New query function ecc_montgomery_is_identity.

To begin with, this allows me to add a regression test for the change
in the previous commit.

ecc.c

@@ -833,6 +833,11 @@ void ecc_montgomery_get_affine(MontgomeryPoint *mp, mp_int **x)
         *x = monty_export(mc->mc, mp->X);
 }
 
+unsigned ecc_montgomery_is_identity(MontgomeryPoint *mp)
+{
+    return mp_eq_integer(mp->Z, 0);
+}
+
 /* ----------------------------------------------------------------------
  * Twisted Edwards curves.
  */

ecc.h

@@ -170,6 +170,11 @@ MontgomeryPoint *ecc_montgomery_multiply(MontgomeryPoint *, mp_int *);
  */
 void ecc_montgomery_get_affine(MontgomeryPoint *mp, mp_int **x);
 
+/*
+ * Test whether a point is the curve identity.
+ */
+unsigned ecc_montgomery_is_identity(MontgomeryPoint *mp);
+
 /* ----------------------------------------------------------------------
  * Twisted Edwards curves.
  *

test/cryptsuite.py

@@ -769,6 +769,12 @@ class ecc(MyTestBase):
         check_point(ecc_montgomery_double(mP), rP + rP)
         check_point(ecc_montgomery_double(mQ), rQ + rQ)
 
+        zero = ecc_montgomery_point_new(mc, 0)
+        self.assertEqual(ecc_montgomery_is_identity(zero), False)
+        identity = ecc_montgomery_double(zero)
+        ecc_montgomery_get_affine(identity)
+        self.assertEqual(ecc_montgomery_is_identity(identity), True)
+
     def testEdwardsSimple(self):
         p, d, a = 3141592661, 2688750488, 367934288
 

testcrypt.h

@@ -110,6 +110,7 @@ FUNC3(val_mpoint, ecc_montgomery_diff_add, val_mpoint, val_mpoint, val_mpoint)
 FUNC1(val_mpoint, ecc_montgomery_double, val_mpoint)
 FUNC2(val_mpoint, ecc_montgomery_multiply, val_mpoint, val_mpint)
 FUNC2(void, ecc_montgomery_get_affine, val_mpoint, out_val_mpint)
+FUNC1(boolean, ecc_montgomery_is_identity, val_mpoint)
 FUNC4(val_ecurve, ecc_edwards_curve, val_mpint, val_mpint, val_mpint, opt_val_mpint)
 FUNC3(val_epoint, ecc_edwards_point_new, val_ecurve, val_mpint, val_mpint)
 FUNC3(val_epoint, ecc_edwards_point_new_from_y, val_ecurve, val_mpint, uint)