@ -1,10 +1,13 @@
use ark_ec ::{
bls12 ::Bls12Config ,
models ::{ short_weierstrass ::SWCurveConfig , CurveConfig } ,
short_weierstrass ::Affine ,
short_weierstrass ::{ Affine , Projective } ,
AffineRepr , CurveGroup , Group ,
} ;
use ark_ff ::{ Field , MontFp , Zero } ;
use ark_std ::ops ::Neg ;
use crate ::{ g1 , Fq , Fq2 , Fr } ;
use crate ::* ;
pub type G2Affine = Affine < Config > ;
#[ derive(Clone, Default, PartialEq, Eq) ]
@ -56,6 +59,36 @@ impl SWCurveConfig for Config {
fn mul_by_a ( _ : Self ::BaseField ) -> Self ::BaseField {
Self ::BaseField ::zero ( )
}
#[ 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)
let x : & 'static [ u64 ] = crate ::Config ::X ;
let p_projective = p . into_group ( ) ;
// [x]P
let x_p = Config ::mul_affine ( p , x ) ;
// 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 ;
tmp + = & psi_p ;
// tmp2 = [x^2]P + [x]ψ(P)
let mut tmp2 : Projective < Config > = tmp ;
tmp2 = tmp2 . mul_bigint ( x ) ;
// 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 ) ;
@ -76,3 +109,118 @@ pub const G2_GENERATOR_Y_C0: Fq = MontFp!("6316029476829207320938136194393519890
/// G2_GENERATOR_Y_C1 =
/// 149157405641012693445398062341192467754805999074082136895788947234480009303640899064710353187729182149407503257491
pub const G2_GENERATOR_Y_C1 : Fq = MontFp ! ( "149157405641012693445398062341192467754805999074082136895788947234480009303640899064710353187729182149407503257491" ) ;
// PSI_X = u^((p-1)/3)
const P_POWER_ENDOMORPHISM_COEFF_0 : Fq2 = Fq2 ::new (
MontFp ! (
"80949648264912719408558363140637477264845294720710499478137287262712535938301461879813459410946"
) ,
Fq ::ZERO ,
) ;
// PSI_Y = u^((p-1)/2)
const P_POWER_ENDOMORPHISM_COEFF_1 : Fq2 = Fq2 ::new (
MontFp ! (
"216465761340224619389371505802605247630151569547285782856803747159100223055385581585702401816380679166954762214499" ) ,
Fq ::ZERO ,
) ;
// PSI_2_X = u^((p^2 - 1)/3)
const DOUBLE_P_POWER_ENDOMORPHISM_COEFF_0 : Fq2 = Fq2 ::new (
MontFp ! ( "80949648264912719408558363140637477264845294720710499478137287262712535938301461879813459410945" ) ,
Fq ::ZERO
) ;
/// psi(x,y) is the untwist-Frobenius-twist endomorhism on E'(Fq2)
fn p_power_endomorphism ( p : & Affine < Config > ) -> Affine < Config > {
// The p-power endomorphism for G2 is defined as follows:
// 1. Note that G2 is defined on curve E': y^2 = x^3 + 1/u.
// To map a point (x, y) in E' to (s, t) in E,
// one set s = x * (u ^ (1/3)), t = y * (u ^ (1/2)),
// because E: y^2 = x^3 + 1.
// 2. Apply the Frobenius endomorphism (s, t) => (s', t'),
// another point on curve E, where s' = s^p, t' = t^p.
// 3. Map the point from E back to E'; that is,
// one set x' = s' / ((u) ^ (1/3)), y' = t' / ((u) ^ (1/2)).
//
// To sum up, it maps
// (x,y) -> (x^p * (u ^ ((p-1)/3)), y^p * (u ^ ((p-1)/2)))
// as implemented in the code as follows.
let mut res = * p ;
res . x . frobenius_map_in_place ( 1 ) ;
res . y . frobenius_map_in_place ( 1 ) ;
res . x * = P_POWER_ENDOMORPHISM_COEFF_0 ;
res . y * = P_POWER_ENDOMORPHISM_COEFF_1 ;
res
}
/// For a p-power endomorphism psi(P), compute psi(psi(P))
fn double_p_power_endomorphism ( p : & Projective < Config > ) -> Projective < Config > {
// p_power_endomorphism(&p_power_endomorphism(&p.into_affine())).into()
let mut res = * p ;
res . x * = DOUBLE_P_POWER_ENDOMORPHISM_COEFF_0 ;
// u^((p^2 - 1)/2) == -1
res . y = res . y . neg ( ) ;
res
}
#[ cfg(test) ]
mod test {
use super ::* ;
use ark_std ::{ rand ::Rng , UniformRand } ;
fn sample_unchecked ( ) -> Affine < g2 ::Config > {
let mut rng = ark_std ::test_rng ( ) ;
loop {
let x1 = Fq ::rand ( & mut rng ) ;
let x2 = Fq ::rand ( & mut rng ) ;
let greatest = rng . gen ( ) ;
let x = Fq2 ::new ( x1 , x2 ) ;
if let Some ( p ) = Affine ::get_point_from_x_unchecked ( x , greatest ) {
return p ;
}
}
}
#[ test ]
fn test_psi_2 ( ) {
let p = sample_unchecked ( ) ;
let psi_p = p_power_endomorphism ( & p ) ;
let psi2_p_composed = p_power_endomorphism ( & psi_p ) ;
let psi2_p_optimised = double_p_power_endomorphism ( & p . into ( ) ) ;
assert_eq ! ( psi2_p_composed , psi2_p_optimised ) ;
}
#[ test ]
fn test_cofactor_clearing ( ) {
let h_eff = & [
0x1e34800000000000 ,
0xcf664765b0000003 ,
0x8e8e73ad8a538800 ,
0x78ba279637388559 ,
0xb85860aaaad29276 ,
0xf7ee7c4b03103b45 ,
0x8f6ade35a5c7d769 ,
0xa951764c46f4edd2 ,
0x53648d3d9502abfb ,
0x1f60243677e306 ,
] ;
const SAMPLES : usize = 10 ;
for _ in 0 . . SAMPLES {
let p : Affine < g2 ::Config > = sample_unchecked ( ) ;
let optimised = p . clear_cofactor ( ) ;
let naive = g2 ::Config ::mul_affine ( & p , h_eff ) ;
assert_eq ! ( optimised . into_group ( ) , naive ) ;
assert ! ( optimised . is_on_curve ( ) ) ;
assert ! ( optimised . is_in_correct_subgroup_assuming_on_curve ( ) ) ;
}
}
}
mmagician
1 year ago
GitHub