Add a conditional negation function.

src/field.rs

@@ -803,6 +803,7 @@ impl FieldElement {
 mod test {
     use field::*;
     use test::Bencher;
+    use util::conditional_negate;
 
     #[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;
+        conditional_negate(&mut x,1u8);
+        assert_eq!(x, minus_one);
+        conditional_negate(&mut x,0u8);
+        assert_eq!(x, minus_one);
+        conditional_negate(&mut x,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 +21,15 @@ pub trait CTAssignable {
     fn conditional_assign(&mut self, other: &Self, choice: u8);
 }
 
+/// Conditionally negate an element if `choice == 1u8`.
+pub fn conditional_negate<T>(x: &mut T, choice: u8)
+    where T: CTAssignable, for<'a> &'a T: Neg<Output=T>
+{
+    // Need to cast to discard mutability
+    let x_neg = -(x as &T);
+    x.conditional_assign(&x_neg, choice);
+}
+
 /// Check equality of two bytes in constant time.
 ///
 /// # Return