No, I am not looking to pirate. I bought a key through the Upgrade assistant (for just $15 due to the upgrade offer), but it downloaded an iso file that was between 2.3 and 2.5 GB. Which doesn't make sense to me, because the Evaluation version of Windows 8 x64 is closer to 3.4 GB in size. I assumed the Upgrade Assistant would be intelligent enough to realize that it is being run on a Windows 7 x64 machine and by extension, download the x64 code.

Previously, I was able to legally download the ISOs (sans the keys, of course) from the Digital River host. I do not see an option to do that.

I'm not interested in risking downloading a tampered ISO. I want to do it through Microsoft channels, but I just don't see how. As you may imagine, search terms such as "Windows 8 official download link" result in a plethora of obviously spyware infested piracy sites.

If there's any non-exposing way for me to prove that I have legally purchased Windows and I'm genuinely looking for this answer, please let me know.

For reference to what I am looking for it is similar to the answer given in this question for Windows 7: Where do I download Windows 7 (legally from Microsoft)?