Faster cofactor clearing for G1 & G2 of bls12-381 + benchmarking (#103)

bls12_381/src/curves/g1.rs

@@ -1,3 +1,4 @@
+use crate::*;
 use ark_ec::{
     bls12,
     bls12::Bls12Parameters,
@@ -5,11 +6,8 @@ use ark_ec::{
     short_weierstrass::{Affine, SWCurveConfig},
     AffineRepr, Group,
 };
-use ark_ff::{Field, MontFp, Zero};
-use ark_std::ops::Neg;
-
-use crate::*;
-
+use ark_ff::{Field, MontFp, PrimeField, Zero};
+use ark_std::{ops::Neg, One};
 pub type G1Affine = bls12::G1Affine<crate::Parameters>;
 pub type G1Projective = bls12::G1Projective<crate::Parameters>;
 
@@ -62,6 +60,21 @@ impl SWCurveConfig for Parameters {
         let endomorphism_p = endomorphism(p);
         minus_x_squared_times_p.eq(&endomorphism_p)
     }
+
+    #[inline]
+    fn clear_cofactor(p: &G1Affine) -> G1Affine {
+        // Using the effective cofactor, as explained in
+        // Section 5 of https://eprint.iacr.org/2019/403.pdf.
+        //
+        // It is enough to multiply by (1 - x), instead of (x - 1)^2 / 3
+        let h_eff = one_minus_x().into_bigint();
+        Parameters::mul_affine(&p, h_eff.as_ref()).into()
+    }
+}
+
+fn one_minus_x() -> Fr {
+    const X: Fr = Fr::from_sign_and_limbs(!crate::Parameters::X_IS_NEGATIVE, crate::Parameters::X);
+    Fr::one() - X
 }
 
 /// G1_GENERATOR_X =
@@ -83,3 +96,33 @@ pub fn endomorphism(p: &Affine<Parameters>) -> Affine<Parameters> {
     res.x *= BETA;
     res
 }
+
+#[cfg(test)]
+mod test {
+
+    use super::*;
+    use ark_std::{rand::Rng, UniformRand};
+
+    fn sample_unchecked() -> Affine<g1::Parameters> {
+        let mut rng = ark_std::test_rng();
+        loop {
+            let x = Fq::rand(&mut rng);
+            let greatest = rng.gen();
+
+            if let Some(p) = Affine::get_point_from_x_unchecked(x, greatest) {
+                return p;
+            }
+        }
+    }
+
+    #[test]
+    fn test_cofactor_clearing() {
+        const SAMPLES: usize = 100;
+        for _ in 0..SAMPLES {
+            let p: Affine<g1::Parameters> = sample_unchecked();
+            let p = p.clear_cofactor();
+            assert!(p.is_on_curve());
+            assert!(p.is_in_correct_subgroup_assuming_on_curve());
+        }
+    }
+}

bls12_381/src/curves/g2.rs

@@ -1,9 +1,11 @@
+use ark_std::ops::Neg;
+
 use ark_ec::{
     bls12,
     bls12::Bls12Parameters,
     models::CurveConfig,
-    short_weierstrass::{Affine, SWCurveConfig},
-    AffineRepr,
+    short_weierstrass::{Affine, Projective, SWCurveConfig},
+    AffineRepr, CurveGroup, Group,
 };
 use ark_ff::{Field, MontFp, Zero};
 
@@ -69,6 +71,40 @@ impl SWCurveConfig for Parameters {
 
         x_times_point.eq(&p_times_point)
     }
+
+    #[inline]
+    fn clear_cofactor(p: &G2Affine) -> G2Affine {
+        // Based on Section 4.1 of https://eprint.iacr.org/2017/419.pdf
+        // [h(ψ)]P = [x^2 − x − 1]P + [x − 1]ψ(P) + (ψ^2)(2P)
+
+        // x = -15132376222941642752
+        // When multiplying, use -c1 instead, and then negate the result. That's much
+        // more efficient, since the scalar -c1 has less limbs and a much lower Hamming
+        // weight.
+        let x: &'static [u64] = crate::Parameters::X;
+        let p_projective = p.into_group();
+
+        // [x]P
+        let x_p = Parameters::mul_affine(p, &x).neg();
+        // ψ(P)
+        let psi_p = p_power_endomorphism(&p);
+        // (ψ^2)(2P)
+        let mut psi2_p2 = double_p_power_endomorphism(&p_projective.double());
+
+        // tmp = [x]P + ψ(P)
+        let mut tmp = x_p.clone();
+        tmp += &psi_p;
+
+        // tmp2 = [x^2]P + [x]ψ(P)
+        let mut tmp2: Projective<Parameters> = tmp;
+        tmp2 = tmp2.mul_bigint(x).neg();
+
+        // add up all the terms
+        psi2_p2 += tmp2;
+        psi2_p2 -= x_p;
+        psi2_p2 += &-psi_p;
+        (psi2_p2 - p_projective).into_affine()
+    }
 }
 
 pub const G2_GENERATOR_X: Fq2 = Fq2::new(G2_GENERATOR_X_C0, G2_GENERATOR_X_C1);
@@ -109,6 +145,11 @@ pub const P_POWER_ENDOMORPHISM_COEFF_1: Fq2 = Fq2::new(
                 "1028732146235106349975324479215795277384839936929757896155643118032610843298655225875571310552543014690878354869257")
 );
 
