| Offset | Size | Field | Description | |--------|------|-------|-------------| | 0x00 | 4 bytes | Magic Number | 0x49445800 ("IDX\0") for validation | | 0x04 | 4 bytes | Version | 1, 2, 3 (determines field sizes) | | 0x08 | 8 bytes | Record Count | Number of index entries | | 0x10 | 2 bytes | Key Length | Bytes per key (e.g., 255) | | 0x12 | 2 bytes | Offset Length | Bytes per pointer (usually 4 or 8) | | 0x14... | Variable | Index Entries | [Key][Offset] repeated |
strings -n 8 unknown.idx Reveals embedded metadata or filenames. | Format | Use Case | Pros | Cons | |--------|----------|------|------| | .idx | Custom binary index | Fast, compact, seekable | Proprietary, opaque | | .ndx (Windows Help) | Compiled index | Rich metadata | Outdated | | .fti (Full-text) | Document search | Supports stemming | Larger file size | | .sha1 (Forensics) | Hash-only | Verifiable | No positional data | what is an idx file
An .idx file is the index at the back of a textbook (pointing to page numbers), while the primary data file is the chapters themselves . Common Contexts & Structures 1. Video Subtitles (VobSub) Primary File: .sub (bitmap images) Idx Role: Timecode-to-position map. Internal Format (Plain Text): | Offset | Size | Field | Description
Executive Summary An .idx file is not a single file type but a generic extension used to denote an index file . Its core purpose is to act as a roadmap or lookup table that allows software to find specific data within a larger, often sequential, primary data file without scanning the entire file. While the extension is consistent, its internal structure varies wildly depending on the application—from video subtitles to database keys and game archives. Core Concept: The Index Pattern In computing, an index separates location from data . Without an index, finding a record in a 10GB file requires reading from byte 0 to the target. With an index, you consult a small .idx file that says: "Search term 'X' starts at byte position 4,502,891" . Common Contexts & Structures 1
timestamp: 00:00:33:568, filepos: 000012800 timestamp: 00:00:35:123, filepos: 000045000 Each line tells the player: "At this timecode, jump to this byte offset in the .sub file to draw the subtitle." Primary File: .db (data rows) Idx Role: B-tree structure for rapid WHERE lookups. Why critical: Without the .idx , a SELECT * FROM users WHERE last_name = 'Smith' requires a full table scan (O(n)). With the index, it becomes O(log n). The .idx contains sorted keys and physical row pointers (ROWIDs). 3. Game Archives (e.g., Unreal Engine, Quake .pak indexes) Primary File: .dat , .pak , .big Idx Role: File allocation table for assets. Structure: | File Path | Offset | Size | CRC32 | |-----------|--------|------|-------| | textures/sky.png | 0x00A4F000 | 1.2MB | 0x8F73... | The game loads the tiny .idx into RAM first, enabling instant random access to assets inside the monolithic archive. 4. Media Servers (e.g., Icecast, Winamp) Primary File: .mp3 (streaming audio) Idx Role: Frame-accurate seeking. Because MP3 is a frame-based format, the .idx stores a table of frame headers and their byte offsets. This allows a listener to skip to 2:15 without decoding from the start. 5. Digital Forensics (e.g., EnCase, FTK) Primary File: Raw disk image ( .e01 or .dd ) Idx Role: Hash database or sector lookup. Forensic .idx files contain SHA-1 hashes of every file block, allowing investigators to instantly flag known contraband (via hash matching) without reviewing the entire drive. Technical Anatomy (Generic) While implementations vary, most .idx files share a binary pattern: