USMP Handshake: Setting Up a Secure Session

The USMP handshake is where the magic starts. It is a mutually authenticated, forward-secret exchange that establishes a secure tunnel in 4 messages. On a standard ESP32, the entire process takes about 200ms to complete.

Here is the high-level choreographic sequence:

Client (ESP32 / Arduino)                        Server (Python Gateway)
      │                                                   │
      │─────────────── 1. PKT_HELLO (0x01) ──────────────▶│
      │           [device_id(6) || pub_C(32)]             │
      │                                                   │
      │◀────────────── 2. PKT_CHALLENGE (0x02) ───────────│
      │             [nonce(32) || pub_S(32)]              │
      │                                                   │
      │         [Both derive session keys locally]        │
      │                                                   │
      │─────────────── 3. PKT_HELLO_ACK (0x03) ──────────▶│
      │                 [hmac_client(32)]                 │
      │                                                   │
      │◀────────────── 4. PKT_SESSION_OK (0x04) ──────────│
      │         [session_id(16) || hmac_server(32)]       │
      │                                                   │
      │═══════════════ SESSION ESTABLISHED ═══════════════│

A Conversational Handshake Dialogue

Let's tell the story of the handshake as a friendly dialogue between the Client and the Server:

1. Client (PKT_HELLO): "Hey there! I want to establish a secure session. Here is my hardware address (device_id) and an ephemeral public key (pub_C) I just generated."
2. Server (PKT_CHALLENGE): "Hi! I see you. To start, here is my own ephemeral public key (pub_S) along with a random value (nonce) we'll use to mix up our keys."
3. Client (PKT_HELLO_ACK): "Awesome. I've computed our shared secret using Curve25519, run it through HKDF, and generated session keys. To prove I know our Pre-Shared Key (PSK), I've signed the nonce and our public keys with an HMAC. Here it is!"
4. Server (PKT_SESSION_OK): "Looks perfect! I've checked your HMAC proof, and it matches. To prove that I also know the PSK (so you know you're not talking to an imposter), here is my own HMAC proof along with a unique session_id to track our connection."

Now both sides are fully authenticated, they share identical symmetric session keys, and they're ready to communicate securely!

Step-by-Step Packet Details

Let's inspect the binary layout and implementation of each handshake step.

Step 1: PKT_HELLO (0x01)

The client initiates the handshake by announcing its physical hardware identity (device_id) and sending its ephemeral Curve25519 public key (pub_C).

Payload Layout (38 bytes)

• Bytes 0..5 (6 bytes): device_id (e.g. WiFi MAC address).
• Bytes 6..37 (32 bytes): pub_C (ephemeral X25519 public key generated fresh for this session).

Implementation

• Client (C): Generates a temporary Curve25519 keypair and writes the packet payload:

  uint8_t pub_buf[65];
  size_t pub_buf_len = 0;
  mbedtls_ecdh_make_public(&ecdh, &pub_buf_len, pub_buf, sizeof(pub_buf), 
                           mbedtls_ctr_drbg_random, &ctr_drbg);
  uint8_t *pub_c = pub_buf + (pub_buf_len - 32);
  
  usmp_packet_t pkt;
  pkt.magic = USMP_MAGIC;
  pkt.version = 1;
  pkt.type = USMP_TYPE_HELLO;
  pkt.seq = 0;
  pkt.length = 38;
  memcpy(pkt.payload, session->device_id, 6);
  memcpy(pkt.payload + 6, pub_c, 32);
  
  // Build and send the packet...
  

• Server (Python): Parses the HELLO frame and matches the client's device_id against the registered Pre-Shared Keys:

  frame = await read_frame(reader, verify_crc=False)
  if frame.type != PacketType.HELLO or frame.length != 38:
      raise HandshakeError("Invalid HELLO frame received")
  
  device_id = frame.payload[:6]
  pub_c = frame.payload[6:38]
  

Step 2: PKT_CHALLENGE (0x02)

The server replies with a random challenge salt (nonce) and its own ephemeral public key (pub_S).

Payload Layout (64 bytes)

• Bytes 0..31 (32 bytes): nonce (cryptographically secure random salt).
• Bytes 32..63 (32 bytes): pub_S (server's ephemeral X25519 public key).

