Crypt-Chat

End-to-end encrypted chat over WiFi, running on a bare-metal RISC-V microcontroller with no OS and no standard library.

A DTEK-V board communicates with a Python client over a bit-banged software UART through an ESP8266 WiFi bridge. All messages are authenticated and encrypted using ChaCha20-Poly1305 AEAD, implemented from scratch in C.

┌──────────────┐   bit-banged    ┌──────────┐   WiFi/TCP    ┌──────────────┐
│   DTEK-V     │───── UART ────▶│ ESP8266  │◀───────────▶│ Python       │
│   (RV32IM)   │   9600 baud     │ (bridge) │   telnet:23   │ Client       │
│              │                 │          │               │              │
│ ChaCha20     │                 │          │               │ Same C code  │
│ Poly1305     │                 │          │               │ via ctypes   │
│ CRC16+frame  │                 │          │               │              │
└──────────────┘                 └──────────┘               └──────────────┘

Features

• Software UART — TX via busy-wait, RX via timer ISR with 8x oversampling and lock-free ring buffer
• ChaCha20-Poly1305 AEAD — RFC 8439 authenticated encryption, implemented without any library dependencies
• Framed protocol — sync bytes, CRC16-CCITT integrity checks, byte stuffing, and timeout recovery
• Shared C crypto library — the Python client calls the exact same C crypto and protocol code via ctypes, ensuring no implementation drift

Building

Requires a riscv32-unknown-elf toolchain, Meson, and Ninja.

# Configure (only needed once)
meson setup build --cross-file cross/riscv32.ini

# Build firmware + host shared library
ninja -C build

This produces:

• build/crypt-chat.bin — firmware binary for the DTEK-V board
• build/libchatcrypto.so — host shared library used by the Python client

To upload firmware to the board:

ninja -C build upload

Running the Python Client

python3 chat_client_protocol.py <ESP8266_IP> [port]

Options:

• --user-id 0x02 — client user ID (default)
• --user-id 0x00 — act as board (send plaintext, let the board encrypt)
• --key <hex> — override the 32-byte shared key (64 hex characters)

Protocol

Wire format:  [0xAA] [0xAA] [LEN] [STUFFED_PAYLOAD]

After unstuffing:  [USER_ID] [MESSAGE...] [CRC_HI] [CRC_LO]

• Sync: two 0xAA bytes mark frame start
• Byte stuffing: 0xAA → 0xAB 0x00, 0xAB → 0xAB 0x01
• CRC16-CCITT over USER_ID + MESSAGE (poly 0x1021, init 0xFFFF)
• Encryption: message payload is nonce(12) + ciphertext(n) + tag(16)
• Timeout: partial frames are dropped after ~1 second of inactivity

See PROTOCOL.md for details.

Source Layout

Firmware (bare-metal C, no libc):

File	Purpose
src/boot.S	Boot, trap vector, ISR dispatch
src/dtekv-lib.c	JTAG UART I/O, exception/interrupt handlers
src/devices.c	LEDs, switches, buttons, 7-segment display, GPIO
src/uart.c	Bit-banged UART: TX busy-wait, RX 8x oversampled ISR, ring buffer
src/chat_protocol.c	CRC16-CCITT, byte stuffing/unstuffing
src/chat_framer.c	Frame state machine with timeout recovery
src/chacha20.c	ChaCha20 stream cipher (RFC 8439)
src/poly1305.c	Poly1305 MAC
src/aead.c	ChaCha20-Poly1305 AEAD encrypt/decrypt
src/main.c	Application: receive frames, encrypt/decrypt, relay

Host (shared library + Python client):

File	Purpose
build/libchatcrypto.so	Crypto + protocol C code compiled for the host
chat_client_protocol.py	Python client using ctypes to call the shared library