MessageDerivedValues = [
    messageId: MessageId,
    hubAcceptedTimestamp: Timestamp,
    mlsGroupId: bstr,
    senderLeafIndex: uint .size 4,
    senderClientUrl: MsgUri,
    senderUserUrl: MsgUri,
    roomUrl: MsgUri
]

MsgUri = tstr
MessageId = bstr .size 32
Timestamp = MsecsSinceEpoch / ExtendedTime
; milliseconds since start of UNIX epoch
MsecsSinceEpoch = uint .size 8
; extended time from RFC9581
ExtendedTime = #6.1001({* name => value })
name = int / tstr .size (1..255)
value = any .size (0..4095)