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Implement CTAssignable for Scalar

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Henry de Valence 2017-01-06 12:56:53 -05:00
parent 74049e039e
commit 63e7e9f8de
1 changed files with 32 additions and 0 deletions
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32
src/scalar.rs
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@ -32,6 +32,7 @@ use std::ops::{Index, IndexMut};
use rand::Rng;
use field::{load3, load4};
use util::CTAssignable;
/// The `Scalar` struct represents an element in ℤ/lℤ, where
///
@ -61,6 +62,37 @@ impl IndexMut<usize> for Scalar {
}
}
impl CTAssignable for Scalar {
/// Conditionally assign another Scalar to this one.
///
/// ```
/// # use curve25519_dalek::scalar::Scalar;
/// # use curve25519_dalek::util::CTAssignable;
/// let a = Scalar([0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
/// 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]);
/// let b = Scalar([1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
/// 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1]);
/// let mut t = a;
/// t.conditional_assign(&b, 0u8);
/// assert!(t[0] == a[0]);
/// t.conditional_assign(&b, 1u8);
/// assert!(t[0] == b[0]);
/// ```
///
/// # Preconditions
///
/// * `choice` in {0,1}
// XXX above test checks first byte because Scalar does not impl Eq
fn conditional_assign(&mut self, other: &Scalar, choice: u8) {
// if choice = 0u8, mask = (-0i8) as u8 = 00000000
// if choice = 1u8, mask = (-1i8) as u8 = 11111111
let mask = -(choice as i8) as u8;
for i in 0..32 {
self[i] ^= mask & (self[i] ^ other[i]);
}
}
}
impl Scalar {
/// Return a `Scalar` chosen uniformly at random using a CSPRNG.
/// Panics if the operating system's CSPRNG is unavailable.