Change externally-exposed API to implement ConditionallySelectable

src/edwards.rs

@@ -70,7 +70,7 @@
 //! The Edwards arithmetic is implemented using the “extended twisted
 //! coordinates” of Hisil, Wong, Carter, and Dawson, and the
 //! corresponding complete formulas.  For more details,
-//! see the [`curve_models` submodule][curve_models] 
+//! see the [`curve_models` submodule][curve_models]
 //! of the internal documentation.
 //!
 //! ## Validity Checking
@@ -92,17 +92,17 @@
 // affine and projective cakes and eat both of them too.
 #![allow(non_snake_case)]
 
+use core::borrow::Borrow;
 use core::fmt::Debug;
 use core::iter::Iterator;
-use core::ops::{Add, Sub, Neg};
+use core::iter::Sum;
+use core::ops::{Add, Neg, Sub};
 use core::ops::{AddAssign, SubAssign};
 use core::ops::{Mul, MulAssign};
-use core::iter::Sum;
-use core::borrow::Borrow;
 
-use subtle::ConditionallyAssignable;
-use subtle::ConditionallyNegatable;
 use subtle::Choice;
+use subtle::ConditionallyNegatable;
+use subtle::ConditionallySelectable;
 use subtle::ConstantTimeEq;
 
 use constants;
@@ -328,10 +328,12 @@ impl CompressedEdwardsY {
 
 impl Identity for EdwardsPoint {
     fn identity() -> EdwardsPoint {
-        EdwardsPoint{ X: FieldElement::zero(),
-                       Y: FieldElement::one(),
-                       Z: FieldElement::one(),
-                       T: FieldElement::zero() }
+        EdwardsPoint {
+            X: FieldElement::zero(),
+            Y: FieldElement::one(),
+            Z: FieldElement::one(),
+            T: FieldElement::zero(),
+        }
     }
 }
 
@@ -358,12 +360,14 @@ impl ValidityCheck for EdwardsPoint {
 // Constant-time assignment
 // ------------------------------------------------------------------------
 
-impl ConditionallyAssignable for EdwardsPoint {
-    fn conditional_assign(&mut self, other: &EdwardsPoint, choice: Choice) {
-        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);
+impl ConditionallySelectable for EdwardsPoint {
+    fn conditional_select(a: &EdwardsPoint, b: &EdwardsPoint, choice: Choice) -> EdwardsPoint {
+        EdwardsPoint {
+            X: FieldElement::conditional_select(&a.X, &b.X, choice),
+            Y: FieldElement::conditional_select(&a.Y, &b.Y, choice),
+            Z: FieldElement::conditional_select(&a.Z, &b.Z, choice),
+            T: FieldElement::conditional_select(&a.T, &b.T, choice),
+        }
     }
 }
 
@@ -373,7 +377,9 @@ impl ConditionallyAssignable for EdwardsPoint {
 
 impl ConstantTimeEq for EdwardsPoint {
     fn ct_eq(&self, other: &EdwardsPoint) -> Choice {
-        self.compress().as_bytes().ct_eq(other.compress().as_bytes())
+        self.compress()
+            .as_bytes()
+            .ct_eq(other.compress().as_bytes())
     }
 }
 

src/ristretto.rs

@@ -157,28 +157,29 @@
 //! [ristretto_main]:
 //! https://ristretto.group/
 
+use core::borrow::Borrow;
 use core::fmt::Debug;
-use core::ops::{Add, Sub, Neg};
+use core::iter::Sum;
+use core::ops::{Add, Neg, Sub};
 use core::ops::{AddAssign, SubAssign};
 use core::ops::{Mul, MulAssign};
-use core::iter::Sum;
-use core::borrow::Borrow;
 
-use rand::{Rng, CryptoRng};
+use rand::{CryptoRng, Rng};
 
-use digest::Digest;
 use digest::generic_array::typenum::U64;
+use digest::Digest;
 
 use constants;
 use field::FieldElement;
 
