Do I need a premium account to play Minecraft offline?

I'm trying to run the offline version of Minecraft (which I'm told doesn't need a premium account?) but the "play offline" button is greyed out, with the text underneath it reading "not downloaded".

Do you need a premium account to play offline?

Note: I've also tried using the assets.minecraft.net/1.8/minecraft.jar but I encounter this (fatal) error: java: symbol lookup error: /usr/lib/jni/libjinput.so: undefined symbol: EVIOCGUSAGE

I'm also a bit worried that what I'm doing might not be allowed - is the minecraft.jar file from assets.minecraft.net legal?

Yes, you do need a premium account to play Minecraft offline.

The first time it is run the client requires you to log in with a premium account to download the necessary files from minecraft.net.

See the FAQ for Minecraft.