On 2013/11/04 17:13, ToddAndMargo wrote:
> On 11/04/2013 05:07 PM, Yasha Karant wrote:
>> On 11/04/2013 04:53 PM, ToddAndMargo wrote:
>>> On 11/04/2013 04:21 PM, Yasha Karant wrote:
>>>> I need to do a media comparison between a data DVD and the .iso file
>>>> that purportedly contains the image of the exact DVD (including any
>>>> bootable or autoload binary files, not for an Intel instruction set
>>>> architecture).
>>>>
>>>> When burning to the DVD, applications such as K3B and Nero (for Linux)
>>>> will do a verify of the burned media.  My understanding is that these
>>>> applications go through the device driver and device controller
>>>> hardware/firmware that may be applying error correction to the raw bit
>>>> stream; any such detected "hardware media" errors typically are reported
>>>> by the driver to a log file, but typically (if corrected) do not cause
>>>> the application to fail.
>>>>
>>>> If one mounts the .iso file, by a command similar to that below,
>>>>
>>>> # mount -t iso9660 -o ro,loop=/dev/loop0 /files/dvdimage.iso
>>>> /media1/virtualdisc
>>>>
>>>> and likewise has the physical DVD in the DVD drive and mounted from,
>>>> say, /dev/sr0
>>>>
>>>> will a diff /dev/loop0 /dev/sr0 suffice?
>>>>
>>>> Is there a utility that will do the same thing that Nero would do as it
>>>> verifies after burning, but not requiring the burn -- that is, verify a
>>>> DVD against an ISO image file?
>>>>
>>>> If /dev/sr0 were mounted on, say, /media/someDVD, and the ISO image
>>>> file on
>>>> /media1/virtualdisk , is there a utility or script to do a "bit by bit"
>>>> comparison via the mount points (not just the "raw" mount as /dev/sr0 )?
>>>>
>>>> Yasha Karant
>>>>
>>>
>>>
>>> Hi Yasha,
>>>
>>> Check the DVD as a raw device.
>>>
>>> After you burn the ISO, eject the DVD (clears out something,
>>> I don't know what, but had to learn the hard way):
>>>     /usr/bin/eject /dev/sr0
>>>
>>> Then inject the DVD (close the door).  Can be on the same
>>> line.
>>>     /usr/bin/eject -t /dev/sr0
>>>
>>> Then make an MD5SUM of each
>>>     md5sum /files/dvdimage.iso /dev/sr0
>>>
>>> Eyeball the sums.  One will be on top of the other.
>>>
>>> If you like, I have some leftover code I can send you.
>>>
>>> -T
>>>
>>>
>>
>>  From http://en.wikipedia.org/wiki/Md5sum
>>
>> As with all such hashing algorithms, there is theoretically an unlimited
>> number of files that will have any given MD5 hash. However, it is very
>> unlikely that any two non-identical files in the real world will have
>> the same MD5 hash, unless they have been specifically created to have
>> the same hash.
>>
>> End quote.
>>
>> I explain the above reality to my students, although I do use MD5SUM
>> myself.  I was hoping for a utility that did a true bit-by-bit
>> comparison of the two files.
>>
>>
>> Aside:  Note that a (very) clever attacker can embed specific issues
>> into a file such that the corrupted (and perhaps infected) file will
>> pass a MD5 hash test.  Note that USA NSA and other entities often do
>> employ such clever persons (do recall the cyber attack on the fissile
>> material enrichment facilities of a Middle Eastern nation state not in
>> full agreement with USA foreign policy, albeit an attack not
>> specifically limited to this mechanism).  I am not suggesting that the
>> DVD and ISO image file I am using are subject to this sort of clever
>> corruption; but, it is important to understand the limitations of
>> certain techniques.
>>
>> Yasha Karant
>>
>
> Hi Yasha,
>
> The likely hood is pretty low.
>
> You could always try using the SHA sums.  Maybe do
> both.
>
> -T

Or use the "cmp" command, "cmp -l infile outfile" if he really wants a
byte by byte comparison of the disks.

{o.o}