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



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