Introduce ecc::BaseFitsInScalarInstructions trait.

halo2_gadgets/src/ecc.rs

@@ -138,6 +138,18 @@ pub trait EccInstructions<C: CurveAffine>:
     ) -> Result<Self::Point, Error>;
 }
 
+/// Instructions that can be implemented for a curve whose base field fits into
+/// its scalar field.
+pub trait BaseFitsInScalarInstructions<C: CurveAffine>: EccInstructions<C> {
+    /// Converts a base field element that exists as a variable in the circuit
+    /// into a scalar to be used in variable-base scalar multiplication.
+    fn scalar_var_from_base(
+        &self,
+        layouter: &mut impl Layouter<C::Base>,
+        base: &Self::Var,
+    ) -> Result<Self::ScalarVar, Error>;
+}
+
 /// Defines the fixed points for a given instantiation of the ECC chip.
 pub trait FixedPoints<C: CurveAffine>: Debug + Eq + Clone {
     /// Fixed points that can be used with full-width scalar multiplication.

halo2_gadgets/src/ecc/chip.rs

@@ -1,6 +1,6 @@
 //! Chip implementations for the ECC gadgets.
 
-use super::{EccInstructions, FixedPoints};
+use super::{BaseFitsInScalarInstructions, EccInstructions, FixedPoints};
 use crate::{
     primitives::sinsemilla,
     utilities::{lookup_range_check::LookupRangeCheckConfig, UtilitiesInstructions},
@@ -533,3 +533,22 @@ where
         )
     }
 }
+
+impl<Fixed: FixedPoints<pallas::Affine>> BaseFitsInScalarInstructions<pallas::Affine>
+    for EccChip<Fixed>
+where
+    <Fixed as FixedPoints<pallas::Affine>>::Base:
+        FixedPoint<pallas::Affine, ScalarKind = BaseFieldElem>,
+    <Fixed as FixedPoints<pallas::Affine>>::FullScalar:
+        FixedPoint<pallas::Affine, ScalarKind = FullScalar>,
+    <Fixed as FixedPoints<pallas::Affine>>::ShortScalar:
+        FixedPoint<pallas::Affine, ScalarKind = ShortScalar>,
+{
+    fn scalar_var_from_base(
+        &self,
+        _layouter: &mut impl Layouter<pallas::Base>,
+        base: &Self::Var,
+    ) -> Result<Self::ScalarVar, Error> {
+        Ok(base.clone())
+    }
+}