Well I believe that question is best answered byTek-micro themselves, I don't know how authentic their products are, I just know it worked for me when I down loaded it, and you have to get ahold of the 'right'person because some times in the past, I have had questions and have used their chat they have on their website, and most of the ppl knew what they were talking about, but a couple of ppl didn't, but overall, I accomplished my goal which was to get windows8.1 up and running on my macbook pro.

Hi James,
I don't remember seeing a guarantee to be authentic. I called Microsoft and gave them my product key number and they said it could not be used for an ISO file. FYI, Tek Micro has an F rating from the BBB in the San Francisco Bay and Northern Coastal California area where they are located. http://www.bbb.org/greater-san-francisco/business-reviews/internet-shopping/tek-micro-in-hayward-ca-539231.
Hope this helps.