fix fp2 inv

ww-rm · Jun 9, 2024 · d76a73f · d76a73f
1 parent 3b9c255
commit d76a73f
Showing 1 changed file with 13 additions and 31 deletions.
diff --git a/src/gmalglib/core/sm9curve.c b/src/gmalglib/core/sm9curve.c
@@ -983,44 +983,26 @@ int SM9FP2_MontHasSqrt(const SM9FP2Mont* x, SM9FP2Mont* y)
 static
 void SM9FP2_MontInv(const SM9FP2Mont* x, SM9FP2Mont* y)
 {
-    // use method of undetermined coefficients
     const SM9FP1Mont* x1 = x->fp1 + 1;
     const SM9FP1Mont* x0 = x->fp1;
 
     SM9FP2Mont y_tmp = { 0 };
     SM9FP1Mont* y1 = y_tmp.fp1 + 1;
     SM9FP1Mont* y0 = y_tmp.fp1;
 
-    if (UInt256_IsZero(x0))
-    {
-        // (-1 / 2x1, 0)
-        UInt256_SetZero(y0);
-        SM9FP1_Add(x1, x1, y1);
-        SM9FP1_MontInv(y1, y1);
-        SM9FP1_Neg(y1, y1);
-    }
-    else if (UInt256_IsZero(x1))
-    {
-        // (0, 1 / x0)
-        UInt256_SetZero(y1);
-        SM9FP1_MontInv(x0, y0);
-    }
-    else
-    {
-        // t = 1 / (x0^2 + 2(x1^2))
-        SM9FP1_MontMul(x0, x0, y0);     // x0^2
-        SM9FP1_MontMul(x1, x1, y1);     // x1^2
-        SM9FP1_Add(y0, y1, y0);
-        SM9FP1_Add(y0, y1, y0);         // x0^2 + 2(x1^2)
-        SM9FP1_MontInv(y0, y0);         // 1 / (x0^2 + 2(x1^2))
-
-        // y1 = -tx1
-        SM9FP1_MontMul(y0, x1, y1);
-        SM9FP1_Neg(y1, y1);
-
-        // y0 = tx0
-        SM9FP1_MontMul(y0, x0, y0);
-    }
+    // det = x0^2 + 2(x1^2)
+    SM9FP1_MontMul(x0, x0, y0);
+    SM9FP1_MontMul(x1, x1, y1);
+    SM9FP1_Add(y0, y1, y0);
+    SM9FP1_Add(y0, y1, y0);
+    SM9FP1_MontInv(y0, y0);
+
+    // y1 =  -x1 / det
+    SM9FP1_MontMul(y0, x1, y1);
+    SM9FP1_Neg(y1, y1);
+
+    // y0 = x0 / det
+    SM9FP1_MontMul(y0, x0, y0);
 
     *y = y_tmp;
 }