fix: Input dependent memory access in statistical constant time execution test

Supplying different memory locations to the memcmp function
in the test is not constant time; this was an issue wit
the test and not with memcmp itself.

The issue mainly showed up in the Release build where the
correlation coefficient was in the ballpark of just below
-0.01 with enough variance to sometimes but not usually fail
the test. The precise reason for this is unknown but some
optimization applied in the release build is most likely the
culprit.

This should increase the stability of our CI which occasionally
was flaky.

constant-time/src/memcmp.rs

@@ -34,6 +34,7 @@ pub fn memcmp(a: &[u8], b: &[u8]) -> bool {
 #[cfg(all(test, feature = "constant_time_tests"))]
 mod tests {
     use super::*;
+    use core::hint::black_box;
     use rand::seq::SliceRandom;
     use rand::thread_rng;
     use std::time::Instant;
@@ -44,20 +45,17 @@ mod tests {
     /// This test function will run an equal amount of comparisons on two different sets of parameters:
     /// - completely equal slices
     /// - completely unequal slices.
-    /// All comparisons are executed in a randomized order. The test will fail if one of the
     /// two sets is checked for equality significantly faster than the other set
     /// (absolute correlation coefficient ≥ 0.01)
     fn memcmp_runs_in_constant_time() {
         // prepare data to compare
         let n: usize = 1E6 as usize; // number of comparisons to run
-        let len = 1024; // length of each slice passed as parameters to the tested comparison function
-        let a1 = "a".repeat(len);
-        let a2 = a1.clone();
-        let b = "b".repeat(len);
+        const LEN: usize = 1024; // length of each slice passed as parameters to the tested comparison function
 
-        let a1 = a1.as_bytes();
-        let a2 = a2.as_bytes();
-        let b = b.as_bytes();
+        let a = [b'a'; LEN];
+        let b = [b'b'; LEN];
+
+        let mut tmp = [0u8; LEN];
 
         // vector representing all timing tests
         //
@@ -71,12 +69,14 @@ mod tests {
 
         // run comparisons / call function to test
         for test in tests.iter_mut() {
+            let src = match test.0 {
+                true => a,
+                false => b,
+            };
+            tmp.copy_from_slice(&src);
+
             let now = Instant::now();
-            if test.0 {
-                memcmp(a1, a2);
-            } else {
-                memcmp(a1, b);
-            }
+            memcmp(black_box(&a), black_box(&tmp));
             test.1 = now.elapsed();
             // println!("eq: {}, elapsed: {:.2?}", test.0, test.1);
         }
@@ -117,6 +117,6 @@ mod tests {
         assert!(
             correlation.abs() < 0.01,
             "execution time correlates with result"
-        );
+        )
     }
 }