rsteg

A minimal-dependency steganography toolkit in Rust — library, CLI, and (soon) full format parity with steghide and stegano-rs. Jump to the live stego demo, install, benchmarks, feature tracker, or read the algorithm explainers.

What is steganography?

Cryptography hides what a message says. Steganography hides that there is a message at all. rsteg takes a carrier file (image or audio) and tweaks its least-significant bits so they spell out a hidden payload. A copy of the carrier that doesn't use rsteg is indistinguishable from a copy that does — unless you know the scheme.

How LSB embedding works

For each byte of the cover, we overwrite the density low bits with one bit of payload. At density 1, the top 7 bits of a pixel are untouched — a pixel value of 0xA3 = 10100011 becomes 0xA2 = 10100010 to carry a 0 bit. That's a 1/256 change, invisible to the eye. At density 4 you get four times the capacity and the change is ±7 (still perceptually tiny but statistically detectable).

See it in action — Munch hidden in Munch

The left is an ordinary PNG of Munch's Starry Night. The right is the same painting with the full JPG of Munch's The Scream — 321 KB of arbitrary binary data — woven into its least significant bits at density 1, sealed with XChaCha20-Poly1305. To the eye and to diff-style byte comparison of visible pixels, they are indistinguishable. Only the bottom bit of every colour channel has been rewritten.

The stego PNG carries a 321 KB hidden JPG payload inside a 1.96 MB cover and ends up only +105 KB larger on disk — the extra bytes are PNG recompression overhead on the noisier bottom bits, not the payload itself.

Reveal the payload that was extracted

Running rsteg extract --in munch_starry_stego.png --password - --out scream.jpg returns a JPG byte-for-byte identical to the one that was embedded (SHA-1 10570e48…):

$ rsteg embed \
    --in  munch_starry.png \
    --payload munch_scream.jpg \
    --out munch_starry_stego.png \
    --password -
embedded 328443B into 2058477B carrier → 2166395B stego (328533 bytes framed)

Install

# Build the CLI with default features (png + bmp + wav + crypto + compat-steghide).
cargo build --workspace --release

# Embed a payload into a BMP cover with a password-derived AEAD key.
cargo run -p rsteg-cli --release -- embed \
  --in cover.bmp --payload secret.txt --out stego.bmp --password -

# Extract it again.
cargo run -p rsteg-cli --release -- extract \
  --in stego.bmp --out recovered.txt --password -

Feature tracker & comparison

Sources: steghide 0.5.1 source, stegano-rs README, rsteg spec set. Phase labels map to specs/09-roadmap.md: phase 1 is the next PR; phase 2 is after it merges.

Try it locally

git clone https://github.com/OpenHackersClub/rsteg
cd rsteg
cargo run -p rsteg-cli --release -- embed \
    --in  cover.bmp --payload secret.txt --out stego.bmp \
    --password -
cargo run -p rsteg-cli --release -- extract \
    --in  stego.bmp --out recovered.txt --password -

How rsteg is built

Plugin architecture via Cargo features

rsteg-core has zero runtime deps. Each carrier format and each crypto scheme is a separate crate gated by a Cargo feature. If you only need PNG-LSB, cargo build --no-default-features --features png pulls exactly one transitive dep (miniz_oxide for DEFLATE).

rsteg-cli
  ├── rsteg-core                  (std only)
  ├── rsteg-png        feat: png  (miniz_oxide)
  ├── rsteg-bmp        feat: bmp  (zero deps)
  ├── rsteg-wav        feat: wav  (zero deps)
  ├── rsteg-jpeg       feat: jpeg (zune-jpeg, phase 2)
  └── rsteg-crypto     feat: crypto + compat-steghide

The 32-byte PayloadHeader

Every rsteg-written carrier starts its embedded bit-stream with a 32-byte frame:

magic "RSTG" │ version │ flags │ crypto_fourcc │ scheme_fourcc │ density │ body_len │ body_crc32 │ reserved
  4           │   1     │  1    │      4        │      4        │    1    │    4     │     4       │    8   = 32 bytes

When a payload is encrypted (XChaCha20-Poly1305 + Argon2id KDF — feature landing next), the full header is bound into the AEAD associated data. Tampering with any field — or a rollback to an older scheme — fails Poly1305 tag verification, not the plaintext CRC. The CRC is there only as an integrity hint for unencrypted payloads.

Supply-chain posture

• Every direct dependency is spec-allowlisted before it enters Cargo.toml.
• No serde_derive, thiserror, clap, or any proc-macro — compile-time arbitrary code is the exact attack surface we're shrinking.
• #![deny(unsafe_code)] workspace-wide, with documented per-module opt-outs only for SIMD hot paths.
• Target budget: ≤ 30 transitive crates with default features; ≤ 2 with --no-default-features --features png.

Source: github.com/OpenHackersClub/rsteg. This page lives at public/index.html and deploys to Cloudflare Pages. Intro / LSB basics / bench headline sections are spliced in from source READMEs by rsteg-site-build.