How to use the bless.ssh.protocol.ssh_protocol.pack_ssh_string function in bless

def _serialize_critical_options(self):
        # Options must be lexically ordered by "name" if they appear in the
        # sequence. Each named option may only appear once in a certificate.
        serialized = b''

        if self.critical_option_force_command is not None:
            serialized += pack_ssh_string('force-command')
            serialized += pack_ssh_string(
                pack_ssh_string(self.critical_option_force_command))

        if self.critical_option_source_address is not None:
            serialized += pack_ssh_string('source-address')
            serialized += pack_ssh_string(
                pack_ssh_string(self.critical_option_source_address))

        return serialized

def _serialize_certificate_body(self):
        body = pack_ssh_string(self.cert_key_type)
        body += pack_ssh_string(self.nonce)
        body += self._serialize_ssh_public_key()
        body += pack_ssh_uint64(self.serial)
        body += pack_ssh_uint32(self.cert_type)
        body += pack_ssh_string(self.key_id)
        body += pack_ssh_string(self._serialize_valid_principals())
        body += pack_ssh_uint64(self.valid_after)
        body += pack_ssh_uint64(self.valid_before)
        body += pack_ssh_string(self._serialize_critical_options())
        body += pack_ssh_string(self._serialize_extensions())
        body += pack_ssh_string('')
        body += pack_ssh_string(self.ca.get_signature_key())
        return body

def _serialize_certificate_body(self):
        body = pack_ssh_string(self.cert_key_type)
        body += pack_ssh_string(self.nonce)
        body += self._serialize_ssh_public_key()
        body += pack_ssh_uint64(self.serial)
        body += pack_ssh_uint32(self.cert_type)
        body += pack_ssh_string(self.key_id)
        body += pack_ssh_string(self._serialize_valid_principals())
        body += pack_ssh_uint64(self.valid_after)
        body += pack_ssh_uint64(self.valid_before)
        body += pack_ssh_string(self._serialize_critical_options())
        body += pack_ssh_string(self._serialize_extensions())
        body += pack_ssh_string('')
        body += pack_ssh_string(self.ca.get_signature_key())
        return body

def _serialize_certificate_body(self):
        body = pack_ssh_string(self.cert_key_type)
        body += pack_ssh_string(self.nonce)
        body += self._serialize_ssh_public_key()
        body += pack_ssh_uint64(self.serial)
        body += pack_ssh_uint32(self.cert_type)
        body += pack_ssh_string(self.key_id)
        body += pack_ssh_string(self._serialize_valid_principals())
        body += pack_ssh_uint64(self.valid_after)
        body += pack_ssh_uint64(self.valid_before)
        body += pack_ssh_string(self._serialize_critical_options())
        body += pack_ssh_string(self._serialize_extensions())
        body += pack_ssh_string('')
        body += pack_ssh_string(self.ca.get_signature_key())
        return body

def _serialize_critical_options(self):
        # Options must be lexically ordered by "name" if they appear in the
        # sequence. Each named option may only appear once in a certificate.
        serialized = b''

        if self.critical_option_force_command is not None:
            serialized += pack_ssh_string('force-command')
            serialized += pack_ssh_string(
                pack_ssh_string(self.critical_option_force_command))

        if self.critical_option_source_address is not None:
            serialized += pack_ssh_string('source-address')
            serialized += pack_ssh_string(
                pack_ssh_string(self.critical_option_source_address))

        return serialized

def _serialize_critical_options(self):
        # Options must be lexically ordered by "name" if they appear in the
        # sequence. Each named option may only appear once in a certificate.
        serialized = b''

        if self.critical_option_force_command is not None:
            serialized += pack_ssh_string('force-command')
            serialized += pack_ssh_string(
                pack_ssh_string(self.critical_option_force_command))

        if self.critical_option_source_address is not None:
            serialized += pack_ssh_string('source-address')
            serialized += pack_ssh_string(
                pack_ssh_string(self.critical_option_source_address))

        return serialized

def get_signature_key(self):
        """
        Get the SSH Public Key associated with this CA.
        Packed per RFC4253 section 6.6.
        :return: SSH Public Key.
        """
        key = pack_ssh_string(self.public_key_type)
        key += pack_ssh_mpint(self.e)
        key += pack_ssh_mpint(self.n)
        return key

def _serialize_extensions(self):
        # Options must be lexically ordered by "name" if they appear in the
        # sequence. Each named option may only appear once in a certificate.
        extensions_list = sorted(self.extensions)

        serialized = b''
        # Format is a series of {extension name}{empty string}
        for extension in extensions_list:
            serialized += pack_ssh_string(extension)
            serialized += pack_ssh_string('')

        return serialized

def _serialize_certificate_body(self):
        body = pack_ssh_string(self.cert_key_type)
        body += pack_ssh_string(self.nonce)
        body += self._serialize_ssh_public_key()
        body += pack_ssh_uint64(self.serial)
        body += pack_ssh_uint32(self.cert_type)
        body += pack_ssh_string(self.key_id)
        body += pack_ssh_string(self._serialize_valid_principals())
        body += pack_ssh_uint64(self.valid_after)
        body += pack_ssh_uint64(self.valid_before)
        body += pack_ssh_string(self._serialize_critical_options())
        body += pack_ssh_string(self._serialize_extensions())
        body += pack_ssh_string('')
        body += pack_ssh_string(self.ca.get_signature_key())
        return body

def _serialize_critical_options(self):
        # Options must be lexically ordered by "name" if they appear in the
        # sequence. Each named option may only appear once in a certificate.
        serialized = b''

        if self.critical_option_force_command is not None:
            serialized += pack_ssh_string('force-command')
            serialized += pack_ssh_string(
                pack_ssh_string(self.critical_option_force_command))

        if self.critical_option_source_address is not None:
            serialized += pack_ssh_string('source-address')
            serialized += pack_ssh_string(
                pack_ssh_string(self.critical_option_source_address))

        return serialized