ODOLDecrypter

Standalone decryptor for encrypted Arma 3 ODOL v75 models — the asset protection Bohemia Interactive introduced for Creators DLC content (Reaction Forces and newer), where binarized .p3d models ship with non-zero encryption fields inside their header.

The tool restores regular, unencrypted ODOL v75 models that can be read by standard modding tools.

Features

• Batch processing: files, wildcards (*.p3d), directories, -r recursion, parallel execution across all CPU cores
• Both known ODOL v75 tweak variants:
  • simple size-derived tweak (enc1 bit 1 clear) — e.g. Reaction Forces
  • FNV-1a name-hash tweak (enc1 bit 1 set) — model-name candidates are derived from the input path automatically and validated
• Output is a valid plaintext ODOL v75 (enc fields zeroed), so re-running the tool is safe and idempotent
• Reports each model's Steam AppID and p3d prefix after decryption

Usage

ODOLDecrypt <inputs...> [-o <outdir>] [-r] [-c] [--overwrite] [--inplace]

<inputs...>       .p3d files, wildcards (*.p3d) or directories
-o, --out <dir>   write decrypted files into <dir>
-r, --recursive   recurse into directories
-f, --overwrite   overwrite existing outputs
    --inplace     replace input files with their decrypted versions
-c, --copy-plain  also copy already-plaintext models into <dir>

Examples:

ODOLDecrypt.exe D:\models\*.p3d -o D:\models\decrypted
ODOLDecrypt.exe "D:\SteamLibrary\steamapps\common\Arma 3\@RF" -r -o out

Note: not every Creators DLC encrypts its models — e.g. Global Mobilization ships regular plaintext ODOL v75 (enc fields are zero). Such files are skipped by default; pass -c/--copy-plain together with -o when batching a mixed directory and you want the complete set in the output folder.

Building

Requires the .NET SDK 10 or newer (download) — no Visual Studio or other dependencies.

git clone https://github.com/Nishikinov/ODOLDecrypter.git
cd ODOLDecrypter
dotnet publish -c Release -r win-x64

The finished standalone executable appears in bin\Release\net10.0\win-x64\publish\ODOLDecrypt.exe (~11 MB). It is fully self-contained (trimmed and compressed): copy it anywhere and run on any Windows x64 machine — no .NET installation required.

For a quick test without publishing you can run directly from source:

dotnet run -c Release -- <inputs...> [-o <dir>]

How it works

The ODOL v75 header stays plaintext:

+0   "ODOL"
+4   version u32 (= 75)
+8   enc1 u32     \
+12  enc2 u32     / non-zero => payload from offset 16 is encrypted

Encryption is a per-4096-byte-block RC4 whose key material is never stored — it is derived at load time from constants baked into arma3_x64.exe (two key tables, a tolower table and a delimiter alphabet) plus per-file values (the enc fields and the stream size):

step	detail
key16	seed walk 0x25, +7 if x%3!=0 else +1, &0x7F XOR table A/B (@0x141B731E8 / 0x141B73200), selected by bit 2 of enc1
tweak	simple (bit1 clear): b = size & 0xFF; s = b>>3 & 7; l = (-b) & 7; ((size >>> s) | (size <<< l)) ^ size
tweak	name-hashed (bit1 set): A ^ B ^ size ^ fnv_low32, where B is the size rotation above and A = low32(rot64(fnv64)); fnv64 = FNV-1a-64 over the lowercased model-name suffix (part after the last \ or /)
block key	key16 XOR repeat_le32(~tweak ^ blockPos)
KSA	standard RC4 KSA over an identity S-box
drop	LCG x = x*0xC1C64E6D + 0x3039 (mod 2^31) seeded with blockPos ^ tweak; a float-trick scales the draw into [256..512] PRGA pre-steps
PRGA	after the drop the index roles i/j are swapped, then the block is XORed

The first 16 stream bytes are always plaintext.

Limitations

• Windows x64 only (as the game itself).
• The name-hash variant depends on the model name the engine holds at load time; the tool probes candidates derived from the input path and validates each result. If none matches, add the expected string to CandidateNames() in Program.cs.
• Alternate key-table mode (enc1 bit 2) is implemented per disassembly but was not exercised against real files.

Disclaimer

This project is intended for interoperability and research (modding, preservation, cryptography study). Use it only on content you legitimately own or have permission to modify. Respect Bohemia Interactive's content licensing terms; no original game data is included in this repository.

License

MIT