group: Introduce Group and PrimeGroup traits

Group represents a cryptographic group with a large prime-order subgroup
and a small cofactor. PrimeGroup further constrains the group to have a
cofactor of one.

bellman/src/groth16/generator.rs

@@ -3,7 +3,7 @@ use std::ops::{AddAssign, MulAssign};
 use std::sync::Arc;
 
 use ff::Field;
-use group::{CurveAffine, CurveProjective, Wnaf};
+use group::{CurveAffine, CurveProjective, Group, Wnaf};
 use pairing::Engine;
 
 use super::{Parameters, VerifyingKey};

bellman/src/groth16/tests/dummy_engine.rs

@@ -1,5 +1,5 @@
 use ff::{Field, PrimeField, ScalarEngine};
-use group::{CurveAffine, CurveProjective, EncodedPoint, GroupDecodingError};
+use group::{CurveAffine, CurveProjective, EncodedPoint, Group, GroupDecodingError, PrimeGroup};
 use pairing::{Engine, PairingCurveAffine};
 
 use rand_core::RngCore;
@@ -352,11 +352,7 @@ impl Engine for DummyEngine {
     }
 }
 
-impl CurveProjective for Fr {
-    type Affine = Fr;
-    type Base = Fr;
-    type Scalar = Fr;
-
+impl Group for Fr {
     fn random<R: RngCore + ?Sized>(rng: &mut R) -> Self {
         <Fr as Field>::random(rng)
     }
@@ -372,6 +368,14 @@ impl CurveProjective for Fr {
     fn is_identity(&self) -> bool {
         <Fr as Field>::is_zero(self)
     }
+}
+
+impl PrimeGroup for Fr {}
+
+impl CurveProjective for Fr {
+    type Affine = Fr;
+    type Base = Fr;
+    type Scalar = Fr;
 
     fn batch_normalization(_: &mut [Self]) {}
 

bellman/src/multiexp.rs

@@ -314,6 +314,7 @@ fn test_with_bls12() {
         acc
     }
 
+    use group::Group;
     use pairing::{bls12_381::Bls12, Engine};
     use rand;
 

group/src/lib.rs

@@ -12,6 +12,26 @@ pub mod tests;
 mod wnaf;
 pub use self::wnaf::Wnaf;
 
+/// This trait represents an element of a cryptographic group.
+pub trait Group:
+    Clone + Copy + fmt::Debug + fmt::Display + Eq + Sized + Send + Sync + 'static
+{
+    /// Returns an element chosen uniformly at random using a user-provided RNG.
+    fn random<R: RngCore + ?Sized>(rng: &mut R) -> Self;
+
+    /// Returns the additive identity, also known as the "neutral element".
+    fn identity() -> Self;
+
+    /// Returns a fixed generator of the prime-order subgroup.
+    fn generator() -> Self;
+
+    /// Determines if this point is the identity.
+    fn is_identity(&self) -> bool;
+}
+
+/// This trait represents an element of a prime-order cryptographic group.
+pub trait PrimeGroup: Group {}
+
 /// A helper trait for types implementing group addition.
 pub trait CurveOps<Rhs = Self, Output = Self>:
     Add<Rhs, Output = Output> + Sub<Rhs, Output = Output> + AddAssign<Rhs> + SubAssign<Rhs>
@@ -30,16 +50,7 @@ impl<T, Rhs, Output> CurveOpsOwned<Rhs, Output> for T where T: for<'r> CurveOps<
 /// Projective representation of an elliptic curve point guaranteed to be
 /// in the correct prime order subgroup.
 pub trait CurveProjective:
-    PartialEq
-    + Eq
-    + Sized
-    + Copy
-    + Clone
-    + Send
-    + Sync
-    + fmt::Debug
-    + fmt::Display
-    + 'static
+    Group
     + Neg<Output = Self>
     + CurveOps
     + CurveOpsOwned
@@ -50,18 +61,6 @@ pub trait CurveProjective:
     type Base: Field;
     type Affine: CurveAffine<Projective = Self, Scalar = Self::Scalar>;
 
