refactor: implement our own TE config

Previously defining the cofactor was done on `AffineCurve`: this
let us set it to 1 since decaf377 provides a prime-order group.
However, the cofactor is now moved to the curve configuration
via the trait `CurveConfig`.

This means we can no longer use the upstream `ark_ed_on_bls12_377`
curve configuration type: we need to define our own and implement
the required traits, and then use that to define `EdwardsAffine`,
`EdwardsProjective`, and `EdwardsVar` (R1CS) instead of the upstream
provided definitions of those types.

Author: redshiftzero

src/constants.rs

@@ -54,6 +54,12 @@ pub static B_T: Lazy<Fq> = Lazy::new(|| {
 });
 pub static B_Z: Lazy<Fq> = Lazy::new(|| ark_ff::MontFp!("1"));
 
+// Canonical basepoint affine coordinates
+pub const GENERATOR_X: Fq =
+    ark_ff::MontFp!("4959445789346820725352484487855828915252512307947624787834978378872129235627");
+pub const GENERATOR_Y: Fq =
+    ark_ff::MontFp!("6060471950081851567114691557659790004756535011754163002297540472747064943288");
+
 // Modulus of basefield
 pub static R: Lazy<Fr> = Lazy::new(|| {
     ark_ff::MontFp!("2111115437357092606062206234695386632838870926408408195193685246394721360383")

src/element.rs

@@ -1,18 +1,75 @@
-use ark_ec::{AffineRepr, CurveGroup, Group, ScalarMul, VariableBaseMSM};
-use ark_ed_on_bls12_377::{EdwardsAffine, EdwardsConfig, EdwardsProjective};
+use ark_ec::{
+    twisted_edwards::{Affine, MontCurveConfig, Projective, TECurveConfig},
+    AffineRepr, CurveConfig, CurveGroup, Group, ScalarMul, VariableBaseMSM,
+};
+use ark_ed_on_bls12_377::EdwardsConfig;
+use ark_ff::MontFp;
 use ark_serialize::Valid;
 
-use crate::{Fq, Fr};
+use crate::{
+    constants::{GENERATOR_X, GENERATOR_Y},
+    Fq, Fr,
+};
 
 pub mod affine;
 pub mod projective;
 
 pub use affine::AffineElement;
 pub use projective::Element;
 
+#[derive(Clone, Default, PartialEq, Eq)]
+pub struct Decaf377EdwardsConfig;
+
+// These types should not be exported. They are similar to `EdwardsAffine` and
+// `EdwardsProjective` from the `ark_ed_on_bls12_377` crate, except using our own
+// `Decaf377Config` that has the cofactor set to 1. Consumers of this
+// library should use the `AffineElement` and `Element` (projective)
+// types.
+pub(crate) type EdwardsAffine = Affine<Decaf377EdwardsConfig>;
+pub(crate) type EdwardsProjective = Projective<Decaf377EdwardsConfig>;
+
+impl CurveConfig for Decaf377EdwardsConfig {
+    type BaseField = Fq;
+    type ScalarField = Fr;
+
+    const COFACTOR: &'static [u64] = &[1];
+
+    const COFACTOR_INV: Fr = MontFp!("1");
+}
+
+impl TECurveConfig for Decaf377EdwardsConfig {
+    /// COEFF_A = -1
+    const COEFF_A: Fq = <EdwardsConfig as ark_ec::twisted_edwards::TECurveConfig>::COEFF_A;
+
+    /// COEFF_D = 3021
+    const COEFF_D: Fq = <EdwardsConfig as ark_ec::twisted_edwards::TECurveConfig>::COEFF_D;
+
+    const GENERATOR: EdwardsAffine = EdwardsAffine::new_unchecked(GENERATOR_X, GENERATOR_Y);
+
+    type MontCurveConfig = EdwardsConfig;
+
+    /// Multiplication by `a` is just negation.
+    #[inline(always)]
+    fn mul_by_a(elem: Self::BaseField) -> Self::BaseField {
+        -elem
+    }
+
+    fn is_in_correct_subgroup_assuming_on_curve(_: &Affine<Self>) -> bool {
+        true
+    }
+}
+
+impl MontCurveConfig for Decaf377EdwardsConfig {
+    const COEFF_A: Fq = <EdwardsConfig as ark_ec::twisted_edwards::MontCurveConfig>::COEFF_A;
+
+    const COEFF_B: Fq = <EdwardsConfig as ark_ec::twisted_edwards::MontCurveConfig>::COEFF_B;
+
+    type TECurveConfig = Decaf377EdwardsConfig;
+}
+
 impl Valid for Element {
     fn check(&self) -> Result<(), ark_serialize::SerializationError> {
-        todo!()
+        Ok(())
     }
 }
 
