diff --git a/build.rs b/build.rs
index 41abfcd4b1..e09a250ddd 100644
--- a/build.rs
+++ b/build.rs
@@ -962,6 +962,7 @@ fn prefix_all_symbols(pp: char, prefix_prefix: &str, prefix: &str) -> String {
         "bn_sqr8x_internal",
         "bn_sqrx8x_internal",
         "bsaes_ctr32_encrypt_blocks",
+        "bssl_constant_time_test_conditional_memxor",
         "bssl_constant_time_test_main",
         "chacha20_poly1305_open",
         "chacha20_poly1305_seal",
diff --git a/crypto/constant_time_test.c b/crypto/constant_time_test.c
index 170525f470..11be4974df 100644
--- a/crypto/constant_time_test.c
+++ b/crypto/constant_time_test.c
@@ -88,36 +88,36 @@ static int test_select_w(crypto_word_t a, crypto_word_t b) {
   return 0;
 }
 
-static crypto_word_t test_values_s[] = {
-  0,
-  1,
-  1024,
-  12345,
-  32000,
+static crypto_word_t test_values_w[] = {
+    0,
+    1,
+    1024,
+    12345,
+    32000,
 #if defined(OPENSSL_64_BIT)
-  0xffffffff / 2 - 1,
-  0xffffffff / 2,
-  0xffffffff / 2 + 1,
-  0xffffffff - 1,
-  0xffffffff,
+    0xffffffff / 2 - 1,
+    0xffffffff / 2,
+    0xffffffff / 2 + 1,
+    0xffffffff - 1,
+    0xffffffff,
 #endif
-  SIZE_MAX / 2 - 1,
-  SIZE_MAX / 2,
-  SIZE_MAX / 2 + 1,
-  SIZE_MAX - 1,
-  SIZE_MAX
+    SIZE_MAX / 2 - 1,
+    SIZE_MAX / 2,
+    SIZE_MAX / 2 + 1,
+    SIZE_MAX - 1,
+    SIZE_MAX
 };
 
 int bssl_constant_time_test_main(void) {
   int num_failed = 0;
 
   for (size_t i = 0;
-       i < sizeof(test_values_s) / sizeof(test_values_s[0]); ++i) {
-    crypto_word_t a = test_values_s[i];
+       i < sizeof(test_values_w) / sizeof(test_values_w[0]); ++i) {
+    crypto_word_t a = test_values_w[i];
     num_failed += test_is_zero_w(a);
     for (size_t j = 0;
-         j < sizeof(test_values_s) / sizeof(test_values_s[0]); ++j) {
-      crypto_word_t b = test_values_s[j];
+         j < sizeof(test_values_w) / sizeof(test_values_w[0]); ++j) {
+      crypto_word_t b = test_values_w[j];
       num_failed += test_binary_op_w(&constant_time_eq_w, a, b, a == b);
       num_failed += test_binary_op_w(&constant_time_eq_w, b, a, b == a);
       num_failed += test_select_w(a, b);
@@ -126,3 +126,10 @@ int bssl_constant_time_test_main(void) {
 
   return num_failed == 0;
 }
+
+// Exposes `constant_time_conditional_memxor` to Rust for tests only.
+void bssl_constant_time_test_conditional_memxor(uint8_t dst[256],
+                                               const uint8_t src[256],
+                                               crypto_word_t b) {
+  constant_time_conditional_memxor(dst, src, 256, b);
+}
diff --git a/src/constant_time.rs b/src/constant_time.rs
index 9ccf8f6653..e41ad6187e 100644
--- a/src/constant_time.rs
+++ b/src/constant_time.rs
@@ -37,7 +37,8 @@ prefixed_extern! {
 
 #[cfg(test)]
 mod tests {
-    use crate::{bssl, error};
+    use crate::limb::LimbMask;
+    use crate::{bssl, error, rand};
 
     #[test]
     fn test_constant_time() -> Result<(), error::Unspecified> {
@@ -46,4 +47,43 @@ mod tests {
         }
         Result::from(unsafe { bssl_constant_time_test_main() })
     }
+
+    #[test]
+    fn constant_time_conditional_memxor() -> Result<(), error::Unspecified> {
+        let rng = rand::SystemRandom::new();
+        for _ in 0..256 {
+            let mut out = rand::generate::<[u8; 256]>(&rng)?.expose();
+            let input = rand::generate::<[u8; 256]>(&rng)?.expose();
+
+            // Mask to 16 bits to make zero more likely than it would otherwise be.
+            let b = (rand::generate::<[u8; 1]>(&rng)?.expose()[0] & 0x0f) != 0;
+
+            let ref_in = input;
+            let mut ref_out = out;
+            if b {
+                ref_out
+                    .iter_mut()
+                    .zip(ref_in.iter())
+                    .for_each(|(out, input)| {
+                        *out ^= input;
+                    });
+            }
+
+            prefixed_extern! {
+                fn bssl_constant_time_test_conditional_memxor(dst: &mut [u8; 256], src: &[u8; 256], b: LimbMask);
+            }
+            unsafe {
+                bssl_constant_time_test_conditional_memxor(
+                    &mut out,
+                    &input,
+                    if b { LimbMask::True } else { LimbMask::False },
+                );
+            }
+
+            assert_eq!(ref_in, input);
+            assert_eq!(ref_out, out);
+        }
+
+        Ok(())
+    }
 }