diff --git a/library/cipher.c b/library/cipher.c
index b51a40b..71fa6f5 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -69,9 +69,13 @@
 #define MBEDTLS_CIPHER_MODE_STREAM
 #endif
 
+
+#if defined(MBEDTLS_GCM_C) || defined(MBEDTLS_AEAD_CHACHA20_POLY1305_C)
 /* Compare the contents of two buffers in constant time.
  * Returns 0 if the contents are bitwise identical, otherwise returns
- * a non-zero value. */
+ * 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;
@@ -84,6 +88,7 @@
 
     return (int)diff;
 }
+#endif /* MBEDTLS_GCM_C || MBEDTLS_AEAD_CHACHA20_POLY1305_C */
 
 static int supported_init = 0;