Merge branch 'feature/scalarmult-lhs' into develop

src/curve.rs

@@ -84,6 +84,8 @@ use core::ops::{Mul, MulAssign};
 use core::ops::Index;
 
 use constants;
+#[cfg(feature = "yolocrypto")]
+use decaf::DecafPoint;
 use field::FieldElement;
 use scalar::Scalar;
 use subtle::arrays_equal_ct;
@@ -445,6 +447,15 @@ impl CTAssignable for AffineNielsPoint {
     }
 }
 
+impl CTAssignable for ExtendedPoint {
+    fn conditional_assign(&mut self, other: &ExtendedPoint, choice: u8) {
+        self.X.conditional_assign(&other.X, choice);
+        self.Y.conditional_assign(&other.Y, choice);
+        self.Z.conditional_assign(&other.Z, choice);
+        self.T.conditional_assign(&other.T, choice);
+    }
+}
+
 // ------------------------------------------------------------------------
 // Constant-time Equality
 // ------------------------------------------------------------------------
@@ -817,6 +828,29 @@ impl<'a, 'b> Mul<&'b Scalar> for &'a ExtendedPoint {
     }
 }
 
+impl<'a, 'b> Mul<&'b ExtendedPoint> for &'a Scalar {
+    type Output = ExtendedPoint;
+
+    /// Scalar multiplication: compute `self * point`.
+    ///
+    /// Uses a window of size 4.  Note: for scalar multiplication of
+    /// the basepoint, `basepoint_mult` is approximately 4x faster.
+    fn mul(self, point: &'b ExtendedPoint) -> ExtendedPoint {
+        point * &self
+    }
+}
+
+#[cfg(feature = "yolocrypto")]
+impl<'a, 'b> Mul<&'b DecafPoint> for &'a Scalar {
+    type Output = DecafPoint;
+
+    /// Scalar multiplication: compute `self * scalar`.
+    fn mul(self, point: &'b DecafPoint) -> DecafPoint {
+        DecafPoint(self * &point.0)
+    }
+}
+
+
 /// Precomputation
 #[derive(Clone)]
 pub struct EdwardsBasepointTable(pub [[AffineNielsPoint; 8]; 32]);
@@ -870,6 +904,39 @@ impl<'a, 'b> Mul<&'b Scalar> for &'a EdwardsBasepointTable {
     }
 }
 
+impl<'a, 'b> Mul<&'a EdwardsBasepointTable> for &'b Scalar {
+    type Output = ExtendedPoint;
+
+    /// Construct an `ExtendedPoint` by via this `Scalar` times
+    /// a the basepoint, `B` included in a precomputed `basepoint_table`.
+    ///
+    /// Precondition: this scalar must be reduced.
+    ///
+    /// The computation proceeds as follows, as described on page 13
+    /// of the Ed25519 paper.  Write this scalar `a` in radix 16 with
+    /// coefficients in [-8,8), i.e.,
+    ///
+    ///    a = a_0 + a_1*16^1 + ... + a_63*16^63,
+    ///
+    /// with -8 ≤ a_i < 8.  Then
+    ///
+    ///    a*B = a_0*B + a_1*16^1*B + ... + a_63*16^63*B.
+    ///
+    /// Grouping even and odd coefficients gives
+    ///
+    ///    a*B =       a_0*16^0*B + a_2*16^2*B + ... + a_62*16^62*B
+    ///              + a_1*16^1*B + a_3*16^3*B + ... + a_63*16^63*B
+    ///        =      (a_0*16^0*B + a_2*16^2*B + ... + a_62*16^62*B)
+    ///          + 16*(a_1*16^0*B + a_3*16^2*B + ... + a_63*16^62*B).
+    ///
+    /// We then use the `select_precomputed_point` function, which
+    /// takes `-8 ≤ x < 8` and `[16^2i * B, ..., 8 * 16^2i * B]`,
+    /// and returns `x * 16^2i * B` in constant time.
+    fn mul(self, basepoint_table: &'a EdwardsBasepointTable) -> ExtendedPoint {
+        basepoint_table * &self
+    }
+}
+
 impl EdwardsBasepointTable {
     /// Create a table of precomputed multiples of `basepoint`.
     pub fn create(basepoint: &ExtendedPoint) -> EdwardsBasepointTable {
@@ -1150,6 +1217,8 @@ pub mod vartime {
 
