1use ark_ec::{
  2    models::CurveConfig,
  3    short_weierstrass::{self, SWCurveConfig},
  4    twisted_edwards::{Affine, MontCurveConfig, Projective, TECurveConfig},
  5};
  6use ark_ff::{Field, MontFp};
  7
  8use crate::{Fq, Fr};
  9
 10#[cfg(test)]
 11mod tests;
 12
 13pub type EdwardsAffine = Affine<BandersnatchParameters>;
 14pub type EdwardsProjective = Projective<BandersnatchParameters>;
 15
 16pub type SWAffine = short_weierstrass::Affine<BandersnatchParameters>;
 17pub type SWProjective = short_weierstrass::Projective<BandersnatchParameters>;
 18
 19/// `bandersnatch` is a twisted Edwards curve. These curves have equations of
 20/// the form: ax² + y² = 1 - dx²y².
 21/// over some base finite field Fq.
 22///
 23/// bandersnatch's curve equation: -5x² + y² = 1 - dx²y²
 24///
 25/// q = 52435875175126190479447740508185965837690552500527637822603658699938581184513.
 26///
 27/// a = -5.
 28/// d = (138827208126141220649022263972958607803/
 29///     171449701953573178309673572579671231137) mod q
 30///   = 45022363124591815672509500913686876175488063829319466900776701791074614335719.
 31///
 32/// Sage script to calculate these:
 33///
 34/// ```text
 35/// q = 52435875175126190479447740508185965837690552500527637822603658699938581184513
 36/// Fq = GF(q)
 37/// d = (Fq(138827208126141220649022263972958607803)/Fq(171449701953573178309673572579671231137))
 38/// ```
 39/// These parameters and the sage script obtained from:
 40/// <https://github.com/asanso/Bandersnatch/>
 41///
 42/// bandersnatch also has a short Weierstrass curve form, following the
 43/// form: y² = x³ + A * x + B
 44/// where
 45///
 46/// A = 10773120815616481058602537765553212789256758185246796157495669123169359657269
 47/// B = 29569587568322301171008055308580903175558631321415017492731745847794083609535
 48///
 49/// Script to transfer between different curves are available
 50/// <https://github.com/zhenfeizhang/bandersnatch/blob/main/bandersnatch/script/bandersnatch.sage>
 51#[derive(Clone, Default, PartialEq, Eq)]
 52pub struct BandersnatchParameters;
 53
 54pub type EdwardsParameters = BandersnatchParameters;
 55pub type SWParameters = BandersnatchParameters;
 56
 57impl CurveConfig for BandersnatchParameters {
 58    type BaseField = Fq;
 59    type ScalarField = Fr;
 60
 61    /// COFACTOR = 4
 62    const COFACTOR: &'static [u64] = &[4];
 63
 64    /// COFACTOR^(-1) mod r =
 65    /// 9831726595336160714896451345284868594481866920080427688839802480047265754601
 66    const COFACTOR_INV: Fr =
 67        MontFp!("9831726595336160714896451345284868594481866920080427688839802480047265754601");
 68}
 69
 70impl TECurveConfig for BandersnatchParameters {
 71    /// COEFF_A = -5
 72    const COEFF_A: Fq = MontFp!("-5");
 73
 74    /// COEFF_D = (138827208126141220649022263972958607803/
 75    /// 171449701953573178309673572579671231137) mod q
 76    const COEFF_D: Fq =
 77        MontFp!("45022363124591815672509500913686876175488063829319466900776701791074614335719");
 78
 79    /// AFFINE_GENERATOR_COEFFS = (GENERATOR_X, GENERATOR_Y)
 80    const GENERATOR: EdwardsAffine = EdwardsAffine::new_unchecked(TE_GENERATOR_X, TE_GENERATOR_Y);
 81
 82    type MontCurveConfig = BandersnatchParameters;
 83
 84    /// Multiplication by `a` is multiply by `-5`.
 85    #[inline(always)]
 86    fn mul_by_a(elem: &Self::BaseField) -> Self::BaseField {
 87        let t = (*elem).double().double();
 88        -(t + *elem)
 89    }
 90}
 91
 92impl MontCurveConfig for BandersnatchParameters {
 93    /// COEFF_A = 29978822694968839326280996386011761570173833766074948509196803838190355340952
 94    const COEFF_A: Fq =
 95        MontFp!("29978822694968839326280996386011761570173833766074948509196803838190355340952");
 96
 97    /// COEFF_B = 25465760566081946422412445027709227188579564747101592991722834452325077642517
 98    const COEFF_B: Fq =
 99        MontFp!("25465760566081946422412445027709227188579564747101592991722834452325077642517");
100
101    type TECurveConfig = BandersnatchParameters;
102}
103
104// The TE form generator is generated following Zcash's fashion:
105//  "The generators of G1 and G2 are computed by finding the lexicographically
106//   smallest valid x-coordinate, and its lexicographically smallest
107//   y-coordinate and scaling it by the cofactor such that the result is not
108//   the point at infinity."
109// The SW form generator is the same TE generator converted into SW form,
110// obtained from the scripts:
111//   <https://github.com/zhenfeizhang/bandersnatch/blob/main/bandersnatch/script/bandersnatch.sage>
112
113/// x coordinate for TE curve generator
114const TE_GENERATOR_X: Fq =
115    MontFp!("18886178867200960497001835917649091219057080094937609519140440539760939937304");
116
117/// y coordinate for TE curve generator
118const TE_GENERATOR_Y: Fq =
119    MontFp!("19188667384257783945677642223292697773471335439753913231509108946878080696678");
120
121/// x coordinate for SW curve generator
122const SW_GENERATOR_X: Fq =
123    MontFp!("30900340493481298850216505686589334086208278925799850409469406976849338430199");
124
125/// y coordinate for SW curve generator
126const SW_GENERATOR_Y: Fq =
127    MontFp!("12663882780877899054958035777720958383845500985908634476792678820121468453298");
128
129impl SWCurveConfig for BandersnatchParameters {
130    /// COEFF_A = 10773120815616481058602537765553212789256758185246796157495669123169359657269
131    const COEFF_A: Self::BaseField =
132        MontFp!("10773120815616481058602537765553212789256758185246796157495669123169359657269");
133
134    /// COEFF_B = 29569587568322301171008055308580903175558631321415017492731745847794083609535
135    const COEFF_B: Self::BaseField =
136        MontFp!("29569587568322301171008055308580903175558631321415017492731745847794083609535");
137
138    /// generators
139    const GENERATOR: SWAffine = SWAffine::new_unchecked(SW_GENERATOR_X, SW_GENERATOR_Y);
140}