stenoxide

Adaptive LSB steganography with cryptographic-grade payload protection.

What is stenoxide?

stenoxide hides an encrypted message inside a lossless PNG image. The message is compressed, encrypted and authenticated before a single pixel is touched, and the bits are then placed by a Syndrome-Trellis coder that is steered by a HILL adaptive cost map, so the changes land in the textured regions where a detector has the hardest time finding them. The keys come from the password and from the image itself: nothing but the payload bits travels in the container — no header, no salt, no nonce, no marker of any kind.

How it works

1. Container validation. The image is loaded, checked for size and format, screened for the 8x8 grid a prior JPEG round trip leaves behind, and measured for perceptual stability. A type-state pattern makes the validated buffer the only thing downstream code can receive, so an unvalidated image cannot reach the embedding path even by mistake.
2. Key derivation. Argon2id stretches the password against a salt derived from a perceptual hash of the container, and HKDF-SHA3-512 expands the result into an encryption key, a nonce and the seed of the embedding permutation. The salt is never stored: the receiver recomputes it from the image.
3. Payload protection. The message is compressed with Zstandard at level 19 and encrypted with XChaCha20-Poly1305. Extraction authenticates before it decompresses, so nothing unverified ever reaches the decoder.
4. Adaptive cost map. HILL assigns every pixel the cost of changing it: low in texture and noise, high in smooth gradients and flat areas. Images too smooth to carry a payload safely are rejected rather than used badly.
5. STC embedding. A Fisher-Yates permutation seeded from the derived key fixes a secret visiting order, and Syndrome-Trellis Codes embed the payload along it while minimising total distortion under the cost map. The embedding rate is capped at 0.02 bits per pixel, a compile-time constant rather than a parameter a caller can raise.

Security model

What it protects. The payload is encrypted and authenticated, so an attacker who suspects the image and cannot guess the password learns nothing about the message and cannot alter it undetected. The embedding is designed for statistical undetectability: adaptive costs, a low fixed rate and a secret permutation are what keep the stego image close enough to the cover for a detector to be unable to separate them.

What it does not protect. A compromised endpoint defeats everything here — the message exists in plaintext on both ends. It hides nothing about the fact that two parties exchanged an image: network metadata, timing and traffic analysis are outside its scope. It also assumes the container is never published elsewhere; an adversary holding the original cover can subtract the two images and see every changed pixel, which no embedding scheme survives.

Assumed adversary. A forensic laboratory running convolutional steganalysis (SRNet, YeNet and the like) against the stego image alone, without access to the original cover, and without the password.

Installation

CLI

cargo install stenoxide-cli

Prebuilt binaries for Linux (x86_64), Windows (x86_64) and macOS (Apple silicon) are attached to every GitHub release if you would rather not compile.

Library

cargo add stenoxide-core

Usage

Both subcommands read the password from the terminal with echo disabled. Neither the password nor the message is ever passed as an argument, so nothing sensitive reaches the shell history or the process table.

Embed

echo "secret message" | stenoxide embed --input photo.png --output stego.png

Extract

stenoxide extract --input stego.png

Extraction writes the recovered message to standard output as raw bytes, and reports every failure — wrong password, image carrying nothing, damaged payload — with the same sentence. Telling them apart is the oracle an attacker holding an intercepted image is looking for.

Requirements

• PNG container image: minimum 2000x2000 pixels, with natural camera texture.
• No prior JPEG compression on the container image. A PNG saved from a JPEG still carries the 8x8 block grid and is refused.

Crates

License

Apache-2.0. See LICENSE.