@@ -84,7 +141,7 @@ impl CurveGroup for Element {
 
 impl Valid for AffineElement {
     fn check(&self) -> Result<(), ark_serialize::SerializationError> {
-        todo!()
+        Ok(())
     }
 }
 

src/element/affine.rs

@@ -1,6 +1,6 @@
 use std::hash::Hash;
 
-use ark_ed_on_bls12_377::EdwardsAffine;
+use crate::element::EdwardsAffine;
 use ark_std::fmt::{Display, Formatter, Result as FmtResult};
 use ark_std::Zero;
 

src/element/projective.rs

@@ -1,13 +1,12 @@
 use std::borrow::Borrow;
 use std::hash::Hash;
 
-use ark_ed_on_bls12_377::EdwardsProjective;
 use ark_ff::Zero;
 use ark_std::fmt::{Display, Formatter, Result as FmtResult};
 
 use zeroize::Zeroize;
 
-use crate::{Fq, Fr};
+use crate::{EdwardsProjective, Fq, Fr};
 
 #[derive(Copy, Clone)]
 pub struct Element {

src/elligator.rs

@@ -1,8 +1,9 @@
 #![allow(non_snake_case)]
 use ark_ec::twisted_edwards::TECurveConfig;
-use ark_ed_on_bls12_377::{EdwardsConfig, EdwardsProjective};
 use ark_ff::Field;
 
+use crate::element::{Decaf377EdwardsConfig, EdwardsProjective};
+
 use crate::{
     constants::{ONE, TWO, ZETA},
     Element, Fq, OnCurve, Sign, SqrtRatioZeta,
@@ -12,8 +13,8 @@ impl Element {
     /// Elligator 2 map to decaf377 point
     fn elligator_map(r_0: &Fq) -> Element {
         // Ref: `Decaf_1_1_Point.elligator` (optimized) in `ristretto.sage`
-        let A = EdwardsConfig::COEFF_A;
-        let D = EdwardsConfig::COEFF_D;
+        let A = Decaf377EdwardsConfig::COEFF_A;
+        let D = Decaf377EdwardsConfig::COEFF_D;
 
         let r = *ZETA * r_0.square();
 
@@ -44,8 +45,8 @@ impl Element {
 
         // Convert point to extended projective (X : Y : Z : T)
         let E = *TWO * s;
-        let F = *ONE + EdwardsConfig::COEFF_A * s.square();
-        let G = *ONE - EdwardsConfig::COEFF_A * s.square();
+        let F = *ONE + Decaf377EdwardsConfig::COEFF_A * s.square();
+        let G = *ONE - Decaf377EdwardsConfig::COEFF_A * s.square();
         let H = t;
         let result = Element {
             inner: EdwardsProjective::new(E * H, F * G, E * G, F * H),
@@ -86,7 +87,7 @@ impl Element {
 
 #[cfg(test)]
 mod tests {
-    use ark_ed_on_bls12_377::EdwardsAffine;
+    use crate::element::EdwardsAffine;
 
     use super::*;
 

src/encoding.rs

@@ -3,11 +3,13 @@
 use std::convert::{TryFrom, TryInto};
 
 use ark_ec::twisted_edwards::TECurveConfig;
-use ark_ed_on_bls12_377::{EdwardsConfig, EdwardsProjective};
 use ark_ff::{Field, One};
 use ark_serialize::{CanonicalDeserialize, CanonicalSerialize};
 
-use crate::{constants::TWO, Element, EncodingError, Fq, OnCurve, Sign, SqrtRatioZeta};
+use crate::{
+    constants::TWO, element::Decaf377EdwardsConfig, EdwardsProjective, Element, EncodingError, Fq,
+    OnCurve, Sign, SqrtRatioZeta,
+};
 
 #[derive(Copy, Clone, Default, Eq, Ord, PartialOrd, PartialEq)]
 pub struct Encoding(pub [u8; 32]);
@@ -35,7 +37,7 @@ impl Encoding {
 
         // This isn't a constant, only because traits don't have const methods
         // yet and multiplication is only implemented as part of the Mul trait.
-        let D4: Fq = EdwardsConfig::COEFF_D * Fq::from(4u32);
+        let D4: Fq = Decaf377EdwardsConfig::COEFF_D * Fq::from(4u32);
 
