Question [Q] How to securely install SuperSU before installing Safestrap

Hello! I want to install Safestrap Recovery v3.71 and i have the HDX8.9 WIFI.

"First Requirement: Rooted w/ SuperSU installed." (Hashcodes HowTo from here http://forum.xda-developers.com/show....php?t=2612772)

I have the stock rom, the only thing i have done ist root it and prevent OTA-Updates.
Before i make a backup/copy of the stock rom, i dont want to make much changes, like installing an alternate market.

Now my question:
How to secure install SuperSU ?

I found the APK can be downloaded from http://www.1mobile.com/supersu-358316.html.
But can i trust that download from 1mobile ?
Is a scan of the apk with https://www.virustotal.com/ usefull ?

THX for Help ...