T-Mo S3, was 4.3, reset to 4.1.2. Trying to root for the first time. topic

Stock T-Mobile S3 started getting slow, so I decided to rebuild. I reset the device, and attempted to root it for the first time and install Archidroid 1.x. Odin 3.09 loaded CWM Recovery and PhilZ (or at least claimed to), but the system booted into the standard Android manager, and it won't install from the media, claiming the file fails verification.

I'm pretty sure the phone was upgraded OTA to 4.3 before I wiped it, but now claims to be 4.1.2. Does this mean that I'm suffering the effects of Knox, or is there something very basic that I'm missing?

Thanks for any replies; this isn't my first rooted phone, but I'm no pro.