 #[cfg(test)]
 mod test {
+    #[cfg(feature = "yolocrypto")]
+    use decaf::DecafPoint;
     use field::FieldElement;
     use scalar::Scalar;
     use subtle::CTAssignable;
@@ -1459,6 +1528,29 @@ mod test {
         }
     }
 
+    #[test]
+    fn scalarmult_extended_point_works_both_ways() {
+        let G: ExtendedPoint = constants::ED25519_BASEPOINT;
+        let s: Scalar = A_SCALAR;
+
+        let P1 = &G * &s;
+        let P2 = &s * &G;
+
+        assert!(P1.compress_edwards().to_bytes() == P2.compress_edwards().to_bytes());
+    }
+
+    #[test]
+    #[cfg(feature = "yolocrypto")]
+    fn scalarmult_decafpoint_works_both_ways() {
+        let P: DecafPoint = DecafPoint(constants::ED25519_BASEPOINT);
+        let s: Scalar = A_SCALAR;
+
+        let P1 = &P * &s;
+        let P2 = &s * &P;
+
+        assert!(P1.compress().as_bytes() == P2.compress().as_bytes());
+    }
+
     mod vartime {
         use super::super::*;
         use super::{A_SCALAR, B_SCALAR, A_TIMES_BASEPOINT, DOUBLE_SCALAR_MULT_RESULT};

src/decaf.rs

@@ -271,6 +271,14 @@ impl<'a, 'b> Mul<&'b Scalar> for &'a DecafBasepointTable {
     }
 }
 
+impl<'a, 'b> Mul<&'a DecafBasepointTable> for &'b Scalar {
+    type Output = DecafPoint;
+
+    fn mul(self, basepoint_table: &'a DecafBasepointTable) -> DecafPoint {
+        DecafPoint(self * &basepoint_table.0)
+    }
+}
+
 impl DecafBasepointTable {
     /// Create a precomputed table of multiples of the given `basepoint`.
     pub fn create(basepoint: &DecafPoint) -> DecafBasepointTable {
@@ -283,6 +291,38 @@ impl DecafBasepointTable {
     }
 }
 
+// ------------------------------------------------------------------------
+// Constant-time conditional assignment
+// ------------------------------------------------------------------------
+
+impl CTAssignable for DecafPoint {
+    /// Conditionally assign `other` to `self`, if `choice == 1u8`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use curve25519_dalek::curve::Identity;
+    /// # use curve25519_dalek::decaf::DecafPoint;
+    /// # use curve25519_dalek::subtle::CTAssignable;
+    /// # use curve25519_dalek::constants;
+    /// let A = DecafPoint::identity();
+    /// let B = constants::DECAF_ED25519_BASEPOINT;
+    ///
+    /// let mut P = A;
+    ///
+    /// P.conditional_assign(&B, 0u8);
+    /// assert!(P == A);
+    /// P.conditional_assign(&B, 1u8);
+    /// assert!(P == B);
+    /// ```
+    fn conditional_assign(&mut self, other: &DecafPoint, choice: u8) {
+        self.0.X.conditional_assign(&other.0.X, choice);
+        self.0.Y.conditional_assign(&other.0.Y, choice);
+        self.0.Z.conditional_assign(&other.0.Z, choice);
+        self.0.T.conditional_assign(&other.0.T, choice);
+    }
+}
+
 // ------------------------------------------------------------------------
 // Debug traits
 // ------------------------------------------------------------------------

src/scalar.rs

@@ -775,10 +775,11 @@ mod test {
         assert_eq!(should_be_one, Scalar::one());
     }
 
+    #[allow(non_snake_case)]
     #[test]
     fn impl_mul() {
-        let should_be_X_TIMES_Y = &X * &Y;
-        assert_eq!(should_be_X_TIMES_Y, X_TIMES_Y);
+        let should_be_X_times_Y = &X * &Y;
+        assert_eq!(should_be_X_times_Y, X_TIMES_Y);
     }
 
     #[test]
@@ -809,6 +810,7 @@ mod test {
         }
     }
 
+    #[allow(non_snake_case)]
     #[test]
     fn invert() {
         let inv_X = X.invert();
@@ -817,6 +819,7 @@ mod test {
     }
 
     // Negating a scalar twice should result in the original scalar.
+    #[allow(non_snake_case)]
     #[test]
     fn neg_twice_is_identity() {
         let negative_X = -&X;