curve: use subtle::Choice for constant-time fixes

Alternative to #659/#661 and #662 which leverages `subtle::Choice` and
`subtle::ConditionallySelectable` as the optimization barriers.

Really the previous masking was there to conditionally add the scalar
field modulus on underflow, so instead of that, we can conditionally
select zero or the modulus using a `Choice` constructed from the
underflow bit.

curve25519-dalek/src/backend/serial/u32/scalar.rs

@@ -12,7 +12,7 @@
 
 use core::fmt::Debug;
 use core::ops::{Index, IndexMut};
-use subtle::BlackBox;
+use subtle::{Choice, ConditionallySelectable};
 
 #[cfg(feature = "zeroize")]
 use zeroize::Zeroize;
@@ -187,23 +187,22 @@ impl Scalar29 {
     /// Compute `a - b` (mod l).
     pub fn sub(a: &Scalar29, b: &Scalar29) -> Scalar29 {
         let mut difference = Scalar29::ZERO;
-        let mask = BlackBox::new((1u32 << 29) - 1);
+        let mask = (1u32 << 29) - 1;
 
         // a - b
         let mut borrow: u32 = 0;
         for i in 0..9 {
             borrow = a[i].wrapping_sub(b[i] + (borrow >> 31));
-            difference[i] = borrow & mask.get();
+            difference[i] = borrow & mask;
         }
 
         // conditionally add l if the difference is negative
-        let underflow_mask = BlackBox::new(((borrow >> 31) ^ 1).wrapping_sub(1));
         let mut carry: u32 = 0;
         for i in 0..9 {
-            // SECURITY: `BlackBox` prevents LLVM from inserting a `jns` conditional on x86(_64)
-            // which can be used to bypass this section when `underflow_mask` is zero.
-            carry = (carry >> 29) + difference[i] + (constants::L[i] & underflow_mask.get());
-            difference[i] = carry & mask.get();
+            let underflow = Choice::from((borrow >> 31) as u8);
+            let addend = u32::conditional_select(&0, &constants::L[i], underflow);
+            carry = (carry >> 29) + difference[i] + addend;
+            difference[i] = carry & mask;
         }
 
         difference

curve25519-dalek/src/backend/serial/u64/scalar.rs

@@ -13,7 +13,7 @@
 
 use core::fmt::Debug;
 use core::ops::{Index, IndexMut};
-use subtle::BlackBox;
+use subtle::{Choice, ConditionallySelectable};
 
 #[cfg(feature = "zeroize")]
 use zeroize::Zeroize;
@@ -176,23 +176,22 @@ impl Scalar52 {
     /// Compute `a - b` (mod l)
     pub fn sub(a: &Scalar52, b: &Scalar52) -> Scalar52 {
         let mut difference = Scalar52::ZERO;
-        let mask = BlackBox::new((1u64 << 52) - 1);
+        let mask = (1u64 << 52) - 1;
 
         // a - b
         let mut borrow: u64 = 0;
         for i in 0..5 {
             borrow = a[i].wrapping_sub(b[i] + (borrow >> 63));
-            difference[i] = borrow & mask.get();
+            difference[i] = borrow & mask;
         }
 
         // conditionally add l if the difference is negative
-        let underflow_mask = BlackBox::new(((borrow >> 63) ^ 1).wrapping_sub(1));
         let mut carry: u64 = 0;
         for i in 0..5 {
-            // SECURITY: `BlackBox` prevents LLVM from inserting a `jns` conditional on x86(_64)
-            // which can be used to bypass this section when `underflow_mask` is zero.
-            carry = (carry >> 52) + difference[i] + (constants::L[i] & underflow_mask.get());
-            difference[i] = carry & mask.get();
+            let underflow = Choice::from((borrow >> 63) as u8);
+            let addend = u64::conditional_select(&0, &constants::L[i], underflow);
+            carry = (carry >> 52) + difference[i] + addend;
+            difference[i] = carry & mask;
         }
 
         difference