its not possible to crack diablo, because if you want to play it, you must be connected on your bnet account, and if you are killing and leveling your character you need to be online on account, so .. its not possible to crack this game. Also there is explanation from blizzard:

Hi! As an expert in the subject i would like to explain why you cant play diablo 3 offline.
 
Diablo 3 was designed to be played as multiplayer, therefore servers must be responsible for item randomization and stash sicronization. 
 
If it wasnt this way, anyone could just break diablo 3 client code and improve his drops, or just assign items into your inventory. Obviously this would ruin game balance.
 
Since its everyone's wish to mantain game as fair as possible only thing blizzard could do would be: make a offline mode available, totally aside from online mode, but even this would require a major client update, because like i said before, client doesnt generate items, they only get and draw on your screen what server's tell them too.
 
"Why blizzard doesnt obfuscate client code:
 
This never worked with anyone. Its just a matter of time until someone breaks into it.
 
Thats pretty much why companies dont build standalone games anymore for computer platforms. Games are too easy to crack, it just demands time.
 
Extra myths:
 
Diablo 3 server is not a single pc. Its a cluster of pc's. Any chance of servers not being able to hold it, they will just add more machines to the server (I saw ppl saying buy a better server xD).
 
Auction House heavyness:
 
I am not sure if regular players know this, and i am not sure, but here it goes: its very likely, from a blizzard perspective that AH is way more heavy in terms of servers resource consumption that the whole game itself. Spamming the AH with several search queries is probably the worst thing you can do to blizzard servers. 
 
Imagine a SQL query searching trough millions of items, hundreds of variation points between them, without a simple way to organize them in a very simple logic way. 
 
Now imagine game, simple coordinates sync between player and server, plus actions.
 
Add to this huge problem the fact that 2 players shouldn't be allowed to buy the same item at the same time, and CPU threads MUST NOT colide and acess the same item at the same time.
 
AH is high tech, there aren't much companies able of doing systems like this one.