tar checksums its headers and never your files

tar is older than most of the people who type it, and the format underneath has barely changed since the tape drives it was named after. I had used it for years without ever looking inside, so I wrote one by hand from the POSIX specification using nothing but Python's struct module, and checked every claim below against GNU tar 1.35 and Python's own tarfile. The format turns out to be simple enough to write in forty lines and odd enough that most of its consequences are not what I would have guessed. The oddest one is that tar carefully checksums every header and does nothing at all to protect