backport BlackBox

src/lib.rs

@@ -13,7 +13,6 @@
 #![deny(missing_docs)]
 #![doc(html_root_url = "https://docs.rs/subtle-ng/2.5.0")]
 
-
 #[cfg(feature = "std")]
 #[macro_use]
 extern crate std;
@@ -145,25 +144,23 @@ impl Not for Choice {
 /// code may break in a non-destructive way in the future, “constant-time” code
 /// is a continually moving target, and this is better than doing nothing.
 #[inline(never)]
-fn black_box(input: u8) -> u8 {
-    debug_assert!((input == 0u8) | (input == 1u8));
-
+fn black_box<T: Copy>(input: T) -> T {
     unsafe {
         // Optimization barrier
         //
-        // Unsafe is ok, because:
-        //   - &input is not NULL;
-        //   - size of input is not zero;
-        //   - u8 is neither Sync, nor Send;
-        //   - u8 is Copy, so input is always live;
-        //   - u8 type is always properly aligned.
-        core::ptr::read_volatile(&input as *const u8)
+        // SAFETY:
+        //   - &input is not NULL because we own input;
+        //   - input is Copy and always live;
+        //   - input is always properly aligned.
+        core::ptr::read_volatile(&input)
     }
 }
 
 impl From<u8> for Choice {
     #[inline]
     fn from(input: u8) -> Choice {
+        debug_assert!((input == 0u8) | (input == 1u8));
+
         // Our goal is to prevent the compiler from inferring that the value held inside the
         // resulting `Choice` struct is really an `i1` instead of an `i8`.
         Choice(black_box(input))
@@ -728,7 +725,7 @@ macro_rules! generate_unsigned_integer_greater {
                 Choice::from((bit & 1) as u8)
             }
         }
-    }
+    };
 }
 
 generate_unsigned_integer_greater!(u8, 8);
@@ -788,3 +785,21 @@ impl ConstantTimeLess for u32 {}
 impl ConstantTimeLess for u64 {}
 #[cfg(feature = "i128")]
 impl ConstantTimeLess for u128 {}
+
+/// Wrapper type which implements an optimization barrier for all accesses.
+#[derive(Clone, Copy, Debug)]
+pub struct BlackBox<T: Copy>(T);
+
+impl<T: Copy> BlackBox<T> {
+    /// Constructs a new instance of `BlackBox` which will wrap the specified value.
+    ///
+    /// All access to the inner value will be mediated by a `black_box` optimization barrier.
+    pub fn new(value: T) -> Self {
+        Self(value)
+    }
+
+    /// Read the inner value, applying an optimization barrier on access.
+    pub fn get(self) -> T {
+        black_box(self.0)
+    }
+}

tests/mod.rs

@@ -266,16 +266,66 @@ fn test_ctoption() {
     ));
 
     // Test (in)equality
-    assert!(CtOption::new(1, Choice::from(0)).ct_eq(&CtOption::new(1, Choice::from(1))).unwrap_u8() == 0);
-    assert!(CtOption::new(1, Choice::from(1)).ct_eq(&CtOption::new(1, Choice::from(0))).unwrap_u8() == 0);
-    assert!(CtOption::new(1, Choice::from(0)).ct_eq(&CtOption::new(2, Choice::from(1))).unwrap_u8() == 0);
-    assert!(CtOption::new(1, Choice::from(1)).ct_eq(&CtOption::new(2, Choice::from(0))).unwrap_u8() == 0);
-    assert!(CtOption::new(1, Choice::from(0)).ct_eq(&CtOption::new(1, Choice::from(0))).unwrap_u8() == 1);
-    assert!(CtOption::new(1, Choice::from(0)).ct_eq(&CtOption::new(2, Choice::from(0))).unwrap_u8() == 1);
-    assert!(CtOption::new(1, Choice::from(1)).ct_eq(&CtOption::new(2, Choice::from(1))).unwrap_u8() == 0);
-    assert!(CtOption::new(1, Choice::from(1)).ct_eq(&CtOption::new(2, Choice::from(1))).unwrap_u8() == 0);
-    assert!(CtOption::new(1, Choice::from(1)).ct_eq(&CtOption::new(1, Choice::from(1))).unwrap_u8() == 1);
-    assert!(CtOption::new(1, Choice::from(1)).ct_eq(&CtOption::new(1, Choice::from(1))).unwrap_u8() == 1);
+    assert!(
+        CtOption::new(1, Choice::from(0))
+            .ct_eq(&CtOption::new(1, Choice::from(1)))
+            .unwrap_u8()
+            == 0
+    );
+    assert!(
+        CtOption::new(1, Choice::from(1))
+            .ct_eq(&CtOption::new(1, Choice::from(0)))
+            .unwrap_u8()
+            == 0
+    );
+    assert!(
+        CtOption::new(1, Choice::from(0))
+            .ct_eq(&CtOption::new(2, Choice::from(1)))
+            .unwrap_u8()
+            == 0
+    );
+    assert!(
+        CtOption::new(1, Choice::from(1))
+            .ct_eq(&CtOption::new(2, Choice::from(0)))
+            .unwrap_u8()
+            == 0
+    );
+    assert!(
+        CtOption::new(1, Choice::from(0))
+            .ct_eq(&CtOption::new(1, Choice::from(0)))
+            .unwrap_u8()
+            == 1
+    );
+    assert!(
+        CtOption::new(1, Choice::from(0))
+            .ct_eq(&CtOption::new(2, Choice::from(0)))
+            .unwrap_u8()
+            == 1
+    );
+    assert!(
+        CtOption::new(1, Choice::from(1))
+            .ct_eq(&CtOption::new(2, Choice::from(1)))
+            .unwrap_u8()
+            == 0
+    );
+    assert!(
+        CtOption::new(1, Choice::from(1))
+            .ct_eq(&CtOption::new(2, Choice::from(1)))
+            .unwrap_u8()
+            == 0
+    );
+    assert!(
+        CtOption::new(1, Choice::from(1))
+            .ct_eq(&CtOption::new(1, Choice::from(1)))
+            .unwrap_u8()
+            == 1
+    );
+    assert!(
+        CtOption::new(1, Choice::from(1))
+            .ct_eq(&CtOption::new(1, Choice::from(1)))
+            .unwrap_u8()
+            == 1
+    );
 }
 
 #[test]
@@ -303,7 +353,7 @@ macro_rules! generate_greater_than_test {
                 assert!(z.unwrap_u8() == 1);
             }
         }
-    }
+    };
 }
 
 #[test]
@@ -337,7 +387,7 @@ fn greater_than_u128() {
 /// gives the correct result. (This fails using the bit-twiddling algorithm that
 /// go/crypto/subtle uses.)
 fn less_than_twos_compliment_minmax() {
-    let z = 1u32.ct_lt(&(2u32.pow(31)-1));
+    let z = 1u32.ct_lt(&(2u32.pow(31) - 1));
 
     assert!(z.unwrap_u8() == 1);
 }
@@ -359,7 +409,7 @@ macro_rules! generate_less_than_test {
                 assert!(z.unwrap_u8() == 1);
             }
         }
-    }
+    };
 }
 
 #[test]
@@ -387,3 +437,10 @@ fn less_than_u64() {
 fn less_than_u128() {
     generate_less_than_test!(u128);
 }
+
+#[test]
+fn black_box_round_trip() {
+    let n = 42u64;
+    let black_box = BlackBox::new(n);
+    assert_eq!(n, black_box.get());
+}