Executive summary — Milcodec Receiver is a zero-install, browser-native receiver that demodulates and decrypts MILCODEC messages carried over sound. It turns any phone or laptop into an acoustic intercept terminal: a near-ultrasonic chirp-spread-spectrum signal arriving at the device microphone is matched-filtered, error-corrected, authenticated, and surfaced as a prioritized message inbox — all client-side, behind a covert FM-radio interface. Built by the Defense Intelligence division of Monarch Castle Technologies for short-range, infrastructure-independent communications where RF is unavailable, monitored, or undesirable.
- Data-over-sound reception in the 14–17 kHz near-ultrasonic band — no radios, no pairing, no network. The air gap is the channel.
- Chirp Spread Spectrum ("The Dolphin") modulation — binary chirp keying (up-chirp = 1, down-chirp = 0) at 20 baud, decoded with a matched-filter correlation receiver for robustness against ambient noise.
- Authenticated encryption via TweetNaCl
secretbox(XSalsa20-Poly1305): every payload is decrypted and integrity-checked before it ever reaches the operator's inbox. - Forward error correction — 3× repetition coding with majority-vote bit recovery, plus a preamble + 16-bit sync word + length-prefixed framing protocol for reliable packet recovery.
- Military message precedence — decoded traffic is tagged
FLASH/IMMEDIATE/ROUTINE, with audible alerting on high-priority receipt. - Covert operating mode — the live UI presents as an ordinary FM tuner; the receiver console unlocks only on a secret gesture and passcode.
- Real-time spectral visualizer — Web Audio FFT spectrum and signal-strength indicator for live link assessment.
Milcodec Receiver is the receive half of an acoustic covert channel. A companion sender plays an encoded waveform through a speaker; this application listens through the microphone, recovers the bitstream, decrypts it, and displays the message.
End-to-end signal flow:
PC Sender → Speaker → Air (acoustic) → Device Mic → Milcodec Receiver → Decode → Decrypt → Inbox
The decoder generates linear up- and down-chirp templates (14 kHz ⇄ 17 kHz over a 50 ms symbol) and slides them across the captured audio as a correlation receiver. It locates the U-U-D-D preamble by spaced peak detection, locks to a 16-bit sync word, reads a 16-bit payload-length field, then recovers each payload bit by majority vote across three transmitted copies before reassembling bytes.
Recovered payloads are NaCl secretbox frames (24-byte nonce + ciphertext). On successful open, the inner packet is parsed as 1-byte type + 64-byte signature field + JSON body, yielding the message text and its precedence. Failed opens are rejected and never displayed.
The console runs entirely on the Web Audio API — getUserMedia capture, an AnalyserNode spectrum, and a ScriptProcessor that batches ~2–3 seconds of audio per decode pass. In covert mode the interface mimics an FM radio (tuner dial, scan, power, stereo indicator); triple-clicking the stereo indicator opens a passcode gate that reveals the receiver, its inbox, and the live decode log.
This is an analytical tool, not a data feed — but Monarch Castle doctrine still governs every message it surfaces: evidence before assertion.
- Input: live acoustic audio from the device microphone (mono, 44.1 kHz), captured only after explicit user permission.
- Output: decoded messages, each stamped at the point of receipt with:
- collected_at — local reception timestamp on the receiving device,
- priority — military precedence (
FLASH/IMMEDIATE/ROUTINE), - method —
MILCODEC CSS · 14–17 kHz · secretbox(XSalsa20-Poly1305), - verification — authentication status of the sender / packet.
- Auditability: a live debug log records preamble locks, extracted bit counts, payload lengths, and decrypt outcomes so every displayed message is traceable from raw audio to plaintext.
- Language: vanilla JavaScript (no framework, no build step) · HTML5 · CSS3
- DSP / capture: Web Audio API —
AudioContext,AnalyserNode,ScriptProcessorNode,getUserMedia; HTML5<canvas>spectrum rendering - Cryptography: TweetNaCl (
nacl-fast) +tweetnacl-util, loaded via jsDelivr CDN - Modulation: custom Chirp Spread Spectrum codec ("The Dolphin") with repetition-code FEC
- Hosting: GitHub Pages (static, HTTPS — required for microphone access);
.nojekyllfor raw asset serving - Test harnesses:
test_crypto.html,test_file.html— in-browser round-trip checks for the crypto and end-to-end paths
Use the live build (recommended):
- Open https://monarchcastletech.github.io/milcodec-receiver/ on a modern browser (Chrome, Safari, or Firefox) — HTTPS is required for microphone access.
Operate the receiver:
- The interface opens as an FM radio tuner (cover mode).
- Triple-click the
STEREOindicator to reveal the passcode gate. - Enter the demo passcode
DELTAto unlock the receiver console. - Press POWER, then grant microphone permission when prompted.
- Decoded transmissions appear in the INBOX, tagged by precedence; high-priority traffic triggers an audible alert.
Run locally:
git clone https://github.com/monarchcastletech/milcodec-receiver.git
cd milcodec-receiver
python -m http.server 8080
# open http://localhost:8080Deploy your own: push the repository contents to a GitHub repo, enable Settings → Pages → Deploy from branch (main), and the receiver is served over HTTPS in about a minute.
Operational note. This build ships with a hardcoded demo key and passcode for evaluation only. For any operational use, replace the symmetric key with a proper key-exchange / key-management scheme and remove the static passcode. The cover interface and default credentials are demonstration defaults, not a security control.
A product of Defense Intelligence · Monarch Castle Technologies — an operating company of Monarch Castle Holdings. Sister companies: Monarch Castle Technologies · Strategic Data Company of Ankara
See LICENSE. © 2026 Monarch Castle Holdings · Ankara, Türkiye.