diff --git a/bn_mp_expt_d.c b/bn_mp_expt_d.c
index 64c1b41..427ef06 100644
--- a/bn_mp_expt_d.c
+++ b/bn_mp_expt_d.c
@@ -18,7 +18,7 @@
 /* calculate c = a**b  using a square-multiply algorithm */
 int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
 {
-  int     res, x;
+  int     res;
   mp_int  g;
 
   if ((res = mp_init_copy (&g, a)) != MP_OKAY) {
@@ -28,23 +28,23 @@ int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
   /* set initial result */
   mp_set (c, 1);
 
-  for (x = 0; x < (int) DIGIT_BIT; x++) {
+  while (b > 0) {
+    /* if the bit is set multiply */
+    if (b & 1) {
+      if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
+        mp_clear (&g);
+        return res;
+      }
+    }
+
     /* square */
-    if ((res = mp_sqr (c, c)) != MP_OKAY) {
+    if (b > 1 && (res = mp_sqr (c, c)) != MP_OKAY) {
       mp_clear (&g);
       return res;
     }
 
-    /* if the bit is set multiply */
-    if ((b & (mp_digit) (((mp_digit)1) << (DIGIT_BIT - 1))) != 0) {
-      if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
-         mp_clear (&g);
-         return res;
-      }
-    }
-
     /* shift to next bit */
-    b <<= 1;
+    b >>= 1;
   }
 
   mp_clear (&g);