nightjar

An offline Android app that demonstrates covert data transmission — hiding data in sound and images — alongside the detection techniques that can catch it.

What it does

nightjar pairs four data-hiding techniques with their detection/steganalysis counterparts, all running locally on-device:

• Acoustic data-over-sound modem — sends data phone-to-phone, speaker to microphone, room-local, using FSK/PSK/MFSK encoding.
• Image steganography — least-significant-bit (LSB) encoding of data inside images, plus a steganalysis screen for detecting it.
• Audio steganography — four techniques (phase inversion, spectrogram-LSB, MFSK, and true phase-coding) for hiding data inside an audio clip.
• Passive acoustic detector — a real-time listener that can flag the modem's own signal, demonstrating that "covert" acoustic channels aren't necessarily undetectable.

The default UI is "Night Jar," a disguise-themed front end where every decoded payload appears as a firefly. The underlying technical module screens (modem, image stego, audio stego, detector) are reachable via a long-press reveal gesture from the jar screen.

Screenshots

Real screenshots from a Pixel 6a. The app opens as Night Jar — a calm, disguise-themed front end where every hidden payload shows up as a "firefly." A long-press on the jar reveals the technical Workshop underneath.

Welcome first-run trail	Night Jar the home shelf	Inside a jar your fireflies
In sound acoustic modem	In an image LSB bit-plane	In audio spectrogram diff
The Meadow passive detector	The Workshop module menu	Image tool embed · extract

Download

Grab the latest signed APK from the Releases page and install it directly — no Play Store needed. On your phone: download the .apk, open it, and allow "install unknown apps" for your browser or file manager if prompted.

Prefer to track updates automatically? Obtainium can watch this GitHub repo and notify you of new releases — add herakles-dev/nightjar as a GitHub source.

Requirements

• Android 12+ (minSdk 31), target/compileSdk 35
• A single permission: RECORD_AUDIO (used by the acoustic modem and detector to capture microphone input)
• Android SDK + JDK 17 to build

Build & run

./gradlew assembleDebug   # build debug APK (output: app-debug.apk)
./gradlew test            # run the JVM unit test suite

Debug builds use Android Gradle Plugin's default auto-generated debug keystore — no signing setup required. If you don't already have local.properties pointing at an Android SDK, create one at the repo root with:

sdk.dir=<path-to-your-android-sdk>

Architecture

Kotlin + Jetpack Compose + Room, built with AGP 8.7.3 / Kotlin 2.0.21. Each technique lives behind a shared CovertModule interface, with its transmit/embed logic in a *Carrier class and its detection logic in a matching *Detector/*Steganalysis class:

modules/fireflyjar   Default disguise-themed UI (jar shelf, firefly log, player)
modules/acoustic      Acoustic FSK/PSK modem screen
modules/audiostego     Audio steganography screen (embed/extract)
modules/imagestego    Image LSB steganography screen
modules/detector       Passive acoustic detector screen
picker                Technical module-picker home screen
trail                 Onboarding "riddle trail" walkthrough
share                 Outbound sharing (FileProvider-based)
incoming              Unified incoming-payload pipeline: sniff, route, decode

Root-level classes (AcousticCarrier, AudioStegoCarrier, ImageStegoCarrier, SturdyImageCarrier, AcousticDetector, AudioStegDetector, ImageSteganalysis, Fft, Spectrogram, WavFile, MicCapture, …) implement the signal-processing and steganalysis logic that the module screens share.

Intended use

nightjar is a research and educational demonstration of steganography and covert-channel techniques, and of the detection methods used against them. It is meant for learning, teaching, and experimentation in controlled settings — not for evading lawful monitoring or transmitting data without the consent of everyone involved. Please use it responsibly and lawfully.

Credits & Acknowledgments

nightjar's techniques are directly inspired by, and in some cases adapt, the published work of others — most notably Benn Jordan's acoustic data-hiding projects (Wavest, BUM16, AlphaSteg) and Georgi Gerganov's ggwave, along with foundational steganography and audio-watermarking research. See CREDITS.md for full acknowledgments.

License

MIT — see LICENSE. Third-party components (ggwave, the Silkscreen font) are used under their own licenses; see NOTICE for details.

Contributing

Issues and pull requests are welcome.