Wednesday, January 28, 2009

Hash of a CD

Recently I had to see if two CDs were identical. I had the hash value of the iso for the desired content so I decided to check the hash for both CDs. There is really not that much to this post, but just in case someone ever needs to know how to do this I'll give you the command line how-to.

In my case, md5sum /dev/cdrom did not work. Now there is no reason to copy the CD to an iso file in order to do this. You can just use dd and pipe the output into md5sum or sha1sum.

I found that just doing a straight dd without extra options did not work. So you should use isoinfo to get the logical block size and the volume size to feed to dd (bs="Logical block size" and count="Volume size"). I decided to put this all into a bash script you can find here. The code is shown below:

INFO=`isoinfo -d -i /dev/cdrom \
|awk '{ if ($1 ~ /Volume/ && $2 ~ /size/ ) print $4; \
else if ($1 ~ /Logical/ && $2 ~ /block/ && $3 ~ /size/) \
print $5 endif }'`


echo "Logical block size: ${INFO[0]}"
echo "Volume size: ${INFO[1]}"
echo "Now executing: "
echo "dd if=/dev/cdrom bs=${INFO[0]} count=${INFO[1]} conv=notrunc,noerror,sync | md5sum"

dd if=/dev/cdrom bs=${INFO[0]} count=${INFO[1]} \
conv=notrunc,noerror,sync | md5sum

It's not the most beautiful solution, but there it is. This uses md5, for other hashes just modify the script as needed. Also if your CD device is not /dev/cdrom modify that as well.

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