The problem is that when you read the CD-ROM as a raw device, it will return more data than the length of the ISO image; there are almost always some additional sectors of padding beyond the end of the image. Even dd'ing from the CD-ROM into shasum won't work unless you tell dd the number of blocks to copy, so that you don't include the padding. Try something like dd if=/dev/sr0 of=foo.img bs=2k count=`isosize -d 2048 /dev/sr0` | sha256sum - Note the backquotes for command substitution of the output of isosize. I haven't tried this just now, so it's possible that I've got the details wrong. Eric