group: Extract compressed encodings from CurveAffine trait

bellman/src/groth16/mod.rs

@@ -2,7 +2,7 @@
 //!
 //! [Groth16]: https://eprint.iacr.org/2016/260
 
-use group::{CurveAffine, UncompressedEncoding};
+use group::{CurveAffine, GroupEncoding, UncompressedEncoding};
 use pairing::{Engine, MultiMillerLoop};
 
 use crate::SynthesisError;
@@ -47,7 +47,7 @@ impl<E: Engine> Proof<E> {
 
     pub fn read<R: Read>(mut reader: R) -> io::Result<Self> {
         let read_g1 = |reader: &mut R| -> io::Result<E::G1Affine> {
-            let mut g1_repr = <E::G1Affine as CurveAffine>::Compressed::default();
+            let mut g1_repr = <E::G1Affine as GroupEncoding>::Compressed::default();
             reader.read_exact(g1_repr.as_mut())?;
 
             let affine = E::G1Affine::from_compressed(&g1_repr);
@@ -70,7 +70,7 @@ impl<E: Engine> Proof<E> {
         };
 
         let read_g2 = |reader: &mut R| -> io::Result<E::G2Affine> {
-            let mut g2_repr = <E::G2Affine as CurveAffine>::Compressed::default();
+            let mut g2_repr = <E::G2Affine as GroupEncoding>::Compressed::default();
             reader.read_exact(g2_repr.as_mut())?;
 
             let affine = E::G2Affine::from_compressed(&g2_repr);

bellman/src/groth16/tests/dummy_engine.rs

@@ -1,5 +1,5 @@
 use ff::{Field, PrimeField};
-use group::{CurveAffine, CurveProjective, Group, PrimeGroup, UncompressedEncoding};
+use group::{CurveAffine, CurveProjective, Group, GroupEncoding, PrimeGroup, UncompressedEncoding};
 use pairing::{Engine, MillerLoopResult, MultiMillerLoop, PairingCurveAffine};
 
 use rand_core::RngCore;
@@ -425,7 +425,6 @@ impl AsRef<[u8]> for FakePoint {
 }
 
 impl CurveAffine for Fr {
-    type Compressed = FakePoint;
     type Projective = Fr;
     type Scalar = Fr;
 
@@ -444,6 +443,10 @@ impl CurveAffine for Fr {
     fn to_projective(&self) -> Self::Projective {
         *self
     }
+}
+
+impl GroupEncoding for Fr {
+    type Compressed = FakePoint;
 
     fn from_compressed(_bytes: &Self::Compressed) -> CtOption<Self> {
         unimplemented!()

group/src/lib.rs

@@ -132,13 +132,13 @@ pub trait CurveAffine:
     + PartialEq
     + Eq
     + 'static
+    + GroupEncoding
     + Neg<Output = Self>
     + Mul<<Self as CurveAffine>::Scalar, Output = <Self as CurveAffine>::Projective>
     + for<'r> Mul<<Self as CurveAffine>::Scalar, Output = <Self as CurveAffine>::Projective>
 {
     type Scalar: PrimeField;
     type Projective: CurveProjective<Affine = Self, Scalar = Self::Scalar>;
-    type Compressed: Default + AsRef<[u8]> + AsMut<[u8]>;
 
     /// Returns the additive identity.
     fn identity() -> Self;
@@ -152,6 +152,11 @@ pub trait CurveAffine:
 
     /// Converts this element into its affine representation.
     fn to_projective(&self) -> Self::Projective;
+}
+
+pub trait GroupEncoding: Sized {
+    /// The encoding of group elements.
+    type Compressed: Default + AsRef<[u8]> + AsMut<[u8]>;
 
     /// Attempts to deserialize an element from its compressed encoding.
     fn from_compressed(bytes: &Self::Compressed) -> CtOption<Self>;

group/src/tests/mod.rs

@@ -3,7 +3,7 @@ use rand::SeedableRng;
 use rand_xorshift::XorShiftRng;
 use std::ops::{Mul, Neg};
 
-use crate::{CurveAffine, CurveProjective, UncompressedEncoding};
+use crate::{CurveAffine, CurveProjective, GroupEncoding, UncompressedEncoding};
 
 pub fn curve_tests<G: CurveProjective>() {
     let mut rng = XorShiftRng::from_seed([

pairing/src/bls12_381/ec.rs

@@ -200,7 +200,6 @@ macro_rules! curve_impl {
         impl CurveAffine for $affine {
             type Scalar = $scalarfield;
             type Projective = $projective;
-            type Compressed = $compressed;
 
             fn identity() -> Self {
                 $affine {
@@ -221,6 +220,10 @@ macro_rules! curve_impl {
             fn to_projective(&self) -> $projective {
                 (*self).into()
             }
+        }
+
+        impl GroupEncoding for $affine {
+            type Compressed = $compressed;
 
             fn from_compressed(bytes: &Self::Compressed) -> CtOption<Self> {
                 Self::from_compressed_unchecked(bytes).and_then(|affine| {
@@ -904,7 +907,9 @@ pub mod g1 {
     use super::{g2::G2Affine, GroupDecodingError};
     use crate::{Engine, PairingCurveAffine};
     use ff::{BitIterator, Field, PrimeField};
-    use group::{CurveAffine, CurveProjective, Group, PrimeGroup, UncompressedEncoding};
+    use group::{
+        CurveAffine, CurveProjective, Group, GroupEncoding, PrimeGroup, UncompressedEncoding,
+    };
     use rand_core::RngCore;
     use std::fmt;
     use std::ops::{AddAssign, MulAssign, Neg, SubAssign};
@@ -1481,7 +1486,9 @@ pub mod g2 {
     use super::{g1::G1Affine, GroupDecodingError};
     use crate::{Engine, PairingCurveAffine};
     use ff::{BitIterator, Field, PrimeField};
-    use group::{CurveAffine, CurveProjective, Group, PrimeGroup, UncompressedEncoding};
+    use group::{
+        CurveAffine, CurveProjective, Group, GroupEncoding, PrimeGroup, UncompressedEncoding,
+    };
     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, UncompressedEncoding};
+use group::{CurveAffine, CurveProjective, GroupEncoding, UncompressedEncoding};
 
 use super::*;
 use crate::*;
@@ -87,7 +87,7 @@ where
 
 fn compressed_test_vectors<G: CurveProjective>(expected: &[u8]) {
     let mut e = G::identity();
-    let encoded_len = <G::Affine as CurveAffine>::Compressed::default()
+    let encoded_len = <G::Affine as GroupEncoding>::Compressed::default()
         .as_ref()
         .len();
 
@@ -99,7 +99,7 @@ fn compressed_test_vectors<G: CurveProjective>(expected: &[u8]) {
             let encoded = e_affine.to_compressed();
             v.extend_from_slice(encoded.as_ref());
 
-            let mut decoded = <G::Affine as CurveAffine>::Compressed::default();
+            let mut decoded = <G::Affine as GroupEncoding>::Compressed::default();
             decoded.as_mut().copy_from_slice(&expected[0..encoded_len]);
             expected = &expected[encoded_len..];
             let decoded = G::Affine::from_compressed(&decoded).unwrap();