SilcBool silc_pkcs_save_private_key(const char *filename,
                                        SilcPrivateKey private_key,
                                        const unsigned char *passphrase,
                                        SilcUInt32 passphrase_len,
                                        SilcPKCSFileEncoding encoding,
                                        SilcRng rng);

    Saves private key into file.  The private key is encrypted into
    the file with the `passphrase' as a key, if PKCS supports encrypted
    private keys.  Returns FALSE on error.