impl ConditionallyNegatable for BoxedUint

Author: dvdplm

src/traits.rs

@@ -77,27 +77,6 @@ impl<T: ConditionallySelectable> ConstantTimeSelect for T {
     }
 }
 
-/// A type which can be conditionally negated in constant time.
-///
-/// Similar to `subtle`'s `ConditionallyNegate` trait, but without the `Copy` bound which allows
-/// it to to be impl'd for heap allocated types.
-pub trait ConstantTimeNegatable {
-    /// Negate `self` according to `choice`.
-    fn ct_negate(&mut self, choice: Choice);
-}
-
-impl<T> ConstantTimeNegatable for T
-where
-    T: ConstantTimeSelect,
-    for<'a> &'a T: Neg<Output = T>,
-{
-    #[inline]
-    fn ct_negate(&mut self, choice: Choice) {
-        let self_neg = -(&*self);
-        self.ct_assign(&self_neg, choice);
-    }
-}
-
 /// Integer trait: represents common functionality of integer types provided by this crate.
 pub trait Integer:
     'static

src/uint/boxed/ct.rs

@@ -2,7 +2,7 @@
 
 use super::BoxedUint;
 use crate::{ConstantTimeSelect, Limb};
-use subtle::{Choice, ConditionallySelectable};
+use subtle::{Choice, ConditionallyNegatable, ConditionallySelectable};
 
 /// NOTE: can't impl `subtle`'s [`ConditionallySelectable`] trait due to its `Copy` bound
 impl ConstantTimeSelect for BoxedUint {
@@ -37,11 +37,18 @@ impl ConstantTimeSelect for BoxedUint {
     }
 }
 
+impl ConditionallyNegatable for BoxedUint {
+    #[inline]
+    fn conditional_negate(&mut self, choice: Choice) {
+        let self_neg = self.wrapping_neg();
+        self.ct_assign(&self_neg, choice)
+    }
+}
+
 #[cfg(test)]
 mod tests {
-    use super::BoxedUint;
-    use crate::ConstantTimeSelect;
-    use subtle::Choice;
+    use crate::{BoxedUint, ConstantTimeSelect};
+    use subtle::{Choice, ConditionallyNegatable};
 
     #[test]
     fn conditional_select() {
@@ -51,4 +58,17 @@ mod tests {
         assert_eq!(a, BoxedUint::ct_select(&a, &b, Choice::from(0)));
         assert_eq!(b, BoxedUint::ct_select(&a, &b, Choice::from(1)));
     }
+
+    #[test]
+    fn conditional_negate() {
+        let mut a = BoxedUint::from(123u64);
+        let control = a.clone();
+
+        a.conditional_negate(Choice::from(0));
+        assert_eq!(a, control);
+
+        a.conditional_negate(Choice::from(1));
+        assert_ne!(a, control);
+        assert_eq!(a, control.wrapping_neg());
+    }
 }