Function header_len 

pub fn header_len(media_type: &str, kek_id: &str) -> usize

How many bytes a header holding these two strings occupies.

A reader already knows both — they are columns on the blob’s row — so it reads exactly this much rather than the 128 KiB a maximal header could reach. Header::decode still re-derives the length from the FILE’s own u16 fields, so a row that disagreed with its container is caught rather than believed.