Basic question about the detection of the encrypted payload's size of an UADP NetworkMessage in OPC UA specifcation 14|OPC UA Standard|Forum|OPC Foundation

Dear all, I have a quite basic question. In specification 14, as I understand: the size of a DataSet Message and the DataSet Message itself are parts of the payload (figure 29 in specification 14). We then can use this information to detect the size of the whole payload. However, the payload is the data to be encrypted (figure 27 in specification 14), so I cannot find a way to calculate the size of the payload after the encryption (encrypted payload). More detail, a subscriber after receiving the UADP Network Message cannot detect the size of the encrypted payload so that it cannot decrypt it? I mean, which information in which field of the UADP Network Message can specify the size of the encrypted payload? Thank you for your answer.