diff --git a/library/rsa_internal.c b/library/rsa_internal.c
index 507009f..9a42d47 100644
--- a/library/rsa_internal.c
+++ b/library/rsa_internal.c
@@ -351,15 +351,20 @@
      */
 
 #if defined(MBEDTLS_GENPRIME)
+    /*
+     * When generating keys, the strongest security we support aims for an error
+     * rate of at most 2^-100 and we are aiming for the same certainty here as
+     * well.
+     */
     if( f_rng != NULL && P != NULL &&
-        ( ret = mbedtls_mpi_is_prime( P, f_rng, p_rng ) ) != 0 )
+        ( ret = mbedtls_mpi_is_prime_ext( P, 50, f_rng, p_rng ) ) != 0 )
     {
         ret = MBEDTLS_ERR_RSA_KEY_CHECK_FAILED;
         goto cleanup;
     }
 
     if( f_rng != NULL && Q != NULL &&
-        ( ret = mbedtls_mpi_is_prime( Q, f_rng, p_rng ) ) != 0 )
+        ( ret = mbedtls_mpi_is_prime_ext( Q, 50, f_rng, p_rng ) ) != 0 )
     {
         ret = MBEDTLS_ERR_RSA_KEY_CHECK_FAILED;
         goto cleanup;