Merge pull request privacy-scaling-explorations#127 from input-output…

…-hk/dev-feature/ensure-non-id

Add EccInstructions API to turn `Point` to `NonIdentityPoint`

halo2_gadgets/src/ecc.rs

@@ -51,6 +51,14 @@ pub trait EccInstructions<C: CurveAffine>:
         b: &Self::Point,
     ) -> Result<(), Error>;
 
+    /// Converts [`EccInstructions::Point`] to [`EccInstructions::NonIdentityPoint`] by
+    /// convincing the verifier that `point` is not identity.
+    fn ensure_non_id(
+        &self,
+        layouter: &mut impl Layouter<C::Base>,
+        point: &Self::Point,
+    ) -> Result<Self::NonIdentityPoint, Error>;
+
     /// Witnesses the given point as a private input to the circuit.
     /// This allows the point to be the identity, mapped to (0, 0) in
     /// affine coordinates.

halo2_gadgets/src/ecc/chip.rs

@@ -499,6 +499,17 @@ where
         )
     }
 
+    fn ensure_non_id(
+        &self,
+        layouter: &mut impl Layouter<pallas::Base>,
+        point: &Self::Point,
+    ) -> Result<Self::NonIdentityPoint, Error> {
+        let non_id_point = self.witness_point_non_id(layouter, point.point())?;
+        self.constrain_equal(layouter, point, &non_id_point.clone().into())?;
+
+        Ok(non_id_point)
+    }
+
     fn witness_point(
         &self,
         layouter: &mut impl Layouter<pallas::Base>,