+
+o MAC (variable length) - This field is present only when
+ the private message payload is encrypted with private
+ message key. The MAC is computed from the Message Flags,
+ Message Data Length, Message Data and Padding fields in
+ that order. The MAC protects the integrity of the channel
+ message. The MAC is computed after encryption from the
+ ciphertext. Note that, this field is not encrypted and
+ thus not included in the padding calculation. When encrypted
+ with normal session keys, this field MUST NOT be included.