diff --git a/ChangeLog b/ChangeLog
index 9b8e3c6..d695d43 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,15 @@
 2014-02-21  Niibe Yutaka  <gniibe@fsij.org>
 
+	* src/ecc.c (compute_kG): Compute higer index at first.
+	(point_is_on_the_curve): Don't use coefficient_a if it's zero.
+
+	* src/jpc.c (jpc_double): Care coefficient A.
+
+	* src/ec_p256r1.c (COEFFICIENT_A_IS_MINUS_3): New.
+	* src/ec_p256k1.c (COEFFICIENT_A_IS_ZERO): New.
+	* src/jpc_p256r1.c (COEFFICIENT_A_IS_MINUS_3): Likewise.
+	* src/jpc_p256k1.c (COEFFICIENT_A_IS_MINUS_3): Likewise.
+
 	* src/modp256k1.c (modp256k1_shift): Bug fix.
 
 2014-02-20  Niibe Yutaka  <gniibe@fsij.org>
diff --git a/src/ec_p256k1.c b/src/ec_p256k1.c
index a8b690d..dba5670 100644
--- a/src/ec_p256k1.c
+++ b/src/ec_p256k1.c
@@ -30,6 +30,7 @@
 #include "ec_p256k1.h"
 
 #define FIELD p256k1
+#define COEFFICIENT_A_IS_ZERO    1
 
 /*
  * a = 0, b = 7
diff --git a/src/ec_p256r1.c b/src/ec_p256r1.c
index c77dcaf..46e37a2 100644
--- a/src/ec_p256r1.c
+++ b/src/ec_p256r1.c
@@ -30,6 +30,7 @@
 #include "ec_p256r1.h"
 
 #define FIELD p256r1
+#define COEFFICIENT_A_IS_MINUS_3 1
 
 /*
  * a = -3 mod p256r1
diff --git a/src/ecc.c b/src/ecc.c
index f82e89a..55cce6b 100644
--- a/src/ecc.c
+++ b/src/ecc.c
@@ -140,11 +140,10 @@ FUNC(compute_kG) (ac *X, const bn256 *K)
   for (i = 31; i >= 0; i--)
     {
       FUNC(jpc_double) (Q, Q);
-
-      FUNC(jpc_add_ac_signed) (Q, Q, &precomputed_KG[index[i]&0x0f],
-			       index[i] >> 7);
       FUNC(jpc_add_ac_signed) (Q, Q, &precomputed_2E_KG[index[i+32]&0x0f],
 			       index[i+32] >> 7);
+      FUNC(jpc_add_ac_signed) (Q, Q, &precomputed_KG[index[i]&0x0f],
+			       index[i] >> 7);
     }
 
   dst = k_is_even ? Q : tmp;
@@ -170,8 +169,10 @@ point_is_on_the_curve (const ac *P)
   MFNC(sqr) (s, P->x);
   MFNC(mul) (s, s, P->x);
 
+#ifdef COEFFICIENT_A_IS_ZERO
   MFNC(mul) (t, coefficient_a, P->x);
   MFNC(add) (s, s, t);
+#endif
   MFNC(add) (s, s, coefficient_b);
 
   MFNC(sqr) (t, P->y);
diff --git a/src/jpc.c b/src/jpc.c
index 5a1093b..c4cf879 100644
--- a/src/jpc.c
+++ b/src/jpc.c
@@ -47,12 +47,20 @@ FUNC(jpc_double) (jpc *X, const jpc *A)
   MFNC(sqr) (b, b);
   MFNC(shift) (b, b, 3);
 
+#if defined(COEFFICIENT_A_IS_MINUS_3)
   MFNC(sqr) (tmp0, A->z);
   MFNC(sub) (c, A->x, tmp0);
   MFNC(add) (tmp0, tmp0, A->x);
   MFNC(mul) (tmp0, tmp0, c);
   MFNC(shift) (c, tmp0, 1);
   MFNC(add) (c, c, tmp0);
+#elif defined (COEFFICIENT_A_IS_ZERO)
+  MFNC(sqr) (tmp0, A->x);
+  MFNC(shift) (c, tmp0, 1);
+  MFNC(add) (c, c, tmp0);
+#else
+#error "not supported."
+#endif
 
   MFNC(sqr) (d, c);
   MFNC(shift) (tmp0, a, 1);
diff --git a/src/jpc_p256k1.c b/src/jpc_p256k1.c
index 0b1915f..ed72c47 100644
--- a/src/jpc_p256k1.c
+++ b/src/jpc_p256k1.c
@@ -29,5 +29,6 @@
 
 #define FIELD p256k1
 #define CONST_P256 P256K1
+#define COEFFICIENT_A_IS_ZERO    1
 
 #include "jpc.c"
diff --git a/src/jpc_p256r1.c b/src/jpc_p256r1.c
index 36ffb4b..656fe27 100644
--- a/src/jpc_p256r1.c
+++ b/src/jpc_p256r1.c
@@ -29,5 +29,6 @@
 
 #define FIELD p256r1
 #define CONST_P256 P256R1
+#define COEFFICIENT_A_IS_MINUS_3 1
 
 #include "jpc.c"