YPF

Used in the following games:

• Eroge! ~Sex and Games Make Sexy Games~

Structure

Header
Size	Content	Description
4Bytes	Magic/ID	"YPF/0"
4Bytes	Unknown1	Static?
4Bytes	File Count	Number of Files in archive
4Bytes	Index Size	Size of Index
16Bytes	Padding
Index Entry
Size	Content	Description
4Bytes	Unknown1
1Bytes	Filename Length	XOR 0xFF, then table lookup.
?Bytes	Filename	XOR 0xFF then XOR 0x40
2Bytes	Unknown2	Static?
4Bytes	FileSize1
4Bytes	FileSize2	Suggests compression is supported
4Bytes	File Offset
4Bytes	Unknown3

Comments

To decode the filenames, each byte in the filename is XOR by 0xFF and then 0x40. However, the filenames are not null terminated so 1 byte is stored before the filename that represents the the filename's length. This value is protected like the filename but is slightly different (as far as I can tell). XOR by 0xFF and then do a table lookup. The table is generated at runtime and swaps around 38 values in the table.

unsigned char test[256],
	eax, ecx, edx;

for (int a = 0; a < 256; a++)
{
	test[a] = a;
}

edx = test[0x09];
eax = test[0x0B];
test[0x09] = eax;
test[0x0B] = edx;
eax = test[0x0C];
ecx = test[0x10];
test[0x0C] = ecx;
test[0x10] = eax;
edx = test[0x0D];
eax = test[0x13];
test[0x0D] = eax;
test[0x13] = edx;
edx = test[0x11];
eax = test[0x19];
test[0x11] = eax;
test[0x19] = edx;
edx = test[0x15];
eax = test[0x1B];
test[0x15] = eax;
test[0x1B] = edx;
edx = test[0x1C];
eax = test[0x1E];
test[0x1C] = eax;
test[0x1E] = edx;
edx = test[0x20];
eax = test[0x23];
test[0x20] = eax;
test[0x23] = edx;
edx = test[0x26];
eax = test[0x29];
test[0x26] = eax;
test[0x29] = edx;
edx = test[0x2C];
eax = test[0x0F];
test[0x2C] = eax;
test[0x2F] = edx;
edx = test[0x2E];
eax = test[0x32];

Example: 0xEC ^ 0xFF = 0x13 Table lookup 0x13 -> 0x0D Which means the filename length is 13.

Nothing else appears protected (file sizes and offset) however there is still a few unknown values that will need to be researched if a packing tool is to be made.

Not the most optimized method and I could still be overlooking something or not realizing something important... oh well. It works, right? ^^;