I have a ISO image file of Office 2013 (Pro) obtained from a torrent site. I have geniune product keys obtained from my MSDN subscription. Is it safe to apply my keys on this torrent image? Or should I download the official image from MSDN? How can I check whether this ISO is geniune or not? I have a quite slow Internet connection so don't want to download again if that isn't necessary.
On the MSDN Subscriber Downloads website (which is browsable without a MSDN subscription), Microsoft publishes SHA1 hashes for all downloadable files (or at least, all that I checked). You can use this hash value to verify that the ISO you have matches the official one; the probability of a modified file having the same hash is incredibly small (although one shouldn't say that the probability is zero), and if the hash doesn't match then it's certainly either modified or corrupted compared to the disk image provided by Microsoft.
Go into Subscriber Downloads and navigate to the product you are looking for, then click the "Details" link next to it. The hexadecimal SHA1 hash is shown under the heading "SHA1".
Then use any SHA1 calculator to calculate the hash of your own ISO file, and compare the two.
For example, the SHA1 hash of the Office Professional Plus 2013 with SP1 (x86 and x64) - DVD (English) ISO with release date 3/18/2014 is given as 48F5A9D5457EF7D9D3BDBFA56BFD515EC8B25615, as shown in the screenshot below: