Function group_message_tbs 

pub fn group_message_tbs(
    group_id: &[u8; 32],
    data: &[u8],
    sender_hmac: &[u8],
    should_push: bool,
) -> Result<Vec<u8>, Error>

The TLS encoding of GroupMessageTBS (API section 3.1), which a group message’s signature covers. It fails only for a field of 2^30 bytes or more.