PubSub with UADP message mapping - determine size of the block to decrypt|OPC UA Standard|Forum|OPC Foundation

Referring to Figure 27 and Table 73 (UADP Network Message) in 1.04 Part 14, when parsing the incoming NetworkMessage, how to determine the size of the block of data to decrypt when I have made it past SecurityHeader? I do not see any field that tells me the size of the encryopted data that follows. 

Is it the right approach then to take the size of the whole received NetworkMessage, minus the length of the Signature, minus the position where the Payload starts?

Thank you