Fix Edwards yz/yzsquared to_model.

Fixes #10.

3 files changed, 23 insertions, 3 deletions

diff --git a/pyecsca/ec/curve.py b/pyecsca/ec/curve.py
index 7351583..e976289 100644
--- a/pyecsca/ec/curve.py
+++ b/pyecsca/ec/curve.py
@@ -167,7 +167,10 @@ class EllipticCurve(object):
             return False
         if self.is_neutral(point):
             return True
-        loc = {**self.parameters, **point.to_affine().coords}
+        if isinstance(point.coordinate_model, AffineCoordinateModel):
+            loc = {**self.parameters, **point.coords}
+        else:
+            loc = {**self.parameters, **point.to_affine().coords}
         return eval(compile(self.model.equation, "", mode="eval"), loc)
 
     def to_affine(self) -> "EllipticCurve":
diff --git a/pyecsca/ec/point.py b/pyecsca/ec/point.py
index 480c47f..a3c388e 100644
--- a/pyecsca/ec/point.py
+++ b/pyecsca/ec/point.py
@@ -103,8 +103,13 @@ class Point(object):
                         result[var] = result[var].inverse()
                 elif var == "Y":
                     result[var] = self.coords["y"]
-                    if isinstance(coordinate_model, EFDCoordinateModel) and coordinate_model.name == "inverted":
-                        result[var] = result[var].inverse()
+                    if isinstance(coordinate_model, EFDCoordinateModel):
+                        if coordinate_model.name == "inverted":
+                            result[var] = result[var].inverse()
+                        elif coordinate_model.name == "yz":
+                            result[var] = result[var] * curve.parameters["r"]
+                        elif coordinate_model.name == "yzsquared":
+                            result[var] = result[var] * curve.parameters["r"]**2
                 elif var.startswith("Z"):
                     result[var] = Mod(1, curve.prime)
                 elif var == "T":
diff --git a/test/ec/test_regress.py b/test/ec/test_regress.py
index e2c1799..26ef921 100644
--- a/test/ec/test_regress.py
+++ b/test/ec/test_regress.py
@@ -49,3 +49,15 @@ class RegressionTests(TestCase):
         formula = coords.formulas["dbl-1987-m-2"]
         res = formula(base, **curve.parameters)[0]
         self.assertIsNotNone(res)
+
+    def test_issue_10(self):
+        model = EdwardsModel()
+        coords = model.coordinates["yz"]
+        p = 0x1d
+        c = Mod(1, p)
+        d = Mod(0x1c, p)
+        r = d.sqrt()
+        neutral = Point(coords, Y=c * r, Z=Mod(1, p))
+        curve = EllipticCurve(model, coords, p, neutral, {"c": c, "d": d, "r": r})
+        neutral_affine = Point(AffineCoordinateModel(model), x=Mod(0, p), y=c)
+        self.assertEqual(neutral, neutral_affine.to_model(coords, curve))