Hahah, wow. I had never thought in my wildest dreams that someone would crack it that quickly. Well, "Nagato" sent me his entry last night. My day had been made.
Anyway, with Nagato's permission, I have been allowed to open source his program under the (new) BSD license. Usage is detailed in the archive. Decrypting is done in (POSIX-compliant) C, but the mass downloader scripts for both Yahoo! Comic and Club Sunday are both written in Python and require Beautiful Soup. Enjoy.
PCViewer Downloader (4SH)
I'll try to get my GUI-based downloader ready as soon as possible, though it makes me inadequate as a programmer to translate his code.