Do I need Half-Live on Steam to use ESF

Do I need half-life on steam? I have the legitimate half-life game that's in a box for PC that I haven't yet opened. I've had it for like 5 years now it's gonna come in use lool.

Open it and get the CD key from it. Type it in on Steam and there you go. Legit copy of Half-life on Steam.

Oh really! thanks mate! quick reply to aha!. But doesn't this system seem a little to easy to hack or something? I mean you can google a serial key for the game and put it in steam and you'll get a copy of the game? That seem's a little to unfair to the people that actually spend their money on the games. Or do I have something wrong here? Anyways thanks mate!

A key can only be used once on Steam.

That and valve/steam was usually quite fast disabling rogue keys