bool silc_change_private_key_passphrase(const char *prv_filename,
                                            const char *old_passphrase,
                                            const char *new_passphrase);

    This routine can be used to change the passphrase of the private
    key file, which is used to encrypt the private key.  If the old
    and new passphrase is not provided for this function this will
    prompt for them.