MKX (Mortal Kombat X) File Format

Binary format specification for MKX .xxx files. File version 677 (0x2A5), licensee version 157 (0x9D), FourCC "MK10".

Header (FPackageFileSummary) — Variable Size

Unlike IJ2's fixed 100-byte header, MKX serializes FString and TArray fields inline, making the header variable-size.

#	Field	Type	Notes
1	Magic	u32	0x9E2A83C1
2	FileVersion	u32	Low u16 = 0x2A5 (677), high u16 = 0x9D (157)
3	TotalHeaderSize	u32	Offset where export data begins
4	MidwayTeamFourCC	4B	"MK10"
5	MidwayTeamVersion	u32	80
6	ShaderVersion	u32	102
7	MetadataVersion	16B	MD5 hash (not in IJ2 or MK11)
8	FolderName	FString	Typically "None\0" (not in IJ2 or MK11)
9	PackageFlags	u32
10	NameCount	u32
11	NameOffset	u64
12	ExportCount	u32
13	ExportOffset	u64
14	ImportCount	u32
15	ImportOffset	u64
16	GameThreadExportCount	u32
17	DependsOffset	u64	Not in IJ2
18	BulkDataOffset	u64	Not in IJ2
19	ImportExportGuidsOffset	u64
20	ImportGuidsCount	u32
21	ExportGuidsCount	u32
22	ThumbnailTableOffset	u64
23	GUID	16B
24	EngineVersion	u32
25	CookedContentVersion	u32
26	CompressionFlags	u32	0x02 = ZLIB
27	CompressedChunks	TArray	u32 count + FCompressedChunk[] (inline)
28	AdditionalPackagesToCook	TArray	u32 count + FString[]

After the TArrays: DiskSizeOfMD5Set (u32+skip), DiskSizeOfClassVersionTable (u32+skip), LinkerRootFullyQualifiedPath (FString, licensee >= 0x75).

FCompressedChunk — 24 bytes

Field	Size	Type
UncompressedOffset	8	u64
UncompressedSize	4	u32
CompressedOffset	8	u64
CompressedSize	4	u32

Export Table Entry (FObjectExport) — 88+ bytes

Variable size due to ComponentMap TMap. Minimum 88 bytes (when ComponentMap count = 0).

Field	Size	Type	Notes
ClassIndex	4	i32	resolve_object
SuperIndex	4	i32	resolve_object
OuterIndex	4	i32	resolve_object
ObjectName	8	FName	index + number
ObjectGuid	16	GUID	Before ArchetypeIndex (IJ2 puts it after ObjectFlags)
ArchetypeIndex	4	i32	resolve_object
ReferencedObjects	4	i32
ObjectFlags	8	u64
SerialSize	4	u32	Export data size
SerialOffset	8	u64	Export data offset
ComponentMap	var	TMap	u32 count + entries (12B each: FName + i32)
ExportFlags	4	u32
PackageGuid	16	GUID	Not in IJ2

Import Table Entry (FObjectImport) — 44 bytes

Field	Size	Type	Notes
ClassPackage	8	FName
ClassName	8	FName
OuterIndex	4	i32
ObjectName	8	FName
ObjectGuid	16	GUID	Not in IJ2 (IJ2 imports are 28 bytes)

Compression

Algorithm: ZLIB (flag 0x02). NOT Oodle.

Block framing is the same as IJ2:

• BlockHeader (32B): Magic (u32) + Padding (u32) + ChunkSize (u64) + CompressedSize (u64) + DecompressedSize (u64)
• ChunkHeader (16B): CompressedSize (u64) + DecompressedSize (u64)

Also supports LZO (flag 0x01).

Bulk Data

Uses TFC companion files (same mechanism as IJ2), not PSF tables (like MK11).

SkeletalMesh Geometry

Vertex data stored in separated buffers (unlike IJ2's interleaved FSoftSkinVertex chunks):

Sections → IndexBuffer → ShadowIndices → ActiveBoneIndices →
Chunks → NumVertices → Edges → RequiredBones →
PositionVertexBuffer → TangentVertexBuffer → WeightVertexBuffer →
UVVertexBuffer → [ColorVertexBuffer] → DQBlendWeightVertexBuffer →
VertexBufferGPUMorphSkin → AdjacencyIndexBuffer

Position buffer applies X-negation for LH→RH coordinate conversion.

MKO (MKScript) Format

Header: 72 bytes (18 × u32), same as IJ2. Key difference: field 13 is tweakvars_size (byte count) in MKX vs tweakvars (item count) in IJ2.

Function headers: 80 bytes. Uses u64 offsets vs u32 + padding in IJ2.

Section order differs from IJ2: MKX puts bytecode at the END (after all tables), IJ2 puts it at the START. Checked pointers are 4 bytes in MKX (vs 8 in IJ2). Parse success: 462/463 files (99.8%).

FMOD Audio

SND packages contain FmodDesignerProject, FmodEventFile, FmodSourceData, and FmodEvent exports.

FSB encryption: type 1 cipher — decrypted[i] = reverse_bits(encrypted[i]) ^ key[i % key_len]. Key: 996164B5FC0F402983F61F220BB51DC6 (32-byte ASCII). The bIsEncrypted flag at project offset +200 bit 2 controls application.

Cross-Game Comparison

Feature	MKX (0x2A5)	IJ2 (0x2DC)	MK11 (0x301)
FourCC	MK10	DCF2	MK11
Header	Variable (FString+TArray inline)	Fixed 100B	Fixed 104B
MetadataVersion	16-byte MD5	Not present	Not present
FolderName	FString in header	Not present	Not present
Compressed chunks	Inline TArray	After header (flat)	Named packages
FObjectExport	88B+ (has PackageGuid)	72B	76B
FObjectImport	44B (has ObjectGuid)	28B	20B
Compression	ZLIB / LZO	Oodle v4	Oodle v5
Bulk data	TFC files	TFC files	PSF tables