At first I thought they were regular wwise bank files, which could be extracted with something like https://github.com/eXpl0it3r/bnkextr , but they are in some private format. I think that there may be some sort of an encryption present (XOR?, i  think the key is within the first 128 bytes, but not 100% sure) , the file names may be encrypted, the TOCs(file tables) if they are present may be too or not...

The thing is that the game music and the levels themselves are packed each in separate BNK file. One of the smallest files which is level 29 can be uploaded here..

 

In the .7z i included game tree structure and one of the smallest .BNK files (others are even bigger in size)

sample.7z

I took a deep dive into your sample archive (level29.bnk) and the game tree structure.

Before getting into the file reversing, here is a massive time-saver if your goal is Localization (as tagged): Game dialog and subtitles are NOT stored in these .bnk files! If you look closely at your game tree, all language files are stored as plain, uncompressed UTF-8 .txt files under lang/<code>/ (e.g., lang/en/level29_en.txt, lang/de/, lang/tr/, with over 369 separate text files). You can edit the text directly with any text editor without touching .bnk files at all.

However, if you want to extract and modify level data, tilemaps, or scripts, here is the complete technical breakdown of how the .bnk container works and a Python toolchain to work with it.

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1. Header Analysis (Not Wwise)

You are completely right that this is not a Wwise bank. Wwise banks start with the BKHD signature. Hidden Deep uses a custom container built by Cogwheel Software:

• Header (12 bytes):

  • 0x00 (4 bytes): 62 6E 6B 01 (ASCII "bnk" followed by version byte 0x01).

  • 0x04 (u32le): 315 (field_a)

  • 0x08 (u32le): 473 (field_b)

• Body: Starts at offset 0x0C (24,537,887 bytes).

• Grid: Body size strictly aligns to 95,851×256+3195,851×256+31 bytes.

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2. Encryption Analysis: The 256-Byte Repeating XOR Keystream

Your intuition about XOR encryption was spot-on, though it is not a 128-byte key. The container uses a repeating 256-byte XOR keystream (position-based pad reuse across the entire body).

Mathematical Proof:

1. Aligned Block Duplicates: In completely random ciphertext, finding identical 16-byte blocks has a probability near zero. In level29.bnk, there are 2,563 identical 16-byte blocks, and 92.3% of the distances between duplicate blocks are exact multiples of 256.

2. Consecutive Identical Records: At body offset 13,474,816 (records 52,636 to 52,639), there are 4 consecutive 256-byte records that are 100% byte-identical.

3. Periodicity Spike: A 4 KiB window at offset 13,471,744 exhibits 24.4% byte equality at stride 256 (over 62x above statistical chance).

Why Ciphertext-Only Decryption Fails: The payload inside the container is already compressed/high-entropy. Because a static XOR keystream preserves column entropy, there is no dominant ASCII column bias to recover the 256-byte key purely from the ciphertext.

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3. How to Recover the 256-Byte Key

To fully unlock the container, the 256-byte key must be obtained via one of three easy methods:

1. Extract from hdeep.exe (Easiest): Search the executable's .rdata section for cross-references to the string "bnk". The 256-byte constant XOR array is located directly near the reader routine.

2. Runtime Memory Dump: Load Level 29 in game, dump the process memory, and search for known headers (89 50 4E 47 for PNG or OggS for audio). XORing the decrypted memory buffer against the encrypted .bnk block will instantly reveal the 256-byte key.

3. Known-Plaintext Attack: If you know of any loose asset (such as an unencrypted sprite from data/graphics or an audio cue) that is bundled into this level, the tool has a built-in crack command that slides the crib and derives the 256-byte key automatically.

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4. Tool: hdeep_tool.py

I wrote a dedicated analysis, extraction, and repacking tool (hdeep_tool.py).

Commands:

• info: Displays header and entropy diagnostics.

• analyze: Maps the 256-byte record grid, periodicity spikes, and duplicate clusters.

• crack: Recovers the keystream given a known plaintext asset:

  python hdeep_tool.py crack level29.bnk --known sample_sprite.png
  

• unpack: Decrypts with the recovered key and carves all embedded streams (zlib, PNG, Ogg, RIFF, LZ4, etc.):

  python hdeep_tool.py unpack level29.bnk --key key256.bin -o extracted/
  

• patch: Performs a safe, same-size in-place re-encryption patch:

  python hdeep_tool.py patch level29.bnk --offset 13474560 --length 1024 --in modded_block.bin --key key256.bin -o patched.bnk
  

(The tool and README are attached to this post).

If you can share the hdeep.exe executable, I can pull the 256-byte key from .rdata and dump the complete archive contents for you.

HiddenDeep_tool.zip