nsZip

Work in progress file format for compressed Nintendo Switch games and a tool to compress/decompress them.

Please note that nsZip currently only supports the depreciated NSZP but not the new homebrew compatible NSZ file format so I highly recommend to use https://github.com/nicoboss/nsz instead.

NSZ file format:

Header:

| Offset|Size |Description | |-------|-----|-------------------------------| |0x00 |0x05 |XOR-Encrypted magic ("nsZip") | |0x05 |0x05 |Random key to decrypt the magic| |0x0A |0x01 |Version (for now 0x00) | |0x0B |0x01 |nsZip Type |

Type 0:

| Offset |Size |Description | |----------------|-----------------|--------------------------| |0x0C |0x01 |Compression algorithm | |0x0D |File size - 0x1D |Full compressed file | |File size - 0x10|0x10 (first half)|SHA256 of everything above|

Type 1:

| Offset |Size |Description | |----------------|-----------------|-----------------------------------| |0x0C |0x05 |bs = Decompressed Block Size | |0x11 |0x04 |Amount of Blocks | |0x15 + x * y |0x01 |Compression algorithm | |0x16 + x * y |y - 1 |cbs = Compressed Block Size | |0x15 + (x+1) * y|sum(cbs) |Concatenated compressed blocks | |File size - 0x10|0x10 (first half)|SHA256 header XOR SHA256 compressed|

y = ceil(log2(bs)/8) + 1

Note: The compressed block isn't allowed to be larger than the decompressed data - please use compression algorithm 0x00 (None) in that case or cbs might overflow!

Type 2:

| Offset |Size |Description | |----------------|-----------------|-----------------------------------| |0x0C |0x04 |Amount of Blocks | |0x10 |0x01 |s = Size of size parameters | |0x11 + x * y |0x01 |Compression algorithm | |0x12 + x * y |s |bs = Decompressed Block Size | |0x12 + x * y + s|s |cbs = Compressed Block Size | |0x12 + (x+1) * y|sum(cbs) |Concatenated compressed blocks | |File size - 0x10|0x10 (first half)|SHA256 header XOR SHA256 compressed|

y = 2 * s + 1

Compression algorithms:

|Value|Algorithm|Recommended Parameters | |-----|---------|-----------------------------| |0x00 |None |None - Just use memcpy | |0x01 |Zstandard|CompressionLevel = 19 | |0x02 |lzma |Dic=1536, WordS=273, cLevel=9|