What is the RSDK file format?
A file with the .rsdk extension is a Retro Engine data archive. It bundles most of a game's assets into one container: character sprites, tile sets, stage layouts, object data, configuration files and Ogg music. The format comes from RSDK, the Retro Software Development Kit built by Christian Whitehead and later used by SEGA for official classic-Sonic releases. You will find these archives, usually named Data.rsdk, in Sonic CD (2011), the Sonic 1 & 2 mobile remasters, Sonic Mania and Sonic Origins.
The engine went through several revisions, and the archive format changed with each. RSDKv1 and v2 date to early fangame builds, v3 shipped with the 2011 Sonic CD remaster, v4 with the 2013 mobile remasters, and v5 (plus a v5U variant) with Sonic Mania and Sonic Origins. In an RSDKv5 file the header starts with the text RSDKv5, followed by a file count and a table of entries. Filenames are stored as MD5 hashes rather than readable paths, and most file contents are scrambled with a custom XOR cipher.
Security & safety
RISK: LOWAn .rsdk file is passive game data, not an executable, so opening or extracting it does not run code. The main practical risk is legal and technical: the assets are copyrighted by SEGA, and repacking a datapack incorrectly can corrupt a game install, so keep a backup of the original Data.rsdk before editing.
Format details
in a nutshell- Renoise Instrument (older builds) - Some legacy references map .rsdk-style names to sampler data, but this is unrelated to the Retro Engine archive and is not the dominant meaning.
Programs that open RSDK files
Windows4 apps
RetroED / RetroED2
Open-source Open RetroED and use Tools > RSDK Unpacker to extract a Data.rsdk datapack into an editable Data folder, choosing the file list that matches the game.
RSDKv5Extract
Open-source Run this command-line tool to unpack and repack RSDKv5 archives such as Sonic Mania's Data.rsdk.
RSDKv5_Extract_Plus
Open-source Use this improved extractor to unpack RSDKv5 and v5U datapacks, including Sonic Origins content.
RSDK-Reverse libraries
Open-source Build against these libraries to parse RSDKv1 through v5 archives programmatically when writing your own tooling. 
macOS1 app
RSDKv5Extract
Open-source Run this cross-platform Python extractor to unpack an RSDKv5 datapack on macOS. 
Linux1 app
RSDKv5Extract
Open-source Run this Python script to decrypt and extract the files stored inside an RSDKv5 .rsdk archive. Technical details
deep spec| Encoding | Binary |
| Byte order | Little-endian |
| Container | Single-file archive holding an entire game's data tree |
| Compression | Per-file; individual assets keep their own encoding (image data, Ogg audio, binary configs). RSDKv5 does not ZIP-wrap the whole archive. |
| Encryption | RSDKv5 obfuscates file contents with a custom XOR cipher keyed from the MD5 of the uppercase filename and the MD5 of the file-size string, plus nibble swapping and rotating key indices. Not a standard cipher. |
| Structure | RSDKv5 layout: 6-byte 'RSDKv5' magic, 2-byte little-endian file count, then one 0x18-byte entry per file (16-byte endian-swapped MD5 of the internal path, 4-byte offset, 4-byte size with bit 31 flagging encryption), followed by the raw/encrypted file data. Filenames are stored as MD5 hashes, so a filename list is needed to recover original paths. |
| Integrity | MD5 hashes of the internal file paths act as lookup keys for each entry. |
| Typical size | Tens of megabytes to over a gigabyte, depending on the game |
| Platforms | Windows, macOS, Linux, Android, iOS, Nintendo Switch, PlayStation, Xbox |
| Notes | Produced by the Retro Engine (originally Retro Sonic Development Kit, 'RSDK'), created by Christian Whitehead. Used by Sonic CD (2011), Sonic 1 & 2 mobile remasters, Sonic Mania and Sonic Origins. Versions RSDKv1 through v5/v5U each differ in header and packing; extraction tools require the matching version and a file list. |
| Released | 2011 (Sonic CD remaster, RSDKv3); RSDK lineage from 2007 |
| Latest version | RSDKv5 / RSDKv5U (Sonic Mania, Sonic Origins). Newer titles use later revisions. |
| Specification | github.com |
RSDK conversions
Community Q&A
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