Well, I'm not interested in decrypting it. If I discover (or guess!) what encryption algorithm it uses, I can just replace the old encrypted string with my encrypted string (for the new CD key, whatever). The way the CD key grabber that floats around on the net grabs the key in a similar way to what you described, iago. It does even less work and grabs it off the network when it's submitted to Battle.net, IIRC.
In fact, this might be useful for giving bogus CD keys to an already-working install. At a LAN party or a friend's computer, I can give them an install with my alternate CD key, and then change it to some stupid made up one like "THIS-ISAFA-KECDK-EYBLAH" or whatever and they're left with a system that'll work on the LAN, which is fine for the party, but there's no risk of them using it online and blocking me from getting onto Battle.net when they take it home.
Does anyone with more skillz than me wanna dive into this and start hunting around for candidate containers/files for the CD key? I can think of a few ways of doing this.
1.) Get a delta (binary diff) of two MPQ files that are the same patch version. Theoretically, the only thing that should be different is the part containing the CD key. Save this diff as an MPQ file "cdkey.mpq" and then extract it using an MPQ editor and giving the listfiles of one of the full MPQs as the listfile.
2.) Similar to above: Take a known working MPQ and back it up before changing the CD key using one of the CD key changers floating around on the internet, and compare changes as in above.
It may be easier to extract them before comparison instead of getting fancy with xdeltas and all that.