Implementation

• Server (Python): Generates a server X25519 keypair and random nonce:

  priv_s, pub_s = generate_keypair()
  nonce = os.urandom(32)
  await write_frame(writer, PacketType.CHALLENGE, nonce + pub_s)
  

• Client (C): Extracts the nonce and server public key from the received packet:

  uint8_t nonce[32];
  uint8_t pub_s[32];
  memcpy(nonce, pkt.payload, 32);
  memcpy(pub_s, pkt.payload + 32, 32);
  

Key Derivation (ECDH & HKDF)

Before proceeding, both sides combine their private keys with the other side's public key to compute a Curve25519 shared secret. They run this secret through HKDF-SHA256:

$\text{shared\_secret} = \text{X25519}(\text{priv\_local}, \text{pub\_peer})$

$\text{session\_key} = \text{HKDF-SHA256}(\text{ikm}=\text{shared\_secret}, \text{salt}=\text{nonce}, \text{info}=\text{"usmp-v1"} \parallel \text{pub\_C} \parallel \text{pub\_S}, \text{len}=32)$

Step 3: PKT_HELLO_ACK (0x03)

The client authenticates itself by sending an HMAC proof. To prevent Man-in-the-Middle (MITM) key-swapping, the HMAC is computed over the challenge nonce, device_id, and both public keys (pub_C and pub_S).

Payload Layout (32 bytes)

• Bytes 0..31 (32 bytes): hmac_client = $\text{HMAC-SHA256}(\text{PSK}, \text{nonce} \parallel \text{device\_id} \parallel \text{pub\_C} \parallel \text{pub\_S})$

Implementation

• Client (C): Concatenates parameters and computes the client HMAC:

  uint8_t hmac_client[32];
  uint8_t input[32 + 6 + 32 + 32];
  memcpy(input, nonce, 32);
  memcpy(input + 32, session->device_id, 6);
  memcpy(input + 32 + 6, pub_c, 32);
  memcpy(input + 32 + 6 + 32, pub_s, 32);
  
  compute_hmac(psk, psk_len, input, sizeof(input), hmac_client);
  

• Server (Python): Computes the expected HMAC locally and performs a constant-time comparison to verify:

  expected_client = _compute_hmac(resolved_psk, nonce, device_id, pub_c, pub_s)
  if not hmac.compare_digest(expected_client, frame.payload):
      raise AuthError("Client HMAC verification failed")
  

Step 4: PKT_SESSION_OK (0x04)

The server authenticates itself by sending back a unique session_id and a server HMAC proof computed over the nonce, session_id, and both public keys.

Payload Layout (48 bytes)

• Bytes 0..15 (16 bytes): session_id (cryptographically random session identifier).
• Bytes 16..47 (32 bytes): hmac_server = $\text{HMAC-SHA256}(\text{PSK}, \text{nonce} \parallel \text{session\_id} \parallel \text{pub\_C} \parallel \text{pub\_S})$

Implementation

• Server (Python): Generates a session ID, computes the HMAC proof, and transmits:

  session_id = os.urandom(16)
  hmac_server = _compute_hmac(resolved_psk, nonce, session_id, pub_c, pub_s)
  await write_frame(writer, PacketType.SESSION_OK, session_id + hmac_server)
  

• Client (C): Computes the expected server HMAC and performs a constant-time memory comparison (mbedtls_ct_memcmp) to verify:

  uint8_t session_id[16];
  uint8_t hmac_server_received[32];
  // ... unpack payload
  
  uint8_t hmac_server_expected[32];
  uint8_t input[32 + 16 + 32 + 32];
  // input = nonce(32) || session_id(16) || pub_c(32) || pub_s(32)
  memcpy(input, nonce, 32);
  memcpy(input + 32, session_id, 16);
  memcpy(input + 32 + 16, pub_c, 32);
  memcpy(input + 32 + 16 + 32, pub_s, 32);
  compute_hmac(psk, psk_len, input, sizeof(input), hmac_server_expected);
  
  if (mbedtls_ct_memcmp(hmac_server_received, hmac_server_expected, 32) != 0) {
      USMP_LOGE(TAG, "Server HMAC verification FAILED — possible rogue server");
      // Close transport and exit...
  }