diff --git a/library/gcm.c b/library/gcm.c
index 786290f..b773529 100644
--- a/library/gcm.c
+++ b/library/gcm.c
@@ -35,6 +35,7 @@
 #include "mbedtls/platform.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
+#include "mbedtls/constant_time.h"
 
 #include <string.h>
 
@@ -601,7 +602,6 @@
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     unsigned char check_tag[16];
-    size_t i;
     int diff;
 
     if ((ret = mbedtls_gcm_crypt_and_tag(ctx, MBEDTLS_GCM_DECRYPT, length,
@@ -611,9 +611,7 @@
     }
 
     /* Check tag in "constant-time" */
-    for (diff = 0, i = 0; i < tag_len; i++) {
-        diff |= tag[i] ^ check_tag[i];
-    }
+    diff = mbedtls_ct_memcmp(tag, check_tag, tag_len);
 
     if (diff != 0) {
         mbedtls_platform_zeroize(output, length);