CD/DVD/ISO images, readcd adding extra bytes at EOF?

[Ben Scott]

Hi everybody!  ("Hi Dr. Nick!")

  So I've got a genuine, official, stamped "Fedora Core 5" DVD here.
I got it from a Red Hat employee (Hi Heather!).  Copyright and
trademark notices and everything right on it.

  I made an image copy of the DVD using the readcd(1) command:

          readcd dev=/dev/scd0 f=FC-5-i386-DVD-readcd.iso

  I mounted the image via loopback mount, and mounted the DVD the
old-fashioned way, and ran diff(1) on the filesystems, and they match
exactly.

  I ran sha1sum(1) on my image file, and compared that to the official
SHA1SUM file distributed via the Red Hat mirrors, and the SHA1
checksums do *not* match.

  So I fired up BitTorrent and downloaded a copy of the disc image
distributed by Red Hat.  That went fine.

  I ran sha1sum(1) on *that* file, and it matches the official SHA1SUM.

  I loopback-mounted the downloaded image and the readcd image, and
diff'ed the filesystems, and they match exactly.

  I ran cmp(1) on the two image files, and discovered that up to byte
3,253,669,888, they match exactly.  That is also the byte at which the
downloaded image ends.  The readcd image goes on for another 28,672
bytes.

-rw-rw-r-- 1 root   pubadmin 3253698560 Sep 18 21:59 FC-5-i386-DVD-readcd.iso
-rw-rw-r-- 1 bscott pubadmin 3253669888 Sep 19 00:57 FC-5-i386-DVD.iso

  This isn't the first time something like this has happened to me,
but it's the first time I've had high confidence in both the original
physical media and the downloaded image.

  Anyone know what that extra 30 KB or so at the end of the readcd
image is, and why this happens?  Is there a way to have produce an
image without extra stuff at the end of it?

-- Ben