Support Pasta curves. (#4)

* Support pasta curves directly.

* Upgrade rand.

* Simplify.

* Don't deny missing docs.

Co-authored-by: porcuquine <porcuquine@users.noreply.github.com>

Cargo.toml

@@ -12,13 +12,11 @@ keywords = ["zkSNARKs", "cryptography", "proofs"]
 
 [dependencies]
 merlin = "2.0.0"
-rand = "0.7.3"
+rand = "0.8.4"
 digest = "0.8.1"
 sha3 = "0.8.2"
 rayon = "1.3.0"
 rand_core = { version = "0.5", default-features = false }
 itertools = "0.9.0"
 subtle = "2.4"
-
-[dev-dependencies]
-curve25519-dalek = {version = "3.0.0", features = ["simd_backend"]}
+pasta_curves = "0.2.1"

src/lib.rs

@@ -6,6 +6,7 @@
 
 mod commitments;
 pub mod errors;
+pub mod pasta;
 pub mod r1cs;
 pub mod traits;
 
@@ -125,88 +126,11 @@ impl FinalSNARK {
 #[cfg(test)]
 mod tests {
   use super::*;
-  use crate::traits::{CompressedGroup, Group, PrimeField};
-  use core::borrow::Borrow;
-  use curve25519_dalek::traits::MultiscalarMul;
-  use rand::{rngs::OsRng, CryptoRng, RngCore};
-
-  type S = curve25519_dalek::scalar::Scalar;
-  type G = curve25519_dalek::ristretto::RistrettoPoint;
-  type C = curve25519_dalek::ristretto::CompressedRistretto;
-
-  impl Group for G {
-    type Scalar = S;
-    type CompressedGroupElement = C;
-
-    fn vartime_multiscalar_mul<I, J>(scalars: I, points: J) -> Self
-    where
-      I: IntoIterator,
-      I::Item: Borrow<Self::Scalar>,
-      J: IntoIterator,
-      J::Item: Borrow<Self>,
-      Self: Clone,
-    {
-      Self::multiscalar_mul(scalars, points)
-    }
-
-    fn compress(&self) -> Self::CompressedGroupElement {
-      self.compress()
-    }
-
-    fn from_uniform_bytes(bytes: &[u8]) -> Option<Self> {
-      if bytes.len() != 64 {
-        None
-      } else {
-        let mut arr = [0; 64];
-        arr.copy_from_slice(&bytes[0..64]);
-        Some(Self::from_uniform_bytes(&arr))
-      }
-    }
-  }
-
-  impl PrimeField for S {
-    fn zero() -> Self {
-      S::zero()
-    }
-
-    fn one() -> Self {
-      S::one()
-    }
-
-    fn from_bytes_mod_order_wide(bytes: &[u8]) -> Option<Self> {
-      if bytes.len() != 64 {
-        None
-      } else {
-        let mut arr = [0; 64];
-        arr.copy_from_slice(&bytes[0..64]);
-        Some(Self::from_bytes_mod_order_wide(&arr))
-      }
-    }
-
-    fn random(rng: &mut (impl RngCore + CryptoRng)) -> Self {
-      S::random(rng)
-    }
-  }
-
-  impl CompressedGroup for C {
-    type GroupElement = G;
+  use crate::traits::PrimeField;
+  use rand::rngs::OsRng;
 
-    fn decompress(&self) -> Option<Self::GroupElement> {
-      self.decompress()
-    }
-
-    fn as_bytes(&self) -> &[u8] {
-      self.as_bytes()
-    }
-  }
-
-  impl ChallengeTrait for S {
-    fn challenge(label: &'static [u8], transcript: &mut Transcript) -> Self {
-      let mut buf = [0u8; 64];
-      transcript.challenge_bytes(label, &mut buf);
-      S::from_bytes_mod_order_wide(&buf)
-    }
-  }
+  type S = pasta_curves::pallas::Scalar;
+  type G = pasta_curves::pallas::Point;
 
   #[test]
   fn test_tiny_r1cs() {

src/pasta.rs

@@ -0,0 +1,86 @@
+//! This module implements the Nova traits for pallas::Point and pallas::Scalar.
+use crate::traits::{ChallengeTrait, CompressedGroup, Group, PrimeField};
+use merlin::Transcript;
+use pasta_curves::arithmetic::{CurveExt, FieldExt, Group as Grp};
+use pasta_curves::group::GroupEncoding;
+use pasta_curves::{self, pallas, Ep, Fq};
+use rand::{CryptoRng, RngCore};
+use std::borrow::Borrow;
+use std::ops::Mul;
+
+impl Group for pallas::Point {
+  type Scalar = pallas::Scalar;
+  type CompressedGroupElement = <pallas::Point as GroupEncoding>::Repr;
+
+  fn vartime_multiscalar_mul<I, J>(scalars: I, points: J) -> Self
+  where
+    I: IntoIterator,
+    I::Item: Borrow<Self::Scalar>,
+    J: IntoIterator,
+    J::Item: Borrow<Self>,
+    Self: Clone,
+  {
+    // Unoptimized.
+    scalars
+      .into_iter()
+      .zip(points)
+      .map(|(scalar, point)| (*point.borrow()).mul(*scalar.borrow()))
+      .fold(Ep::group_zero(), |acc, x| acc + x)
+  }
+
+  fn compress(&self) -> Self::CompressedGroupElement {
+    self.to_bytes()
+  }
+
+  fn from_uniform_bytes(bytes: &[u8]) -> Option<Self> {
+    if bytes.len() != 64 {
+      None
+    } else {
+      let mut arr = [0; 32];
+      arr.copy_from_slice(&bytes[0..32]);
+
+      let hash = Ep::hash_to_curve("from_uniform_bytes");
+      Some(hash(&arr))
+    }
+  }
+}
+
+impl PrimeField for pallas::Scalar {
+  fn zero() -> Self {
+    Fq::zero()
+  }
+  fn one() -> Self {
+    Fq::one()
+  }
+  fn from_bytes_mod_order_wide(bytes: &[u8]) -> Option<Self> {
+    if bytes.len() != 64 {
+      None
+    } else {
+      let mut arr = [0; 64];
+      arr.copy_from_slice(&bytes[0..64]);
+      Some(Fq::from_bytes_wide(&arr))
+    }
+  }
+
+  fn random(_rng: &mut (impl RngCore + CryptoRng)) -> Self {
+    Fq::rand()
+  }
+}
+
+impl ChallengeTrait for pallas::Scalar {
+  fn challenge(label: &'static [u8], transcript: &mut Transcript) -> Self {
+    let mut buf = [0u8; 64];
+    transcript.challenge_bytes(label, &mut buf);
+    pallas::Scalar::from_bytes_mod_order_wide(&buf).unwrap()
+  }
+}
+
+impl CompressedGroup for <pallas::Point as GroupEncoding>::Repr {
+  type GroupElement = pallas::Point;
+  fn decompress(&self) -> Option<<Self as CompressedGroup>::GroupElement> {
+    Some(Ep::from_bytes(&self).unwrap())
+  }
+  fn as_bytes(&self) -> &[u8] {
+    self
+  }
+}