diff --git a/library/cipher.c b/library/cipher.c
index 546cace..a53609e 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -29,6 +29,7 @@
 #include "cipher_wrap.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
+#include "constant_time.h"
 
 #include <stdlib.h>
 #include <string.h>
@@ -74,27 +75,6 @@
 #define CIPHER_VALIDATE( cond )        \
     MBEDTLS_INTERNAL_VALIDATE( cond )
 
-#if defined(MBEDTLS_GCM_C) || defined(MBEDTLS_CHACHAPOLY_C)
-/* Compare the contents of two buffers in constant time.
- * Returns 0 if the contents are bitwise identical, otherwise returns
- * a non-zero value.
- * This is currently only used by GCM and ChaCha20+Poly1305.
- */
-static int mbedtls_constant_time_memcmp( const void *v1, const void *v2,
-                                         size_t len )
-{
-    const unsigned char *p1 = (const unsigned char*) v1;
-    const unsigned char *p2 = (const unsigned char*) v2;
-    size_t i;
-    unsigned char diff;
-
-    for( diff = 0, i = 0; i < len; i++ )
-        diff |= p1[i] ^ p2[i];
-
-    return( (int)diff );
-}
-#endif /* MBEDTLS_GCM_C || MBEDTLS_CHACHAPOLY_C */
-
 static int supported_init = 0;
 
 const int *mbedtls_cipher_list( void )