mirror of https://github.com/zcash/halo2.git
chip::witness_point.rs: Allow witnessing the identity.
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36d7888c1c
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@ -68,6 +68,7 @@ pub trait EccInstructions<C: CurveAffine>: Chip<C::Base> {
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) -> Result<Self::ScalarFixedShort, Error>;
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/// Witnesses the given point as a private input to the circuit.
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/// This maps the identity to (0, 0) in affine coordinates.
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fn witness_point(
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&self,
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layouter: &mut impl Layouter<C::Base>,
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@ -447,7 +448,13 @@ mod tests {
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assert_ne!(p_val, q_val);
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// Generate a (0,0) point to be used in other tests.
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let zero = p.add(layouter.namespace(|| "P + (-P)"), &p_neg)?;
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let zero = {
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super::Point::new(
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chip.clone(),
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layouter.namespace(|| "identity"),
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Some(C::identity()),
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)?
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};
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// Test complete addition
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{
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@ -1,5 +1,6 @@
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use super::{CellValue, EccConfig, EccPoint, Var};
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use ff::Field;
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use halo2::{
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arithmetic::CurveAffine,
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circuit::Region,
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@ -33,14 +34,22 @@ impl<C: CurveAffine> From<&EccConfig<C>> for Config<C> {
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impl<C: CurveAffine> Config<C> {
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pub(super) fn create_gate(&self, meta: &mut ConstraintSystem<C::Base>) {
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meta.create_gate("witness point", |meta| {
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// Check that either the point being witness is either:
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// - the identity, which is mapped to (0, 0) in affine coordinates; or
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// - a valid curve point y^2 = x^3 + b, where b = 5 in the Pallas equation
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let q_point = meta.query_selector(self.q_point);
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let x = meta.query_advice(self.x, Rotation::cur());
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let y = meta.query_advice(self.y, Rotation::cur());
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// Check that y^2 = x^3 + b, where b = 5 in the Pallas equation
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// y^2 = x^3 + b
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let curve_eqn = y.clone() * y.clone()
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- (x.clone() * x.clone() * x.clone())
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- Expression::Constant(C::b());
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vec![
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q_point
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* (y.clone() * y - (x.clone() * x.clone() * x) - Expression::Constant(C::b())),
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q_point.clone() * x * curve_eqn.clone(),
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q_point * y * curve_eqn,
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]
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});
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}
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@ -54,10 +63,18 @@ impl<C: CurveAffine> Config<C> {
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// Enable `q_point` selector
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self.q_point.enable(region, offset)?;
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let value = value.map(|value| value.coordinates().unwrap());
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let value = value.map(|value| {
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// Map the identity to (0, 0).
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if value == C::identity() {
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(C::Base::zero(), C::Base::zero())
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} else {
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let value = value.coordinates().unwrap();
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(*value.x(), *value.y())
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}
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});
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// Assign `x` value
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let x_val = value.map(|value| *value.x());
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let x_val = value.map(|value| value.0);
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let x_var = region.assign_advice(
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|| "x",
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self.x,
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@ -66,7 +83,7 @@ impl<C: CurveAffine> Config<C> {
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)?;
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// Assign `y` value
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let y_val = value.map(|value| *value.y());
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let y_val = value.map(|value| value.1);
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let y_var = region.assign_advice(
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|| "y",
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self.y,
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