Feature Request StegoMeow

[Capitali]

Feature Request: Cat-Image Passkeys (Steganographic QR Replacement)

Summary

Introduce an optional cat-image passkey feature: short-lived, server-signed authentication payloads invisibly embedded in cat images using robust steganography (QIM in DCT domain with ECC).
This would serve as a QR-code replacement for cross-device login while maintaining cryptographic assurances.
It aligns with MeowPassword’s branding and provides a novel, engaging UX.

---

Motivation

• Current cross-device passkey flows often rely on QR codes. Functional, but visually uninspired.
• Cat images are consistent with project branding and offer higher user delight.
• Steganography allows us to keep images looking normal while still carrying an out-of-band (OOB) login payload.
• This creates a differentiator: “scan the cat, not the QR.”

---

Proposed Methodology

Payload Structure

• Server generates a short-lived signed object (JWT or CBOR):

  {
    "ver": 1,
    "rp": "example.com",
    "nonce": "base64url(16B)",
    "exp": 60,
    "webauthn": {
      "challenge": "base64url(32B)",
      "session": "opaque-session-id"
    },
    "sig": "Ed25519 or ECDSA signature"
  }

• Size: ≤ 512 bytes compressed and base64url-encoded.
• TTL: 60–120 seconds.
• Signed by server, verified by client.

Embedding Algorithm

• Convert RGB → YCbCr, operate on Y (luma) channel.
• Tile into 8×8 blocks, compute 2D DCT.
• Select mid-band coefficients (zig-zag positions ~10–20).
• Scatter coefficients via PRNG seeded with a watermark key (wmKey).
• Encode bits using dithered QIM (Quantization Index Modulation):

  @inline(__always)
  func qimEmbed(_ c: Float, bit: Int, step: Float) -> Float {
      let d = (bit == 0) ? -step/4 : step/4
      return step * roundf((c - d)/step) + d
  }
  
  @inline(__always)
  func qimExtract(_ c: Float, step: Float) -> Int {
      let d0 = -step/4, d1 = step/4
      let e0 = abs(c - (step * roundf((c - d0)/step) + d0))
      let e1 = abs(c - (step * roundf((c - d1)/step) + d1))
      return e1 < e0 ? 1 : 0
  }

• Apply Reed–Solomon ECC and add a sync preamble (e.g., 64-bit Barker code).
• Save as PNG or high-quality JPEG (Q ≥ 90).

Extraction

• Reverse the pipeline:
  • YCbCr → DCT → extract coefficients per PRNG path.
  • QIM decode bits → sync align → ECC decode → verify CRC.
  • Decompress + base64url decode → verify server signature.
• If valid and not expired, use payload for WebAuthn completion.

---

Software Architecture

New Swift Package Target: MeowStego

• StegoEncoder / StegoDecoder
• DCT8x8Provider with:
  • Accelerate/vDSP backend (Apple platforms)
  • Naive fallback or FFTW shim (Linux)
• ECC module: Reed–Solomon RS(255,k)
• PRNG: ChaCha20/AES-CTR seeded with wmKey

CLI Additions

# Embed payload into a cat
meowpass steg-embed --in cats/tabby.png --out cats/auth.png   --payload-file payload.jwt --wm-key hex:001122...

# Extract payload from a cat
meowpass steg-extract --in cats/auth.png --wm-key hex:001122... --raw

---

Acceptance Criteria

• Encode/decode round trip with BER < 1% after JPEG Q=85, ±5% scale, ±5° rotation.
• Screen→camera capture (1080p monitor → iPhone/Android) succeeds ≥95% within 2 seconds.
• Payload verified with Ed25519/ECDSA signature and rejected if expired.
• CLI subcommands steg-embed and steg-extract added with clear flags.
• Graceful fallback to QR if extraction fails after N retries.
• Unit tests:
  • Deterministic PRNG mapping
  • RS encode/decode
  • QIM property tests (bit-flip robustness)
  • End-to-end payload extraction

---

References

• Chen & Wornell, Quantization Index Modulation: A Class of Provably Good Methods for Digital Watermarking and Information Embedding, IEEE TIT, 2001.
• Robust DCT watermarking literature.
• Existing QR-based WebAuthn out-of-band flows.

---

Requested Outcome

A new MeowStego library target and meowpass steg-* subcommands to generate and verify cat-image passkeys, enabling fun, branded, and secure OOB authentication.