Add component_sub_point to composer

Author: xevisalle

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Add component to subtract points in-circuit [#842]
+
 ## [0.20.2] - 2024-11-14
 
 ### Changed 
@@ -601,6 +605,7 @@ is necessary since `rkyv/validation` was required as a bound.
 - Proof system module.
 
 <!-- ISSUES -->
+[#842]: https://github.com/dusk-network/plonk/issues/842
 [#834]: https://github.com/dusk-network/plonk/issues/834
 [#831]: https://github.com/dusk-network/plonk/issues/831
 [#819]: https://github.com/dusk-network/plonk/issues/819

src/composer.rs

@@ -749,6 +749,23 @@ impl Composer {
         );
     }
 
+    /// Subtracts a curve point from another by consuming 3 gates.
+    pub fn component_sub_point(
+        &mut self,
+        a: WitnessPoint,
+        b: WitnessPoint,
+    ) -> WitnessPoint {
+        // We negate the 'x' coordinate of the point 'b', so that neg_b = (-b.x,
+        // b.y)
+        let constraint = Constraint::new().left(-BlsScalar::one()).a(*b.x());
+        let neg_b_x = self.gate_mul(constraint);
+
+        let neg_b = WitnessPoint::new(neg_b_x, *b.y());
+
+        // We perform a - b = (a.x, a.y) + (-b.x, b.y)
+        self.component_add_point(a, neg_b)
+    }
+
     /// Adds two curve points by consuming 2 gates.
     pub fn component_add_point(
         &mut self,

tests/ecc.rs

@@ -108,6 +108,90 @@ fn component_add_point() {
     check_unsatisfied_circuit(&prover, &circuit, &mut rng, msg);
 }
 
+#[test]
+fn component_sub_point() {
+    pub struct TestCircuit {
+        p1: JubJubExtended,
+        p2: JubJubExtended,
+        sub: JubJubExtended,
+    }
+
+    impl TestCircuit {
+        pub fn new(
+            p1: JubJubExtended,
+            p2: JubJubExtended,
+            sub: JubJubExtended,
+        ) -> Self {
+            Self { p1, p2, sub }
+        }
+    }
+
+    impl Default for TestCircuit {
+        fn default() -> Self {
+            let p1 = JubJubExtended::identity();
+            let p2 = JubJubExtended::identity();
+            let sub = JubJubExtended::identity();
+            Self::new(p1.into(), p2.into(), sub.into())
+        }
+    }
+
+    impl Circuit for TestCircuit {
+        fn circuit(&self, composer: &mut Composer) -> Result<(), Error> {
+            let w_p1 = composer.append_point(self.p1);
+            let w_p2 = composer.append_point(self.p2);
+            let w_sub = composer.append_point(self.sub);
+
+            let sub_circuit = composer.component_sub_point(w_p1, w_p2);
+
+            composer.assert_equal_point(w_sub, sub_circuit);
+
+            Ok(())
+        }
+    }
+
+    // Compile common circuit descriptions for the prover and verifier to be
+    // used by all tests
+    let label = b"component_sub_point";
+    let mut rng = StdRng::seed_from_u64(0xcafe);
+    let capacity = 1 << 4;
+    let pp = PublicParameters::setup(capacity, &mut rng)
+        .expect("Creation of public parameter shouldn't fail");
+    let (prover, verifier) = Compiler::compile::<TestCircuit>(&pp, label)
+        .expect("Circuit should compile");
+
+    // Test default works:
+    let msg = "Default circuit verification should pass";
+    let circuit = TestCircuit::default();
+    let pi = vec![];
+    check_satisfied_circuit(&prover, &verifier, &pi, &circuit, &mut rng, &msg);
+
+    // Test identity works:
+    let msg = "Random point subtraction should satisfy the circuit";
+    let p1 = dusk_jubjub::GENERATOR_EXTENDED * &JubJubScalar::random(&mut rng);
+    let p2 = JubJubExtended::identity();
+    let sub = p1.clone();
+    let circuit = TestCircuit::new(p1, p2, sub);
+    let pi = vec![];
+    check_satisfied_circuit(&prover, &verifier, &pi, &circuit, &mut rng, &msg);
+
+    // Test random works:
+    let msg = "Random point subtraction should satisfy the circuit";
+    let p1 = dusk_jubjub::GENERATOR_EXTENDED * &JubJubScalar::random(&mut rng);
+    let p2 = dusk_jubjub::GENERATOR_EXTENDED * &JubJubScalar::random(&mut rng);
+    let sub = p1 - p2;
+    let circuit = TestCircuit::new(p1, p2, sub);
+    let pi = vec![];
+    check_satisfied_circuit(&prover, &verifier, &pi, &circuit, &mut rng, &msg);
+
+    // Unsatisfied circuit
+    let msg = "Unsatisfied circuit should not pass";
+    let p1 = dusk_jubjub::GENERATOR_EXTENDED * &JubJubScalar::from(0xdecafu64);
+    let p2 = dusk_jubjub::GENERATOR_EXTENDED * &JubJubScalar::from(0xcafeu64);
+    let sub = dusk_jubjub::GENERATOR_EXTENDED * &JubJubScalar::from(0xcabu64);
+    let circuit = TestCircuit::new(p1, p2, sub);
+    check_unsatisfied_circuit(&prover, &circuit, &mut rng, msg);
+}
+
 #[test]
 fn component_mul_generator() {
     pub struct TestCircuit {