feat: support serialization of MsmHandle to a file (PROOF-920) (#45)

* bump blitzar version

* support constructing a msm handle from a file

* add write method

* add test

* format

* format

Cargo.toml

@@ -21,7 +21,7 @@ ark-ff = { version = "0.4.0" }
 ark-serialize = { version = "0.4.2" }
 ark-std = { version = "0.4.0" }
 rayon = { version = "1.5" }
-blitzar-sys = { version = "1.78.0" }
+blitzar-sys = { version = "1.81.0" }
 curve25519-dalek = { version = "4", features = ["serde"] }
 merlin = "2"
 serde = { version = "1", features = ["serde_derive"] }
@@ -33,6 +33,7 @@ criterion = { version = "0.3", features = ["html_reports"] }
 curve25519-dalek = { version = "4", features = ["rand_core"] }
 rand = "0.8"
 rand_core = "0.6"
+tempfile = "3.13.0"
 
 [[bench]]
 harness = false

src/compute/fixed_msm.rs

@@ -2,7 +2,7 @@ use super::backend::init_backend;
 use crate::compute::{curve::SwCurveConfig, CurveId, ElementP2};
 use ark_ec::short_weierstrass::Affine;
 use rayon::prelude::*;
-use std::marker::PhantomData;
+use std::{ffi::CString, marker::PhantomData};
 
 fn count_scalars_per_output(scalars_len: usize, output_bit_table: &[u32]) -> u32 {
     let bit_sum: usize = output_bit_table.iter().map(|s| *s as usize).sum();
@@ -46,6 +46,34 @@ impl<T: CurveId> MsmHandle<T> {
         }
     }
 
+    /// New handle from a serialized file.
+    ///
+    /// Note: any MSMs computed with the handle must have length less than or equal
+    /// to the number of generators used to create the handle.
+    pub fn new_from_file(filename: &str) -> Self {
+        init_backend();
+        let filename = CString::new(filename).expect("filename cannot have null bytes");
+        unsafe {
+            let handle =
+                blitzar_sys::sxt_multiexp_handle_new_from_file(T::CURVE_ID, filename.as_ptr());
+            Self {
+                handle,
+                phantom: PhantomData,
+            }
+        }
+    }
+
+    /// Serialize the handle to a file.
+    ///
+    /// This function can be used together with new_from_file to reduce
+    /// the cost of creating a handle.
+    pub fn write(&self, filename: &str) {
+        let filename = CString::new(filename).expect("filename cannot have null bytes");
+        unsafe {
+            blitzar_sys::sxt_multiexp_handle_write_to_file(self.handle, filename.as_ptr());
+        }
+    }
+
     /// Compute an MSM using pre-specified generators.
     ///
     /// Suppose g_1, ..., g_n are pre-specified generators and

src/compute/fixed_msm_tests.rs

@@ -4,6 +4,7 @@ use ark_bls12_381::G1Affine;
 use ark_std::UniformRand;
 use curve25519_dalek::ristretto::RistrettoPoint;
 use rand_core::OsRng;
+use tempfile::TempDir;
 
 #[test]
 fn we_can_compute_msms_using_a_single_generator() {
@@ -48,6 +49,33 @@ fn we_can_compute_msms_using_multiple_generator() {
     assert_eq!(res[0], generators[0] + generators[1] + generators[1]);
 }
 
+#[test]
+fn we_can_serialize_a_handle_to_a_file() {
+    let mut rng = OsRng;
+
+    let mut res = vec![RistrettoPoint::default(); 1];
+
+    // randomly obtain the generator points
+    let generators: Vec<RistrettoPoint> =
+        (0..2).map(|_| RistrettoPoint::random(&mut rng)).collect();
+
+    // create handle
+    let handle = MsmHandle::new(&generators);
+
+    // write the handle to a file
+    let tmp_dir = TempDir::new().unwrap();
+    let filename = tmp_dir.path().join("t").to_str().unwrap().to_string();
+    handle.write(&filename);
+
+    // read the handle back from file
+    let handle = MsmHandle::<RistrettoPoint>::new_from_file(&filename);
+
+    // we can compute a multiexponentiation
+    let scalars: Vec<u8> = vec![1, 2];
+    handle.msm(&mut res, 1, &scalars);
+    assert_eq!(res[0], generators[0] + generators[1] + generators[1]);
+}
+
 #[test]
 fn we_can_compute_msms_using_multiple_outputs() {
     let mut rng = OsRng;