Probably? I'll grant you that the output of SHA-512 is going to be longer than combining several small hashes, but I don't intuitively see that it's necessarily more secure.

See Joux's work on multicollision attacks. While it was thought (before he developed this attack) that taking a (secure) N-bit hash and a (secure) M-bit hash and concatenating the outputs was equivalent to a secure (N+M)-bit hash, it turns out this is not the case - it's more like a max(N,M)-bit hash, for (some) security purposes, such as collision resistance.

It's not intuitive, though - at least not until after you see the attack. We had been designing and using hashes based on the Miyaguchi-Preneel and Davies-Meyer methods for well over a decade before Joux noticed the problem.

I've been in Japan for about six months now and I've wrote some iAppli; for games in particular, it's tough. The jar size has a 10k restriction (I kid you not), there is a really bizzare file you have to carefully write (called a .jam... yum) in order to download the applet, and the DoCoMo API is almost useful (I have some severe critisms of that, but I suspect it was rushed so what can you do?).

The jar size is the most limiting factor. That 10k holds everything you don't want to d/l from the net while playing your game; class files, graphics, music, data; everything. If you choose to d/l your content from the net, that limits where you can play your game and (at least here in Japan where we are billed by the PACKET) can REALLY jack up your bill- most of the "commercial" games for iMode phones do this, and it gets super-annoying FAST (but even a few extra k really boost the experience).

All this sounds dark, but it is so incredibly cool to play a game or use an app you've written while waiting for the train, that it's worth it. Well, sort of. ;)