Use a variable time multiplication for the torsion check

As the group order is anyway public, there is no reason to prevent
it from leaking to a side channel.

curve25519-dalek/benches/dalek_benchmarks.rs

@@ -29,6 +29,13 @@ mod edwards_benches {
         });
     }
 
+    fn is_torsion_free<M: Measurement>(c: &mut BenchmarkGroup<M>) {
+        let B = &constants::ED25519_BASEPOINT_POINT;
+        c.bench_function("EdwardsPoint is_torsion_free", move |b| {
+            b.iter(|| B.is_torsion_free())
+        });
+    }
+
     fn consttime_fixed_base_scalar_mul<M: Measurement>(c: &mut BenchmarkGroup<M>) {
         let s = Scalar::from(897987897u64).invert();
         c.bench_function("Constant-time fixed-base scalar mul", move |b| {
@@ -62,6 +69,7 @@ mod edwards_benches {
 
         compress(&mut g);
         decompress(&mut g);
+        is_torsion_free(&mut g);
         consttime_fixed_base_scalar_mul(&mut g);
         consttime_variable_base_scalar_mul(&mut g);
         vartime_double_base_scalar_mul(&mut g);

curve25519-dalek/src/backend/mod.rs

@@ -236,6 +236,21 @@ pub fn variable_base_mul(point: &EdwardsPoint, scalar: &Scalar) -> EdwardsPoint
     }
 }
 
+/// Perform variable-time, variable-base scalar multiplication.
+pub fn vartime_variable_base_mul(point: &EdwardsPoint, scalar: &Scalar) -> EdwardsPoint {
+    match get_selected_backend() {
+        #[cfg(curve25519_dalek_backend = "simd")]
+        BackendKind::Avx2 => {
+            vector::scalar_mul::variable_base::spec_avx2::vartime_mul(point, scalar)
+        }
+        #[cfg(all(curve25519_dalek_backend = "simd", nightly))]
+        BackendKind::Avx512 => {
+            vector::scalar_mul::variable_base::spec_avx512ifma_avx512vl::vartime_mul(point, scalar)
+        }
+        BackendKind::Serial => serial::scalar_mul::variable_base::vartime_mul(point, scalar),
+    }
+}
+
 /// Compute \\(aA + bB\\) in variable time, where \\(B\\) is the Ed25519 basepoint.
 #[allow(non_snake_case)]
 pub fn vartime_double_base_mul(a: &Scalar, A: &EdwardsPoint, b: &Scalar) -> EdwardsPoint {

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

@@ -1,10 +1,12 @@
 #![allow(non_snake_case)]
 
-use crate::backend::serial::curve_models::ProjectiveNielsPoint;
+use core::cmp::Ordering;
+
+use crate::backend::serial::curve_models::{ProjectiveNielsPoint, ProjectivePoint};
 use crate::edwards::EdwardsPoint;
 use crate::scalar::Scalar;
 use crate::traits::Identity;
-use crate::window::LookupTable;
+use crate::window::{LookupTable, NafLookupTable5};
 
 /// Perform constant-time, variable-base scalar multiplication.
 #[rustfmt::skip] // keep alignment of explanatory comments
@@ -46,3 +48,39 @@ pub(crate) fn mul(point: &EdwardsPoint, scalar: &Scalar) -> EdwardsPoint {
     }
     tmp1.as_extended()
 }
+
+/// Perform variable-time, variable-base scalar multiplication.
+pub fn vartime_mul(point: &EdwardsPoint, scalar: &Scalar) -> EdwardsPoint {
+    let naf = scalar.non_adjacent_form(5);
+    let table = NafLookupTable5::<ProjectiveNielsPoint>::from(point);
+
+    // Find starting index
+    let mut i: usize = 255;
+    for j in (0..256).rev() {
+        i = j;
+        if naf[i] != 0 {
+            break;
+        }
+    }
+
+    let mut r = ProjectivePoint::identity();
+
+    loop {
+        let mut t = r.double();
+
+        match naf[i].cmp(&0) {
+            Ordering::Greater => t = &t.as_extended() + &table.select(naf[i] as usize),
+            Ordering::Less => t = &t.as_extended() - &table.select(-naf[i] as usize),
+            Ordering::Equal => {}
+        }
+
+        r = t.as_projective();
+
+        if i == 0 {
+            break;
+        }
+        i -= 1;
+    }
+
+    r.as_extended()
+}

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

@@ -6,6 +6,8 @@
 )]
 pub mod spec {
 
+    use core::cmp::Ordering;
+
     #[for_target_feature("avx2")]
     use crate::backend::vector::avx2::{CachedPoint, ExtendedPoint};
 
@@ -15,7 +17,7 @@ pub mod spec {
     use crate::edwards::EdwardsPoint;
     use crate::scalar::Scalar;
     use crate::traits::Identity;
-    use crate::window::LookupTable;
+    use crate::window::{LookupTable, NafLookupTable5};
 
     /// Perform constant-time, variable-base scalar multiplication.
     pub fn mul(point: &EdwardsPoint, scalar: &Scalar) -> EdwardsPoint {
@@ -41,4 +43,42 @@ pub mod spec {
         }
         Q.into()
     }
+
+    /// Perform variable-time, variable-base scalar multiplication.
+    pub fn vartime_mul(point: &EdwardsPoint, scalar: &Scalar) -> EdwardsPoint {
+        let naf = scalar.non_adjacent_form(5);
+        let table = NafLookupTable5::<CachedPoint>::from(point);
+
+        // Find starting index
+        let mut i: usize = 255;
+        for j in (0..256).rev() {
+            i = j;
+            if naf[i] != 0 {
+                break;
+            }
+        }
+
+        let mut Q = ExtendedPoint::identity();
+
+        loop {
+            Q = Q.double();
+
+            match naf[i].cmp(&0) {
+                Ordering::Greater => {
+                    Q = &Q + &table.select(naf[i] as usize);
+                }
+                Ordering::Less => {
+                    Q = &Q - &table.select(-naf[i] as usize);
+                }
+                Ordering::Equal => {}
+            }
+
+            if i == 0 {
+                break;
+            }
+            i -= 1;
+        }
+
+        Q.into()
+    }
 }

curve25519-dalek/src/edwards.rs

@@ -1255,7 +1255,8 @@ impl EdwardsPoint {
     /// assert_eq!((P+Q).is_torsion_free(), false);
     /// ```
     pub fn is_torsion_free(&self) -> bool {
-        (self * constants::BASEPOINT_ORDER_PRIVATE).is_identity()
+        crate::backend::vartime_variable_base_mul(self, &constants::BASEPOINT_ORDER_PRIVATE)
+            .is_identity()
     }
 }