+use subtle::Choice;
 use subtle::ConditionallyAssignable;
 use subtle::ConditionallyNegatable;
+use subtle::ConditionallySelectable;
 use subtle::ConstantTimeEq;
-use subtle::Choice;
 
-use edwards::EdwardsPoint;
 use edwards::EdwardsBasepointTable;
+use edwards::EdwardsPoint;
 
 #[allow(unused_imports)]
 use prelude::*;
@@ -961,11 +962,11 @@ impl RistrettoBasepointTable {
 }
 
 // ------------------------------------------------------------------------
-// Constant-time conditional assignment
+// Constant-time conditional selection
 // ------------------------------------------------------------------------
 
-impl ConditionallyAssignable for RistrettoPoint {
-    /// Conditionally assign `other` to `self`, if `choice == Choice(1)`.
+impl ConditionallySelectable for RistrettoPoint {
+    /// Conditionally select between `self` and `other`.
     ///
     /// # Example
     ///
@@ -973,7 +974,7 @@ impl ConditionallyAssignable for RistrettoPoint {
     /// # extern crate subtle;
     /// # extern crate curve25519_dalek;
     /// #
-    /// use subtle::ConditionallyAssignable;
+    /// use subtle::ConditionallySelectable;
     /// use subtle::Choice;
     /// #
     /// # use curve25519_dalek::traits::Identity;
@@ -986,17 +987,18 @@ impl ConditionallyAssignable for RistrettoPoint {
     ///
     /// let mut P = A;
     ///
-    /// P.conditional_assign(&B, Choice::from(0));
+    /// P = RistrettoPoint::conditional_select(&A, &B, Choice::from(0));
     /// assert_eq!(P, A);
-    /// P.conditional_assign(&B, Choice::from(1));
+    /// P = RistrettoPoint::conditional_select(&A, &B, Choice::from(1));
     /// assert_eq!(P, B);
     /// # }
     /// ```
-    fn conditional_assign(&mut self, other: &RistrettoPoint, choice: Choice) {
-        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);
+    fn conditional_select(
+        a: &RistrettoPoint,
+        b: &RistrettoPoint,
+        choice: Choice,
+    ) -> RistrettoPoint {
+        RistrettoPoint(EdwardsPoint::conditional_select(&a.0, &b.0, choice))
     }
 }
 

src/scalar.rs

@@ -138,26 +138,26 @@
 //! The resulting `Scalar` has exactly the specified bit pattern,
 //! **except for the highest bit, which will be set to 0**.
 
+use core::borrow::Borrow;
+use core::cmp::{Eq, PartialEq};
 use core::fmt::Debug;
+use core::iter::{Product, Sum};
+use core::ops::Index;
 use core::ops::Neg;
 use core::ops::{Add, AddAssign};
-use core::ops::{Sub, SubAssign};
 use core::ops::{Mul, MulAssign};
-use core::ops::{Index};
-use core::cmp::{Eq, PartialEq};
-use core::iter::{Product, Sum};
-use core::borrow::Borrow;
+use core::ops::{Sub, SubAssign};
 
 #[allow(unused_imports)]
 use prelude::*;
 
-use rand::{Rng, CryptoRng};
+use rand::{CryptoRng, Rng};
 
-use digest::Digest;
 use digest::generic_array::typenum::U64;
+use digest::Digest;
 
 use subtle::Choice;
-use subtle::ConditionallyAssignable;
+use subtle::ConditionallySelectable;
 use subtle::ConstantTimeEq;
 
 use backend;
@@ -343,11 +343,13 @@ impl<'a> Neg for Scalar {
     }
 }
 
-impl ConditionallyAssignable for Scalar {
-    fn conditional_assign(&mut self, other: &Scalar, choice: Choice) {
+impl ConditionallySelectable for Scalar {
+    fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
+        let mut bytes = [0u8; 32];
         for i in 0..32 {
-            self.bytes[i].conditional_assign(&other.bytes[i], choice);
+            bytes[i] = u8::conditional_select(&a.bytes[i], &b.bytes[i], choice);
         }
+        Scalar { bytes }
     }
 }