online-phase: algebra: scalar: Optimize (de)serialization

This has large effects throughout any stack built on top of the `Scalar`
type, so we use the `ark_serialize` `CanonicalSerialize` and
`CanonicalDeserialize` traits.

online-phase/Cargo.toml

@@ -31,6 +31,11 @@ path = "integration/main.rs"
 harness = false
 required-features = ["test_helpers"]
 
+[[bench]]
+name = "scalar_serialization"
+harness = false
+required-features = ["benchmarks", "test_helpers"]
+
 [[bench]]
 name = "batch_ops"
 harness = false

online-phase/benches/scalar_serialization.rs

@@ -0,0 +1,61 @@
+use std::time::{Duration, Instant};
+
+use ark_mpc::{algebra::Scalar, test_helpers::TestCurve};
+use criterion::{black_box, criterion_group, criterion_main, Criterion, Throughput};
+use rand::thread_rng;
+
+/// Benchmark the serialization of scalars
+fn bench_scalar_serialization(c: &mut Criterion) {
+    let mut rng = thread_rng();
+    let mut group = c.benchmark_group("scalar_serialization");
+    group.throughput(Throughput::Elements(1));
+
+    group.bench_function("serialize", |b| {
+        b.iter_custom(|n_iters| {
+            let mut total_time = Duration::from_secs(0);
+            for _ in 0..n_iters {
+                let scalar = Scalar::<TestCurve>::random(&mut rng);
+
+                let start = Instant::now();
+                let bytes = serde_json::to_vec(&scalar).unwrap();
+                total_time += start.elapsed();
+
+                black_box(bytes);
+            }
+            total_time
+        })
+    });
+}
+
+/// Benchmark the deserialization of scalars
+fn bench_scalar_deserialization(c: &mut Criterion) {
+    let mut rng = thread_rng();
+    let mut group = c.benchmark_group("scalar_serialization");
+    group.throughput(Throughput::Elements(1));
+
+    group.bench_function("deserialize", |b| {
+        b.iter_custom(|n_iters| {
+            let mut total_time = Duration::from_secs(0);
+            for _ in 0..n_iters {
+                let scalar = Scalar::<TestCurve>::random(&mut rng);
+                let serialized = serde_json::to_vec(&scalar).unwrap();
+
+                // Time deserialization only
+                let start = Instant::now();
+                let deserialized: Scalar<TestCurve> = serde_json::from_slice(&serialized).unwrap();
+                total_time += start.elapsed();
+
+                black_box(deserialized);
+            }
+
+            total_time
+        })
+    });
+}
+
+criterion_group! {
+    name = scalar_serialization;
+    config = Criterion::default();
+    targets = bench_scalar_serialization, bench_scalar_deserialization
+}
+criterion_main!(scalar_serialization);

online-phase/src/algebra/scalar/scalar.rs

@@ -13,6 +13,7 @@ use std::{
 use ark_ec::CurveGroup;
 use ark_ff::{batch_inversion, FftField, Field, One, PrimeField, Zero};
 use ark_poly::EvaluationDomain;
+use ark_serialize::{CanonicalDeserialize, CanonicalSerialize};
 use ark_std::UniformRand;
 use itertools::Itertools;
 use num_bigint::BigUint;
@@ -168,16 +169,18 @@ impl<C: CurveGroup> Display for Scalar<C> {
 
 impl<C: CurveGroup> Serialize for Scalar<C> {
     fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
-        let bytes = self.to_bytes_be();
-        bytes.serialize(serializer)
+        let mut bytes = Vec::with_capacity(n_bytes_field::<C::ScalarField>());
+        self.0.serialize_uncompressed(&mut bytes).map_err(serde::ser::Error::custom)?;
+        serializer.serialize_bytes(&bytes)
     }
 }
 
 impl<'de, C: CurveGroup> Deserialize<'de> for Scalar<C> {
     fn deserialize<D: serde::Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
         let bytes = <Vec<u8>>::deserialize(deserializer)?;
-        let scalar = Scalar::from_be_bytes_mod_order(&bytes);
-        Ok(scalar)
+        let inner = C::ScalarField::deserialize_uncompressed(bytes.as_slice())
+            .map_err(serde::de::Error::custom)?;
+        Ok(Scalar(inner))
     }
 }
 
@@ -720,6 +723,17 @@ mod test {
     use itertools::Itertools;
     use rand::{thread_rng, Rng, RngCore};
 
+    /// Tests serialization and deserialization of scalars
+    #[test]
+    fn test_scalar_serialization() {
+        let mut rng = thread_rng();
+        let scalar = Scalar::<TestCurve>::random(&mut rng);
+
+        let bytes = serde_json::to_vec(&scalar).unwrap();
+        let deserialized: Scalar<TestCurve> = serde_json::from_slice(&bytes).unwrap();
+        assert_eq!(scalar, deserialized);
+    }
+
     /// Tests addition of raw scalars in a circuit
     #[tokio::test]
     async fn test_scalar_add() {