Introduce a more typesafe wNAF API, and remove the unstable-wnaf feature.

Cargo.toml

@@ -15,7 +15,6 @@ byteorder = "1.1.0"
 clippy = { version = "0.0.151", optional = true }
 
 [features]
-unstable-wnaf = []
+unstable-features = []
-unstable-features = ["unstable-wnaf"]
 u128-support = []
 default = ["u128-support"]

src/lib.rs

@@ -24,8 +24,8 @@ pub mod tests;
 
 pub mod bls12_381;
 
-#[cfg(feature = "unstable-wnaf")]
+mod wnaf;
-pub mod wnaf;
+pub use self::wnaf::Wnaf;
 
 use std::fmt;
 use std::error::Error;

src/tests/curve.rs

@@ -1,4 +1,4 @@
-use rand::{SeedableRng, XorShiftRng, Rand};
+use rand::{SeedableRng, XorShiftRng, Rand, Rng};
 
 use ::{CurveProjective, CurveAffine, Field, EncodedPoint};
 
@@ -62,16 +62,13 @@ pub fn curve_tests<G: CurveProjective>()
     random_encoding_tests::<G::Affine>();
 }
 
-#[cfg(not(feature = "unstable-wnaf"))]
-fn random_wnaf_tests<G: CurveProjective>() { }
-
-#[cfg(feature = "unstable-wnaf")]
 fn random_wnaf_tests<G: CurveProjective>() {
     use ::wnaf::*;
     use ::PrimeField;
 
     let mut rng = XorShiftRng::from_seed([0x5dbe6259, 0x8d313d76, 0x3237db17, 0xe5bc0654]);
 
+    {
         let mut table = vec![];
         let mut wnaf = vec![];
 
@@ -91,6 +88,93 @@ fn random_wnaf_tests<G: CurveProjective>() {
         }
     }
 
+    {
+        fn only_compiles_if_send<S: Send>(_: &S) { }
+
+        for _ in 0..100 {
+            let g = G::rand(&mut rng);
+            let s = G::Scalar::rand(&mut rng).into_repr();
+            let mut g1 = g;
+            g1.mul_assign(s);
+
+            let g2 = {
+                let mut wnaf = Wnaf::new();
+                wnaf.base(g, 1).scalar(s)
+            };
+            let g3 = {
+                let mut wnaf = Wnaf::new();
+                wnaf.scalar(s).base(g)
+            };
+            let g4 = {
+                let mut wnaf = Wnaf::new();
+                let mut shared = wnaf.base(g, 1).shared();
+
+                only_compiles_if_send(&shared);
+
+                shared.scalar(s)
+            };
+            let g5 = {
+                let mut wnaf = Wnaf::new();
+                let mut shared = wnaf.scalar(s).shared();
+
+                only_compiles_if_send(&shared);
+
+                shared.base(g)
+            };
+
+            let g6 = {
+                let mut wnaf = Wnaf::new();
+                {
+                    // Populate the vectors.
+                    wnaf.base(rng.gen(), 1).scalar(rng.gen());
+                }
+                wnaf.base(g, 1).scalar(s)
+            };
+            let g7 = {
+                let mut wnaf = Wnaf::new();
+                {
+                    // Populate the vectors.
+                    wnaf.base(rng.gen(), 1).scalar(rng.gen());
+                }
+                wnaf.scalar(s).base(g)
+            };
+            let g8 = {
+                let mut wnaf = Wnaf::new();
+                {
+                    // Populate the vectors.
+                    wnaf.base(rng.gen(), 1).scalar(rng.gen());
+                }
+                let mut shared = wnaf.base(g, 1).shared();
+
+                only_compiles_if_send(&shared);
+
+                shared.scalar(s)
+            };
+            let g9 = {
+                let mut wnaf = Wnaf::new();
+                {
+                    // Populate the vectors.
+                    wnaf.base(rng.gen(), 1).scalar(rng.gen());
+                }
+                let mut shared = wnaf.scalar(s).shared();
+
+                only_compiles_if_send(&shared);
+
+                shared.base(g)
+            };
+
+            assert_eq!(g1, g2);
+            assert_eq!(g1, g3);
+            assert_eq!(g1, g4);
+            assert_eq!(g1, g5);
+            assert_eq!(g1, g6);
+            assert_eq!(g1, g7);
+            assert_eq!(g1, g8);
+            assert_eq!(g1, g9);
+        }
+    }
+}
+
 fn random_negation_tests<G: CurveProjective>() {
     let mut rng = XorShiftRng::from_seed([0x5dbe6259, 0x8d313d76, 0x3237db17, 0xe5bc0654]);
 

src/wnaf.rs

@@ -1,4 +1,4 @@
-use super::{CurveProjective, PrimeFieldRepr};
+use super::{CurveProjective, PrimeFieldRepr, PrimeField};
 