+pub const DOUBLE_P_POWER_ENDOMORPHISM: Fq2 = Fq2::new(
+    MontFp!("4002409555221667392624310435006688643935503118305586438271171395842971157480381377015405980053539358417135540939436"),
+    Fq::ZERO
+);
+
 pub fn p_power_endomorphism(p: &Affine<Parameters>) -> Affine<Parameters> {
     // The p-power endomorphism for G2 is defined as follows:
     // 1. Note that G2 is defined on curve E': y^2 = x^3 + 4(u+1).
@@ -135,3 +176,47 @@ pub fn p_power_endomorphism(p: &Affine<Parameters>) -> Affine<Parameters> {
 
     res
 }
+
+/// For a p-power endomorphism psi(P), compute psi(psi(P))
+pub fn double_p_power_endomorphism(p: &Projective<Parameters>) -> Projective<Parameters> {
+    let mut res = *p;
+
+    res.x *= DOUBLE_P_POWER_ENDOMORPHISM;
+    res.y = res.y.neg();
+
+    res
+}
+
+#[cfg(test)]
+mod test {
+
+    use super::*;
+    use ark_std::UniformRand;
+
+    #[test]
+    fn test_cofactor_clearing() {
+        // multiplying by h_eff and clearing the cofactor by the efficient
+        // endomorphism-based method should yield the same result.
+        let h_eff: &'static [u64] = &[
+            0xe8020005aaa95551,
+            0x59894c0adebbf6b4,
+            0xe954cbc06689f6a3,
+            0x2ec0ec69d7477c1a,
+            0x6d82bf015d1212b0,
+            0x329c2f178731db95,
+            0x9986ff031508ffe1,
+            0x88e2a8e9145ad768,
+            0x584c6a0ea91b3528,
+            0xbc69f08f2ee75b3,
+        ];
+
+        let mut rng = ark_std::test_rng();
+        const SAMPLES: usize = 10;
+        for _ in 0..SAMPLES {
+            let p = Affine::<g2::Parameters>::rand(&mut rng);
+            let optimised = p.clear_cofactor().into_group();
+            let naive = g2::Parameters::mul_affine(&p, h_eff);
+            assert_eq!(optimised, naive);
+        }
+    }
+}

cp6_782/src/curves/mod.rs

@@ -1,5 +1,7 @@
-use ark_ec::pairing::{MillerLoopOutput, PairingOutput};
-use ark_ec::{models::short_weierstrass::SWCurveConfig, pairing::Pairing};
+use ark_ec::{
+    models::short_weierstrass::SWCurveConfig,
+    pairing::{MillerLoopOutput, Pairing, PairingOutput},
+};
 use ark_ff::{
     biginteger::BigInteger832,
     fields::{BitIteratorBE, Field},

curve-constraint-tests/src/lib.rs

@@ -512,8 +512,10 @@ pub mod curves {
 }
 
 pub mod pairing {
-    use ark_ec::pairing::PairingOutput;
-    use ark_ec::{pairing::Pairing, CurveGroup};
+    use ark_ec::{
+        pairing::{Pairing, PairingOutput},
+        CurveGroup,
+    };
     use ark_ff::{BitIteratorLE, Field, PrimeField};
     use ark_r1cs_std::prelude::*;
     use ark_relations::r1cs::{ConstraintSystem, SynthesisError};

ed_on_bls12_381_bandersnatch/src/curves/mod.rs

@@ -16,8 +16,8 @@ pub type EdwardsProjective = Projective<BandersnatchParameters>;
 pub type SWAffine = short_weierstrass::Affine<BandersnatchParameters>;
 pub type SWProjective = short_weierstrass::Projective<BandersnatchParameters>;
 
-/// `bandersnatch` is an incomplete twisted Edwards curve. These curves have equations of
-/// the form: ax² + y² = 1 + dx²y².
+/// `bandersnatch` is an incomplete twisted Edwards curve. These curves have
+/// equations of the form: ax² + y² = 1 + dx²y².
 /// over some base finite field Fq.
 ///
 /// bandersnatch's curve equation: -5x² + y² = 1 + dx²y²