 16) & 0xff;
-  dst[off + 7] = (left >>> 24) & 0xff;
+  new bn(num).toArray().reverse().forEach(function(ch, i) {
+    dst[off + i] = ch;
+  });
 
   return 8;
 };
@@ -219,19 +209,9 @@ utils.writeU64BE = function writeU64BE(dst, num, off) {
   if (!off)
     off = 0;
 
-  var num = new bn(num);
-  var left = +num.shrn(32).toString(10);
-  var right = +num.maskn(32).toString(10);
-
-  dst[off] = (left >>> 24) & 0xff;
-  dst[off + 1] = (left >>> 16) & 0xff;
-  dst[off + 2] = (left >>> 8) & 0xff;
-  dst[off + 3] = left & 0xff;
-
-  dst[off + 4] = (right >>> 24) & 0xff;
-  dst[off + 5] = (right >>> 16) & 0xff;
-  dst[off + 6] = (right >>> 8) & 0xff;
-  dst[off + 7] = right & 0xff;
+  new bn(num).toArray().forEach(function(ch, i) {
+    dst[off + i] = ch;
+  });
 
   return 8;
 };