Merge remote-tracking branch 'hdevalence/feature/conditional-negation' into develop

src/curve.rs

@@ -87,6 +87,7 @@ use field::FieldElement;
 use scalar::Scalar;
 use util::bytes_equal_ct;
 use util::CTAssignable;
+use util::CTNegateable;
 
 // ------------------------------------------------------------------------
 // Compressed points
@@ -734,9 +735,8 @@ fn select_precomputed_point<T>(x: i8, points: &[T; 8]) -> T
     }
     // Now t == |x| * P.
 
-    let minus_t = -(&t);
     let neg_mask = (xmask & 1) as u8;
-    t.conditional_assign(&minus_t, neg_mask);
+    t.conditional_negate(neg_mask);
     // Now t == x * P.
 
     t

src/field.rs

@@ -803,6 +803,7 @@ impl FieldElement {
 mod test {
     use field::*;
     use test::Bencher;
+    use util::CTNegateable;
 
     #[bench]
     fn bench_fieldelement_a_mul_a(b: &mut Bencher) {
@@ -932,4 +933,17 @@ mod test {
         // high bit is set to zero in to_bytes
         assert!(test_bytes[31] == (B_BYTES[31] & 127u8));
     }
+
+    #[test]
+    fn test_conditional_negate() {
+        let       one = FieldElement([ 1,0,0,0,0,0,0,0,0,0]);
+        let minus_one = FieldElement([-1,0,0,0,0,0,0,0,0,0]);
+        let mut x = one;
+        x.conditional_negate(1u8);
+        assert_eq!(x, minus_one);
+        x.conditional_negate(0u8);
+        assert_eq!(x, minus_one);
+        x.conditional_negate(1u8);
+        assert_eq!(x, one);
+    }
 }

src/util.rs

@@ -11,6 +11,8 @@
 
 //! Utility functions and tools for constant-time comparisons.
 
+use core::ops::Neg;
+
 /// Trait for items which can be conditionally assigned in constant time.
 pub trait CTAssignable {
     /// If `choice == 1u8`, assign `other` to `self`.
@@ -19,6 +21,26 @@ pub trait CTAssignable {
     fn conditional_assign(&mut self, other: &Self, choice: u8);
 }
 
+/// Trait for items which can be conditionally negated in constant time.
+///
+/// Note: it is not necessary to implement this trait, as a generic
+/// implementation is provided.
+pub trait CTNegateable
+{
+    /// Conditionally negate an element if `choice == 1u8`.
+    fn conditional_negate(&mut self, choice: u8);
+}
+
+impl<T> CTNegateable for T
+    where T: CTAssignable, for<'a> &'a T: Neg<Output=T>
+{
+    fn conditional_negate(&mut self, choice: u8) {
+        // Need to cast to eliminate mutability
+        let self_neg: T = -(self as &T);
+        self.conditional_assign(&self_neg, choice);
+    }
+}
+
 /// Check equality of two bytes in constant time.
 ///
 /// # Return