curve: add precomputation length to MSM structs (#685)

curve25519-dalek/src/backend/mod.rs

@@ -128,6 +128,32 @@ impl VartimePrecomputedStraus {
         }
     }
 
+    /// Return the number of static points in the precomputation.
+    pub fn len(&self) -> usize {
+        use crate::traits::VartimePrecomputedMultiscalarMul;
+
+        match self {
+            #[cfg(curve25519_dalek_backend = "simd")]
+            VartimePrecomputedStraus::Avx2(inner) => inner.len(),
+            #[cfg(all(curve25519_dalek_backend = "unstable_avx512", nightly))]
+            VartimePrecomputedStraus::Avx512ifma(inner) => inner.len(),
+            VartimePrecomputedStraus::Scalar(inner) => inner.len(),
+        }
+    }
+
+    /// Determine if the precomputation is empty.
+    pub fn is_empty(&self) -> bool {
+        use crate::traits::VartimePrecomputedMultiscalarMul;
+
+        match self {
+            #[cfg(curve25519_dalek_backend = "simd")]
+            VartimePrecomputedStraus::Avx2(inner) => inner.is_empty(),
+            #[cfg(all(curve25519_dalek_backend = "unstable_avx512", nightly))]
+            VartimePrecomputedStraus::Avx512ifma(inner) => inner.is_empty(),
+            VartimePrecomputedStraus::Scalar(inner) => inner.is_empty(),
+        }
+    }
+
     pub fn optional_mixed_multiscalar_mul<I, J, K>(
         &self,
         static_scalars: I,

curve25519-dalek/src/backend/serial/scalar_mul/precomputed_straus.rs

@@ -46,6 +46,14 @@ impl VartimePrecomputedMultiscalarMul for VartimePrecomputedStraus {
         }
     }
 
+    fn len(&self) -> usize {
+        self.static_lookup_tables.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.static_lookup_tables.is_empty()
+    }
+
     fn optional_mixed_multiscalar_mul<I, J, K>(
         &self,
         static_scalars: I,

curve25519-dalek/src/backend/vector/scalar_mul/precomputed_straus.rs

@@ -57,6 +57,14 @@ pub mod spec {
             }
         }
 
+        fn len(&self) -> usize {
+            self.static_lookup_tables.len()
+        }
+
+        fn is_empty(&self) -> bool {
+            self.static_lookup_tables.is_empty()
+        }
+
         fn optional_mixed_multiscalar_mul<I, J, K>(
             &self,
             static_scalars: I,

curve25519-dalek/src/edwards.rs

@@ -879,6 +879,14 @@ impl VartimePrecomputedMultiscalarMul for VartimeEdwardsPrecomputation {
         Self(crate::backend::VartimePrecomputedStraus::new(static_points))
     }
 
+    fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
     fn optional_mixed_multiscalar_mul<I, J, K>(
         &self,
         static_scalars: I,
@@ -2136,6 +2144,9 @@ mod test {
 
         let precomputation = VartimeEdwardsPrecomputation::new(static_points.iter());
 
+        assert_eq!(precomputation.len(), 128);
+        assert!(!precomputation.is_empty());
+
         let P = precomputation.vartime_mixed_multiscalar_mul(
             &static_scalars,
             &dynamic_scalars,

curve25519-dalek/src/ristretto.rs

@@ -1027,6 +1027,14 @@ impl VartimePrecomputedMultiscalarMul for VartimeRistrettoPrecomputation {
         ))
     }
 
+    fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
     fn optional_mixed_multiscalar_mul<I, J, K>(
         &self,
         static_scalars: I,
@@ -1852,6 +1860,9 @@ mod test {
 
         let precomputation = VartimeRistrettoPrecomputation::new(static_points.iter());
 
+        assert_eq!(precomputation.len(), 128);
+        assert!(!precomputation.is_empty());
+
         let P = precomputation.vartime_mixed_multiscalar_mul(
             &static_scalars,
             &dynamic_scalars,

curve25519-dalek/src/traits.rs

@@ -299,6 +299,12 @@ pub trait VartimePrecomputedMultiscalarMul: Sized {
         I: IntoIterator,
         I::Item: Borrow<Self::Point>;
 
+    /// Return the number of static points in the precomputation.
+    fn len(&self) -> usize;
+
+    /// Determine if the precomputation is empty.
+    fn is_empty(&self) -> bool;
+
     /// Given `static_scalars`, an iterator of public scalars
     /// \\(b_i\\), compute
     /// $$