Why Chimera?

How It Works

Chimera applies six processing stages to every audio input:

Input audio
    |
    v  [1] VAD -- isolates speech from noise, music, silence
    |
    v  [2] Diarization -- MFCC k-means, assigns segments to speakers
    |
    v  [3] Key Derivation -- HKDF-SHA256 per speaker -> 5 parameters
    |
    v  [4] Voice Transform (Praat LPC source-filter model)
    |       Pitch shift     via PSOLA resynthesis
    |       Formant shift   via LPC vocal-tract length scaling
    |
    v  [5] COWL -- Cryptographic One-Way Layer
    |       Sub-perceptual spectral noise (SSNI)
    |       Micro phase perturbation (MPP)
    |
    v  [6] Cross-fade stitching + level matching
    |
Output audio (mono float64, same sample rate)

All randomness is derived from the key via HKDF, making the transformation fully deterministic and fully one-way: same key -> same output; output -> original is computationally infeasible.

Installation

pip install chimera-voice

With real-time microphone support:

pip install "chimera-voice[realtime]"

From source:

git clone https://github.com/Ohswedd/chimera
cd chimera
pip install -e ".[dev]"

Quick Start

Anonymise a file

import chimera

chimera.mask_file("interview.wav", "anonymous.wav", key="my-secret", preset="strong")

Work with NumPy arrays

import chimera
import soundfile as sf

audio, sr = sf.read("interview.wav")
result = chimera.mask_array(audio, sr, key="my-secret", preset="strong")
sf.write("anonymous.wav", result.audio, sr)

Multi-speaker: independent key per speaker

result = chimera.mask_array(
    audio, sr,
    key        = "my-secret",
    preset     = "moderate",
    mode       = chimera.MaskMode.ALL_UNIQUE,   # default
    n_speakers = 3,
)
print(result.speakers_masked)    # ['SPEAKER_0', 'SPEAKER_1', 'SPEAKER_2']
print(f"Processed in {result.processing_time_s:.2f}s")

Mask only selected speakers

result = chimera.mask_array(
    audio, sr,
    key          = "my-secret",
    preset       = "strong",
    mode         = chimera.MaskMode.SELECTED,
    speaker_ids  = ["SPEAKER_0"],   # only mask speaker 0
)

Real-time microphone

from chimera.realtime import RealtimeAnonymiser

anon = RealtimeAnonymiser(key="my-secret", preset="moderate")
anon.start()
input("Recording -- press Enter to stop...")
anon.stop()
anon.save("recorded_anonymous.wav")

Streaming (generator-based)

from chimera.realtime import mask_stream

for masked_chunk in mask_stream(my_chunk_generator, sr=22050,
                                key="my-secret", preset="strong"):
    send_to_output(masked_chunk)

Inspect parameters

p = chimera.get_params("my-secret", preset="strong")
print(p.summary())

#        Chimera MaskParams
# --------------------------------------------
#   Speaker label      : (none)
#   Pitch shift        : +5.743 st
#   Formant warp       : 0.91234x
#   Spectral tilt      : -1.214 dB/kHz
#   Breathiness        : 0.0812
#   Master intensity   : 0.780
# --------------------------------------------

Presets

Indicative Equal Error Rate against ECAPA-TDNN (VoicePrivacy 2024 protocol).

MaskMode

Security

Chimera is designed with three security properties:

Determinism -- F(audio, key) always returns the same output. Every bit of randomness is derived from the key via HKDF-SHA256.

One-way -- Given F(audio, key), recovering the original speaker's acoustic identity requires simultaneously inverting:

1. Key-seeded micro phase perturbation (requires HKDF seed)
2. Key-derived sub-perceptual noise injection (requires HMAC sub-key)
3. Praat LPC pitch + formant transform (non-invertible without all params)

Key independence -- HKDF-SHA256 provides 128-bit second-preimage resistance. Per-speaker keys are domain-separated: key + ":chimera:spk:" + speaker_id.

Chimera is a privacy-enhancing tool, not an encryption scheme. For high-stakes deployments, combine it with access control, key rotation, and additional anonymisation measures. See the Security wiki page for the full threat model.

Supported Audio Formats

Any format supported by libsndfile: WAV, FLAC, OGG, AIFF, and more.

Supported sample rates: 8 000, 16 000, 22 050, 24 000, 44 100, 48 000 Hz.

Project Structure

chimera/
|-- chimera/              # Library source
|   |-- __init__.py       # Public API surface
|   |-- core.py           # High-level functions: mask_file, mask_array, get_params
|   |-- pipeline.py       # ChimeraPipeline -- full orchestration
|   |-- keygen.py         # HKDF-SHA256 parameter derivation
|   |-- vad.py            # Voice Activity Detector
|   |-- diarize.py        # MFCC k-means speaker diarizer
|   |-- transform.py      # Praat LPC source-filter voice transform
|   |-- irreversible.py   # Cryptographic One-Way Layer (COWL)
|   |-- realtime.py       # Real-time microphone / streaming engine
|   |-- presets.py        # Named intensity presets
|   |-- types.py          # MaskParams, ChimeraResult, MaskMode, ...
|   |-- exceptions.py     # Exception hierarchy
|   `-- py.typed          # PEP 561 marker
|-- tests/                # Full test suite
|-- examples/             # Runnable usage examples
|-- docs/                 # Documentation
|-- benchmarks/           # Performance benchmarks
|-- paper/                # Academic paper (PDF)
|-- .github/workflows/    # CI and release workflows
|-- pyproject.toml
|-- CHANGELOG.md
|-- CONTRIBUTING.md
`-- LICENSE

Development

Setup

git clone https://github.com/Ohswedd/chimera
cd chimera
python -m venv .venv && source .venv/bin/activate
pip install -e ".[dev]"

Run tests

pytest                         # all tests
pytest -m "not slow"           # skip slow tests
pytest --cov=chimera           # with coverage

Lint and format

black chimera tests examples
ruff check chimera tests examples
mypy chimera

Build distribution

pip install build
python -m build

Academic Paper

The full technical paper is available at paper/chimera_paper.pdf.

It covers the full threat model (A1-A4), HKDF key derivation, voice transformation layers, COWL security argument, diarization architecture, real-time latency profile, comparison with state-of-the-art, and 15 references.

Citation:

@software{chimera2026,
  title   = {Chimera: Cryptographically Irreversible Speaker-Aware Voice Anonymisation},
  author  = {Ohswedd},
  year    = {2026},
  url     = {https://github.com/Ohswedd/chimera},
  version = {0.2.0},
  license = {MIT}
}

Changelog

See CHANGELOG.md.

Contributing

See CONTRIBUTING.md.

License

MIT 2026 Ohswedd