There is a "code phrase" which is used to decode the ULL files.

That is:

#### For our everlasting love. From 24 Dec, 1994. ####        Ooops. Who are you? You are not supposed to read this message. Please close this, Enjoy our game A3, and always be happy~~!         - The Extra-Terrestrial -

So after that we start reading the encoded file

current = getbyte from file
previous = current - 1
xorkey1 = substring(codephrase,ascii_code(previous),1)
result1 = xor previous,xorkey1
xorkey2 = filesize AND 0FF
result2 = xor result1, xorkey2
result3 = current_byte_position AND 0ff
finalchar = result2 XOR result3

And voila, your file is decoded