diff --git a/phpseclib/Math/BigInteger.php b/phpseclib/Math/BigInteger.php
index 9df0bf0f..fc24b914 100644
--- a/phpseclib/Math/BigInteger.php
+++ b/phpseclib/Math/BigInteger.php
@@ -658,11 +658,11 @@ class BigInteger
     {
         $hex = $this->toHex($twos_compliment);
         $bits = '';
-        for ($i = strlen($hex) - 8, $start = strlen($hex) & 7; $i >= $start; $i-=8) {
-            $bits = str_pad(decbin(hexdec(substr($hex, $i, 8))), 32, '0', STR_PAD_LEFT) . $bits;
+        for ($i = strlen($hex) - 6, $start = strlen($hex) % 6; $i >= $start; $i-=6) {
+            $bits = str_pad(decbin(hexdec(substr($hex, $i, 6))), 24, '0', STR_PAD_LEFT) . $bits;
         }
         if ($start) { // hexdec('') == 0
-            $bits = str_pad(decbin(hexdec(substr($hex, 0, $start))), 8, '0', STR_PAD_LEFT) . $bits;
+            $bits = str_pad(decbin(hexdec(substr($hex, 0, $start))), 8 * $start, '0', STR_PAD_LEFT) . $bits;
         }
         $result = $this->precision > 0 ? substr($bits, -$this->precision) : ltrim($bits, '0');