-    /// Returns an element chosen uniformly at random using a user-provided RNG.
-    fn random<R: RngCore + ?Sized>(rng: &mut R) -> Self;
-
-    /// Returns the additive identity.
-    fn identity() -> Self;
-
-    /// Returns a fixed generator of unknown exponent.
-    fn generator() -> Self;
-
-    /// Determines if this point is the point at infinity.
-    fn is_identity(&self) -> bool;
-
     /// Normalizes a slice of projective elements so that
     /// conversion to affine is cheap.
     fn batch_normalization(v: &mut [Self]);

pairing/benches/bls12_381/ec.rs

@@ -5,7 +5,7 @@ pub(crate) mod g1 {
     use std::ops::AddAssign;
 
     use ff::Field;
-    use group::CurveProjective;
+    use group::{CurveProjective, Group};
     use pairing::bls12_381::*;
 
     fn bench_g1_mul_assign(c: &mut Criterion) {
@@ -92,7 +92,7 @@ pub(crate) mod g2 {
     use std::ops::AddAssign;
 
     use ff::Field;
-    use group::CurveProjective;
+    use group::{CurveProjective, Group};
     use pairing::bls12_381::*;
 
     fn bench_g2_mul_assign(c: &mut Criterion) {

pairing/benches/bls12_381/mod.rs

@@ -8,7 +8,7 @@ use criterion::{criterion_group, Criterion};
 use rand_core::SeedableRng;
 use rand_xorshift::XorShiftRng;
 
-use group::CurveProjective;
+use group::Group;
 use pairing::bls12_381::*;
 use pairing::{Engine, PairingCurveAffine};
 

pairing/src/bls12_381/ec.rs

@@ -508,11 +508,7 @@ macro_rules! curve_impl {
             }
         }
 
-        impl CurveProjective for $projective {
-            type Scalar = $scalarfield;
-            type Base = $basefield;
-            type Affine = $affine;
-
+        impl Group for $projective {
             fn random<R: RngCore + ?Sized>(rng: &mut R) -> Self {
                 loop {
                     let x = $basefield::random(rng);
@@ -548,6 +544,14 @@ macro_rules! curve_impl {
             fn is_identity(&self) -> bool {
                 self.z.is_zero()
             }
+        }
+
+        impl PrimeGroup for $projective {}
+
+        impl CurveProjective for $projective {
+            type Scalar = $scalarfield;
+            type Base = $basefield;
+            type Affine = $affine;
 
             fn is_normalized(&self) -> bool {
                 self.is_identity() || self.z == $basefield::one()
@@ -748,7 +752,9 @@ pub mod g1 {
     use super::g2::G2Affine;
     use crate::{Engine, PairingCurveAffine};
     use ff::{BitIterator, Field, PrimeField};
-    use group::{CurveAffine, CurveProjective, EncodedPoint, GroupDecodingError};
+    use group::{
+        CurveAffine, CurveProjective, EncodedPoint, Group, GroupDecodingError, PrimeGroup,
+    };
     use rand_core::RngCore;
     use std::fmt;
     use std::ops::{AddAssign, MulAssign, Neg, SubAssign};
@@ -1356,7 +1362,9 @@ pub mod g2 {
     use super::g1::G1Affine;
     use crate::{Engine, PairingCurveAffine};
     use ff::{BitIterator, Field, PrimeField};
-    use group::{CurveAffine, CurveProjective, EncodedPoint, GroupDecodingError};
+    use group::{
+        CurveAffine, CurveProjective, EncodedPoint, Group, GroupDecodingError, PrimeGroup,
+    };
     use rand_core::RngCore;
     use std::fmt;
     use std::ops::{AddAssign, MulAssign, Neg, SubAssign};

pairing/src/bls12_381/tests/mod.rs

@@ -1,5 +1,5 @@
 use ff::PrimeField;
-use group::{CurveAffine, CurveProjective, EncodedPoint, GroupDecodingError};
+use group::{CurveAffine, CurveProjective, EncodedPoint, Group, GroupDecodingError};
 
 use super::*;
 use crate::*;

pairing/src/tests/engine.rs

@@ -1,5 +1,5 @@
 use ff::{Endianness, Field, PrimeField};
-use group::{CurveAffine, CurveProjective};
+use group::{CurveAffine, CurveProjective, Group};
 use rand_core::SeedableRng;
 use rand_xorshift::XorShiftRng;
 use std::ops::MulAssign;