Harden constant-time equality on arrays and slices

Author: AaronFeickert

src/lib.rs

@@ -299,12 +299,6 @@ pub trait ConstantTimeEq {
 impl<T: ConstantTimeEq> ConstantTimeEq for [T] {
     /// Check whether two slices of `ConstantTimeEq` types are equal.
     ///
-    /// # Note
-    ///
-    /// This function short-circuits if the lengths of the input slices
-    /// are different.  Otherwise, it should execute in time independent
-    /// of the slice contents.
-    ///
     /// Since arrays coerce to slices, this function works with fixed-size arrays:
     ///
     /// ```
@@ -321,23 +315,17 @@ impl<T: ConstantTimeEq> ConstantTimeEq for [T] {
     /// ```
     #[inline]
     fn ct_eq(&self, _rhs: &[T]) -> Choice {
-        let len = self.len();
-
-        // Short-circuit on the *lengths* of the slices, not their
-        // contents.
-        if len != _rhs.len() {
-            return Choice::from(0);
-        }
+        // Determine if the lengths are equal in constant time
+        let len_ct_eq = self.len().ct_eq(&_rhs.len());
 
-        // This loop shouldn't be shortcircuitable, since the compiler
-        // shouldn't be able to reason about the value of the `u8`
-        // unwrapped from the `ct_eq` result.
-        let mut x = 1u8;
+        // Check each byte for equality in constant time
+        let mut contents_ct_eq = Choice::from(1u8);
         for (ai, bi) in self.iter().zip(_rhs.iter()) {
-            x &= ai.ct_eq(bi).unwrap_u8();
+            contents_ct_eq &= ai.ct_eq(bi);
         }
 
-        x.into()
+        // Now check that the length and bytes are both equal in constant time
+        len_ct_eq & contents_ct_eq
     }
 }
 

tests/mod.rs

@@ -6,12 +6,11 @@ use rand::RngCore;
 use subtle::*;
 
 #[test]
-#[should_panic]
-fn slices_equal_different_lengths() {
+fn slices_different_lengths() {
     let a: [u8; 3] = [0, 0, 0];
     let b: [u8; 4] = [0, 0, 0, 0];
 
-    assert_eq!((&a).ct_eq(&b).unwrap_u8(), 1);
+    assert_eq!((&a).ct_eq(&b).unwrap_u8(), 0);
 }
 
 #[test]