PROTOCOL.md

Protocol

Frame Format

Wire:       [0xAA] [0xAA] [LEN] [STUFFED_DATA]

Unstuffed:  [USER_ID] [MESSAGE...] [CRC_HI] [CRC_LO]

• LEN — length of stuffed data (1-256)
• CRC16-CCITT — polynomial 0x1021, initial 0xFFFF, computed over USER_ID + MESSAGE

Byte Stuffing

Escape byte 0xAB is used to avoid sync byte collisions in the payload:

Raw byte	Encoded as
0xAA	0xAB 0x00
0xAB	0xAB 0x01

Encryption

Message payloads are encrypted with ChaCha20-Poly1305 AEAD (RFC 8439):

MESSAGE = [NONCE (12 bytes)] [CIPHERTEXT (n bytes)] [TAG (16 bytes)]

• Nonce is generated per message (counter-based on board, random on client)
• Tag authenticates the ciphertext; messages with invalid tags are dropped

RX Flow

1. Detect two consecutive 0xAA sync bytes
2. Read length byte
3. Collect LEN bytes of stuffed data
4. Unstuff, verify CRC, deliver message
5. Timeout: drop partial frame after ~1 second of inactivity

TX Flow

1. Prepend USER_ID to message
2. Compute CRC16 over USER_ID + MESSAGE
3. Append CRC (big-endian)
4. Byte-stuff the result
5. Send [0xAA] [0xAA] [LEN] [STUFFED_DATA]

User IDs

ID	Role
0x00	Board
0x02	Client

The client filters frames containing its own user ID to suppress echo.