 /// Replaces the contents of `table` with a w-NAF window table for the given window size.
 ///
@@ -82,3 +82,118 @@ pub fn wnaf_exp<G: CurveProjective>(table: &[G], wnaf: &[i64]) -> G
 
     result
 }
+
+/// A wNAF exponentiation context.
+#[derive(Debug)]
+pub struct Wnaf<W, B, S> {
+    base: B,
+    scalar: S,
+    window_size: W
+}
+
+impl<G: CurveProjective> Wnaf<(), Vec<G>, Vec<i64>> {
+    /// Construct a new wNAF context without allocating.
+    pub fn new() -> Self {
+        Wnaf {
+            base: vec![],
+            scalar: vec![],
+            window_size: ()
+        }
+    }
+
+    /// Given a base and a number of scalars, compute a window table and return a `Wnaf` object that
+    /// can perform exponentiations with `.scalar(..)`.
+    pub fn base<'a>(
+        &'a mut self,
+        base: G,
+        num_scalars: usize
+    ) -> Wnaf<usize, &'a [G], &'a mut Vec<i64>>
+    {
+        // Compute the appropriate window size based on the number of scalars.
+        let window_size = G::recommended_wnaf_for_num_scalars(num_scalars);
+
+        // Compute a wNAF table for the provided base and window size.
+        wnaf_table(&mut self.base, base, window_size);
+
+        // Return a Wnaf object that immutably borrows the computed base storage location,
+        // but mutably borrows the scalar storage location.
+        Wnaf {
+            base: &self.base,
+            scalar: &mut self.scalar,
+            window_size: window_size
+        }
+    }
+
+    /// Given a scalar, compute its wNAF representation and return a `Wnaf` object that can perform
+    /// exponentiations with `.base(..)`.
+    pub fn scalar<'a>(
+        &'a mut self,
+        scalar: <<G as CurveProjective>::Scalar as PrimeField>::Repr
+    ) -> Wnaf<usize, &'a mut Vec<G>, &'a [i64]>
+    {
+        // Compute the appropriate window size for the scalar.
+        let window_size = G::recommended_wnaf_for_scalar(scalar).unwrap_or(2); // TODO
+
+        // Compute the wNAF form of the scalar.
+        wnaf_form(&mut self.scalar, scalar, window_size);
+
+        // Return a Wnaf object that mutably borrows the base storage location, but
+        // immutably borrows the computed wNAF form scalar location.
+        Wnaf {
+            base: &mut self.base,
+            scalar: &self.scalar,
+            window_size: window_size
+        }
+    }
+}
+
+impl<'a, G: CurveProjective> Wnaf<usize, &'a [G], &'a mut Vec<i64>> {
+    /// Constructs new space for the scalar representation while borrowing
+    /// the computed window table, for sending the window table across threads.
+    pub fn shared(&self) -> Wnaf<usize, &'a [G], Vec<i64>> {
+        Wnaf {
+            base: self.base,
+            scalar: vec![],
+            window_size: self.window_size
+        }
+    }
+}
+
+impl<'a, G: CurveProjective> Wnaf<usize, &'a mut Vec<G>, &'a [i64]> {
+    /// Constructs new space for the window table while borrowing
+    /// the computed scalar representation, for sending the scalar representation
+    /// across threads.
+    pub fn shared(&self) -> Wnaf<usize, Vec<G>, &'a [i64]> {
+        Wnaf {
+            base: vec![],
+            scalar: self.scalar,
+            window_size: self.window_size
+        }
+    }
+}
+
+impl<B, S: AsRef<[i64]>> Wnaf<usize, B, S> {
+    /// Performs exponentiation given a base.
+    pub fn base<G: CurveProjective>(
+        &mut self,
+        base: G
+    ) -> G
+        where B: AsMut<Vec<G>>
+    {
+        wnaf_table(self.base.as_mut(), base, self.window_size);
+        wnaf_exp(self.base.as_mut(), self.scalar.as_ref())
+    }
+}
+
+impl<B, S: AsMut<Vec<i64>>> Wnaf<usize, B, S> {
+    /// Performs exponentiation given a scalar.
+    pub fn scalar<G: CurveProjective>(
+        &mut self,
+        scalar: <<G as CurveProjective>::Scalar as PrimeField>::Repr
+    ) -> G
+        where B: AsRef<[G]>
+    {
+        wnaf_form(self.scalar.as_mut(), scalar, self.window_size);
+        wnaf_exp(self.base.as_ref(), self.scalar.as_mut())
+    }
+}