Attestation Protocol

The AuroraSOC attestation protocol provides end-to-end firmware integrity verification for all CPS/IoT devices. This document describes the cross-platform protocol implemented across three firmware stacks and verified by the Rust Core Engine.

Protocol Overview

Message Format

Attestation Payload (Device → Broker)

{
  "device_id": "stm32_pac_01",
  "firmware_hash": "a3f2b8c1d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0",
  "boot_count": 42,
  "signature_hex": "304402201a2b3c4d...f0e1d2c3",
  "nonce": "abc123def456",
  "board_family": "stm32",
  "firmware_stack": "ada_spark",
  "firmware_version": "2.0.0"
}

Signed Message Construction

The signed message is a simple concatenation (no delimiters):

message = device_id + firmware_hash + boot_count + nonce

Example:

stm32_pac_01a3f2b8c1d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f042abc123def456

This message is then SHA-256 hashed and signed with ECDSA P-256.

Per-Platform Implementation

STM32 (Ada/SPARK)

Hardware:  PKA (Public Key Accelerator) + HASH peripheral
Key:       Stored in OTP (One-Time Programmable) area
Transport: Ethernet → MQTT (TLS 1.3, mTLS)
Interval:  Every main loop iteration (5s)
Extras:    SPARK-proven relay safety interlock

nRF52840 (Rust Embassy)

Hardware:  CryptoCell CC310
Key:       Stored in UICR (User Information Configuration Registers)
Transport: BLE → MQTT-SN → ESP32-S3 gateway → MQTT broker
Interval:  5 minutes
Extras:    USB honeypot alerts

ESP32-S3 (Zephyr C)

Hardware:  Software ECDSA (no hardware crypto accelerator)
Key:       Stored in NVS (Non-Volatile Storage) partition
Transport: WiFi → MQTT (TLS)
Interval:  5 minutes
Extras:    Edge AI anomaly detection

Key Management

Vault PKI Configuration

# Device certificate role
vault write pki/roles/aurorasoc-devices \
    key_type=ec \
    key_bits=256 \
    allow_any_name=true \
    enforce_hostnames=false \
    max_ttl=8760h \           # 1 year
    client_flag=true \
    server_flag=false

Security Properties

What Attestation Proves

Property	How
Firmware authenticity	SHA-256 hash matches known-good binary
Device identity	ECDSA signature verifiable only with device's key
Freshness	Monotonic boot count prevents replay
Liveness	Regular attestation proves device is running

What Attestation Does NOT Prove

Limitation	Mitigation
Runtime memory integrity	Not covered (would need TrustZone/SGX)
Physical key extraction	OTP/UICR fuses are one-time write
Side-channel attacks	CC310 has countermeasures
Supply chain compromise	Out of scope (separate process)

Boot Count Anti-Replay

The boot count is stored in hardware fuses that can only increase:

• STM32: OTP area (one-time programmable bits)
• nRF52840: UICR registers (erasable only via full chip erase)
• ESP32-S3: eFuse (one-time programmable)

Failure Handling

Failure	System Response
Signature mismatch	Alert: severity=critical, device marked failed
Boot count regression	Alert: severity=critical, possible reflash attack
Certificate expired	Device marked expired, cert renewal required
No response (>10 min)	Device marked offline, Prometheus alert fires
Structural pass only	Device accepted with lower confidence, flagged for key enrollment

Certificate Revocation

When a device is compromised, its certificate can be revoked via the API:

# Revoke device certificate
curl -X POST http://localhost:8000/api/v1/cps/devices/stm32_pac_01/revoke \
  -H "Authorization: Bearer $TOKEN" \
  -d '{"reason": "Compromised - tamper detected"}'

This triggers:

1. Certificate revoked in Vault CRL
2. Device attestation_status set to revoked
3. STM32 relay opens immediately (SPARK-proven safety interlock)
4. Alert generated for SOC analysts
5. NATS federation notifies other sites

Monitoring

Prometheus alert rules monitor attestation health:

- alert: CPSAttestationFailureRate
  expr: rate(aurora_attestation_failures_total[5m]) / rate(aurora_attestation_total[5m]) > 0.1
  for: 5m
  labels:
    severity: critical
  annotations:
    summary: "CPS attestation failure rate exceeds 10%"

- alert: CPSDeviceOffline
  expr: time() - aurora_device_last_seen_timestamp > 300
  for: 5m
  labels:
    severity: warning
  annotations:
    summary: "CPS device hasn't reported in 5+ minutes"