         // 1/2. Reject unless s is canonically encoded and nonnegative.
         let s =
@@ -90,7 +92,7 @@ impl Element {
     pub fn vartime_compress_to_field(&self) -> Fq {
         // This isn't a constant, only because traits don't have const methods
         // yet and subtraction is only implemented as part of the Sub trait.
-        let A_MINUS_D = EdwardsConfig::COEFF_A - EdwardsConfig::COEFF_D;
+        let A_MINUS_D = Decaf377EdwardsConfig::COEFF_A - Decaf377EdwardsConfig::COEFF_D;
         let p = &self.inner;
 
         // 1.

src/lib.rs

@@ -14,9 +14,9 @@ pub mod rand;
 pub mod serialize;
 mod sign;
 
-use ark_ed_on_bls12_377::EdwardsProjective;
 pub use constants::ZETA;
 pub use element::{AffineElement, Element};
+pub(crate) use element::{Decaf377EdwardsConfig, EdwardsProjective};
 pub use encoding::Encoding;
 pub use error::EncodingError;
 pub use field_ext::FieldExt;

src/ops/affine.rs

@@ -1,9 +1,8 @@
 use core::ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign};
 
 use ark_ec::twisted_edwards::Projective;
-use ark_ed_on_bls12_377::EdwardsConfig;
 
-use crate::{element::AffineElement, Element, Fr};
+use crate::{element::AffineElement, Decaf377EdwardsConfig, Element, Fr};
 
 impl<'a, 'b> Add<&'b AffineElement> for &'a AffineElement {
     type Output = AffineElement;
@@ -102,7 +101,7 @@ impl Neg for AffineElement {
 
 impl<'b> MulAssign<&'b Fr> for AffineElement {
     fn mul_assign(&mut self, point: &'b Fr) {
-        let mut p: Projective<EdwardsConfig> = self.inner.into();
+        let mut p: Projective<Decaf377EdwardsConfig> = self.inner.into();
         p *= *point;
         *self = AffineElement { inner: p.into() }
     }
@@ -118,7 +117,7 @@ impl<'a, 'b> Mul<&'b Fr> for &'a AffineElement {
     type Output = AffineElement;
 
     fn mul(self, point: &'b Fr) -> AffineElement {
-        let mut p: Projective<EdwardsConfig> = self.inner.into();
+        let mut p: Projective<Decaf377EdwardsConfig> = self.inner.into();
         p *= *point;
         AffineElement { inner: p.into() }
     }

src/ops/projective.rs

@@ -105,7 +105,7 @@ impl Neg for Element {
 
 impl<'b> MulAssign<&'b Fr> for Element {
     // Scalar multiplication is performed through the implementation
-    // of `MulAssign` on `EdwardsProjective` which is a type alias for
+    // of `MulAssign` on `ProjectiveDecaf377` which is a type alias for
     // `Group<EdwardsConfig>`.
     fn mul_assign(&mut self, point: &'b Fr) {
         let mut p = self.inner;

src/r1cs/element.rs

@@ -2,17 +2,16 @@
 use std::borrow::Borrow;
 
 use ark_ec::AffineRepr;
-use ark_ed_on_bls12_377::{
-    constraints::{EdwardsVar, FqVar},
-    EdwardsAffine,
-};
+use ark_ed_on_bls12_377::constraints::FqVar;
 use ark_r1cs_std::{alloc::AllocVar, eq::EqGadget, prelude::*, R1CSVar};
 use ark_relations::r1cs::{ConstraintSystemRef, SynthesisError};
 
-use crate::r1cs::inner::ElementVar as InnerElementVar;
 use crate::r1cs::lazy::LazyElementVar;
+use crate::{element::EdwardsAffine, r1cs::inner::ElementVar as InnerElementVar};
 use crate::{AffineElement, Element, Fq};
 
+use super::inner::Decaf377EdwardsVar;
+
 #[derive(Clone, Debug)]
 /// Represents the R1CS equivalent of a `decaf377::Element`
 ///
@@ -115,7 +114,7 @@ impl CondSelectGadget<Fq> for ElementVar {
         let y = cond.select(&true_element.inner.y, &false_element.inner.y)?;
 
         let new_element = InnerElementVar {
-            inner: EdwardsVar::new(x, y),
+            inner: Decaf377EdwardsVar::new(x, y),
         };
         Ok(Self {
             inner: LazyElementVar::new_from_element(new_element),