Mark internal function as static
diff --git a/library/bignum.c b/library/bignum.c
index bf0314b..db2b931 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2142,7 +2142,7 @@
 /*
  * Pseudo-primality test: small factors, then Miller-Rabin
  */
-int mpi_is_prime_internal( const mbedtls_mpi *X, int rounds,
+static int mpi_is_prime_internal( const mbedtls_mpi *X, int rounds,
                   int (*f_rng)(void *, unsigned char *, size_t),
                   void *p_rng )
 {