Difference between revisions of "NKit/NKitFormat"

Revision as of 12:40, 9 September 2019

The NKit Format is a non-lossy format for shrinking and restoring Wii and GameCube images. It supports clean / Redump images as well as scrubbed and hacked images. Some corrupt and bad images are supported too, although these quite often error.

There are 2 NKit output formats, raw (iso) and compressed (gcz). Both were designed with 1:1 preservation, size and playability in mind. Hardware support for Wii was sacrificed for smaller images. WBFS is currently the best available and Recoverable to Redump where not hacked or corrupt.

NKit Format images contain the bare minimum of data. All junk and scrubbing is removed. Non-uniform data is preserved in 256 byte blocks with a 4 byte header. Wii encryption and hashes are fully recreatable and removed. Meaning any remaining data is as compressible possible.

Header

NKit places its header at 0x200 in the Wii and GameCube disc headers.

NKit also modifies the Wii header to set the bytes at 0x60 and 0x61 to 0x1. This indicates to a Dev Kit and Dolphin that the encryption and hashes aren't present.

GameCube

The disc header (Boot.bin) length is 0x440. It's modified as detailed above. The other items remain untouched.

The filesystem and fst.bin are modified when converting to NKit.

If the fst.bin is invalid then the whole filesystem (post fst.bin) is encoded as a gap. This is known as a Bad Conversion. Typically for corrupt images. Some customs may have this issue. Bad Conversions often error too as not all possibilities are convertable

Wii

The disc header length is 0x50000. The partition tables are modified to point to new partition offsets.

Removed Update Partition

The update partition may have been removed in order to save space. In this case offset 0x218 in the disc header will be the CRC32 of the removed partition. The removed partition will have been backed up in the 'Redump' recovery folder or in the 'NkitExtracted' recovery folder if it's not a known partition.

The original partition table is backed up at 0x50000. The next partition (Data or Channel) is located at 0x58000

Partitions

The partition header length is always 0x20000. The NKit Format only modifies 1 4 byte value - the partition length (offset 0x2bc). The length is the new size of the compacted partition data and is used to restore the partition.

The partition data is unencrypted and hashes have been removed.

The new partition data length is calculated without the hashes, whereas the original includes them. The original value is preserved in the partition data at 0x210. The real RVT-H format stores 0 for the length in the header. It's unconfirmed if a Dev Kit uses this value

Partition Processing

NKit partition data information:

• Decrypted groups
• Hashes are verified and removed - if any hash sector fails in a group then all the group's hashes are cached and stored at the end of the partition
• System items are copied as is up to the fst.bin. The fst.bin will be modified to reflect new file offsets
• Immediately after the fst.bin follows a bit mask of flags indicating preserved hashes that didn't verify correctly. The length of this mask is 1 byte for every 8 groups in the partition. Add 1 byte if the number of groups isn't divisible by 8
• The filesystem is compacted with the NKit Format
• If the fst.bin is invalid then the whole filesystem (post fst) is encoded as a gap. This is known as a Bad Conversion. Typically for corrupt images. Some customs may have this issue. Bad Conversions often error too as not all possibilities are convertable
• Hashes that failed to verify are stored at the end of the NKit Format partition data. The length can be calculated from the hash mask. Care must be taken to calculate the last group. Only read as many hashes are there are blocks
• If the partition data ID is 4 null bytes then 0x440 bytes for the header is written followed by 4 bytes indicating the original partition length. The remainder of the partition is encoded as a gap. The header is not modified. This is to cater for 2 system discs that have missing update partition data that's filled with sequential values. Decryption and hashes are valid and removed
• Junk following the partition is encoded and stored in the NKit Format partition
• Partitions are then padded to 32KiB with 0x00 where the next partition follows (if any)

FileSystem

Both the GameCube and Wii use the same code to encode the filesystem. The only real difference is that Wii fst.bin offsets and lengths must be multiplied by 4 when mapping to files. The NKit Format uses a custom Run Length Encoding (RLE) to remove the gaps between files and shrink gaps to 4 bytes in most cases.

Gaps are processed on the 4 byte boundary. Any data from a non aligned file remains intact (it should be 1-3 bytes of 0x00). The gaps also contain trailing nulls that follow the previous file. See the filesystem rules for more information.

Data in a gap is categorised as:

• Junk: a gap with the correct nulls and junk
• Block: Scrubbed or filled with the same byte. Wii scrubbed data when decrypted is a repeating 16 byte sequence
• NonJunk: Data that is not junk or scrubbed data

The Gap / 0x00 Scrubbed / Preserved Junk File RLE format is:

If the gap is set to mixed there will be multiple encoded blocks with a 4 byte header

Repeat is used if the size or block count does not fit in the value mask specified. It's never occurred in tests as yet.