RAC-Hide

Robust Attribute Coding for images — hide one image inside another so it survives JPEG re-compression and proportional rescaling.

RAC-Hide is a dependency-free JavaScript implementation of robust image steganography. A secret image is compressed to JPEG and embedded into the relationships between low-frequency DCT coefficients of a cover image. The result is visually indistinguishable from the cover and can be recovered after the cover has passed through the lossy re-encoding pipelines used by messaging apps and social platforms.

Everything runs locally: the library never uploads an image anywhere.

Dual use. Steganography is a general information-hiding technique. This project is released for research, digital watermarking / copyright protection and lawful covert communication. Do not use it to distribute illegal content or to evade lawful moderation. See Security and ethics.

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Live demo

Try the full tool in your browser — nothing to install:

https://tuoPzf.github.io/rac-hide/demo/

The demo exposes every knob: capacity read-out, cover enhancement, the quality ↔ anti-compression slider, custom embedding parameters, progress feedback and previews. All computation stays on your device.

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Why coefficient relationships?

JPEG quantises every DCT coefficient independently, so absolute coefficient values are not stable under re-compression. But if two coefficients receive almost the same quantisation step, their difference is stable — quantisation shifts both members in nearly the same direction. RAC-Hide therefore encodes one bit as the sign of C1 - C2 for a carefully chosen coefficient pair, and actively pushes that difference away from zero by an adaptive margin.

That single idea, plus error-correction and interleaving, is what makes the payload survive the channel.

Pipeline

EMBED
  secret image ─▶ JPEG ─▶ 11-byte header ─▶ Reed–Solomon ─▶ bit repeat ─▶ interleave
                                                                              │
  cover image ─▶ YCbCr ─▶ Cb + spatial ruler ─▶ luma ─▶ 8×8 DCT ─▶ pair modulation
                                                                              │
                                                             IDCT ─▶ RGB ─▶ stego PNG

EXTRACT
  stego image ─▶ read ruler from Cb ─▶ normalise scale ─▶ 8×8 DCT ─▶ pair differences
                                                                          │
        search (ppb, repeat, nsym) ─▶ RS decode ─▶ header check ─▶ JPEG ─▶ secret image

See docs/algorithm.md for the full design, or the src/ modules — each one is self-contained and documented.

Features

• Anti-JPEG. Payload lives in quantisation-robust coefficient differences.
• Anti-rescale. A resolution-independent spatial ruler in the chroma channel lets the decoder recover the original dimensions at any scale.
• Blind extraction. The payload is self-describing; no original cover and no side channel are needed.
• Zero dependencies. Pure ES modules, no build step.
• Isomorphic core. The src/core.js codec runs in Node, a Web Worker or a browser; only src/browser.js touches the DOM.

Quick start

Node

import { buildPayload, embed, extract, capacityBytes } from 'rac-hide';
import { readFile, writeFile } from 'node:fs/promises';

// `imageData` is any { data, width, height } with RGBA bytes
const cover = await loadImage('cover.png');
const secretJpeg = await readFile('secret.jpg');

const options = { ppb: 2, repeat: 1, nsym: 48 };
console.log(capacityBytes(cover.width, cover.height, options), 'bytes available');

const payload = buildPayload(secretJpeg, options);
const stego = embed(cover, payload, options);      // -> { data, width, height }
const result = extract(stego);                     // -> { jpeg, ppb, repeat, nsym }
await writeFile('recovered.jpg', result.jpeg);

Browser

import { embedImage, extractImage, decodeImageFile, imageDataToBlob } from 'rac-hide/browser';

const cover = await decodeImageFile(coverFile);
const secret = await decodeImageFile(secretFile);

const { stego, secretSize } = await embedImage(cover, secret, { ppb: 2, repeat: 1, nsym: 48 });
const blob = await imageDataToBlob(stego, 'image/png');

const recovered = await extractImage(await decodeImageFile(blob));

Run the demo locally

The hosted demo is at https://tuoPzf.github.io/rac-hide/demo/ (the site root redirects there). To run it from a checkout, serve the repository over HTTP (ES modules do not load from file://):

python3 -m http.server 8000
# then open http://localhost:8000/demo/

Operating points

Preset	ppb	repeat	nsym	Trade-off
capacity	6	1	32	Largest, sharpest secret; weakest against attacks
balanced	2	1	48	Recommended default
robust	1	3	48	Strongest against compression/rescaling; smallest

Parameter	Effect when increased
ppb	Capacity rises linearly; per-bit margin is spread thinner
repeat	Random-error resilience rises; capacity falls by the same factor
nsym	Corrects more errors; payload shrinks (RS rate (255-nsym)/255)
marginMin	Robustness rises, visibility rises
marginGain	Textured blocks get larger margins (more robust, less invisible)
marginMax	Caps the distortion in high-energy blocks

Capacity in bytes:

total slots   = (W/8) · (H/8) · ppb
codewords     = floor(total slots / 2040)
capacity      = floor(codewords / repeat) · (255 - nsym) - 11

Robustness

Measured with the bundled test-suite and manual platform round-trips:

• Lossless for a clean embed → extract cycle, and for mild additive noise.
• The spatial ruler is recovered after proportional rescaling and JPEG/WebP re-compression.
• Survives aggressive JPEG quality reduction and rescaling on the robust operating point.

Known limits:

• Cropping breaks the lattice alignment and is not recoverable.
• Watermarks / redrawing destroy the signal.
• Heavily rescaled and heavily re-compressed images can exceed the error budget, especially on the capacity preset. Robustness is content-dependent: flat covers are harder than textured ones.

If the spatial ruler cannot be read, the extractor accepts the original cover dimensions manually as a fallback.

Project structure

src/
  constants.js      coefficient pairs, magic, defaults
  dct.js            orthonormal 8x8 DCT-II / inverse
  color.js          BT.601 helpers and chroma-preserving luma reconstruction
  reed-solomon.js   GF(256) RS codec
  interleave.js     global bit scattering
  ruler.js          spatial ruler (anti-rescale metadata)
  core.js           the codec: embed() / extract() / capacityBytes()
  browser.js        DOM helpers and high-level embedImage() / extractImage()
  index.js          public entry point
demo/               standalone browser demo
test/               node:test suite
docs/algorithm.md   detailed design notes

Testing

npm test

The suite covers the RS codec, the DCT round-trip, interleaving, the spatial ruler (including a simulated rescale) and full embed/extract round-trips across several operating points.

Security and ethics

• All computation is local. No network requests, no telemetry.
• This is a steganography, not an encryption, tool: the embedded payload is obfuscated, not cryptographically protected. Encrypt sensitive data first.
• Do not use this software to hide illegal content or to circumvent lawful content moderation. You are responsible for how you use it.
• The authors provide no warranty and accept no liability; see LICENSE.

Citation

If you use RAC-Hide in academic work, please cite it via CITATION.cff.

Contributing

Contributions are welcome. Please read CONTRIBUTING.md and CODE_OF_CONDUCT.md first, and report security issues privately as described in SECURITY.md.

License

GNU General Public License v3.0 or later.