sovereign-sensor 0.1.0

Bare-metal Write-Side Custody enforcement for MicroPython sensor nodes with hardware crypto abstraction

sovereign-sensor — Phase 9

A lightweight, MicroPython-compatible Hardware Abstraction Layer (HAL) that enforces data custody at the Point of Genesis by sealing each sensor observation into a versioned, tamper-evident, replay-protected JSON transmission envelope before any network transit or cloud ingestion occurs.

Zero external dependencies. Internal library code is restricted to standard MicroPython built-ins (json, sys, hashlib, hmac, binascii). Targets ESP32 and Raspberry Pi Pico via MicroPython; fully exercisable on CPython 3.12 for desktop CI.

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Architecture

Hardware Abstraction Layer

SovereignCryptoDriver (interface.py) defines a three-method contract:

Method	Contract
initialize_hardware() -> None	Load key material; configure accelerator subsystem.
sign(payload: bytes) -> bytes	Return raw binary signature bytes (no encoding).
algorithm() -> str	Return a canonical algorithm identifier string.

Two concrete drivers are provided:

• SoftwareFallbackDriver — HMAC-SHA256 over a VFS-resident binary key file. Used on all non-ESP32 targets (desktop CI, Raspberry Pi Pico, etc.). The class-level MOCK_KEY_SENTINEL = "/mock/test_gateway.key" opts into a deterministic stub key for desktop testing; every other path that cannot be opened raises RuntimeError immediately, eliminating silent key substitution. A zero-byte key file raises ValueError to prevent HMAC keyed with b"".

• ESP32HardwareDriver — Skeleton placeholder for the on-chip ECC accelerator. initialize_hardware() raises NotImplementedError until register-level engineering is complete; bootstrap_sensor_node() catches this and falls back to SoftwareFallbackDriver automatically so the sealing and VFS layers remain exercisable on the workbench.

Seven-Step Sealing Pipeline (SovereignEnvelope.seal())

1. Monotonic sequence counter — computed transiently as _sequence + 1 and bound into the preimage. The in-memory counter and VFS file (default .sovereign_sequence) are advanced only after signing succeeds, so a sign() failure never consumes a sequence position or introduces a gap in the on-disk custody timeline. A truncated (0-byte) file resets to 0; a negative stored value is clamped to 0. VFS write failures degrade gracefully to RAM-only tracking.

2. Algorithm identifier — queried from the active driver via algorithm() and embedded in the authenticated preimage, providing protocol agility without a schema change.

3. Canonical payload serialization — json.dumps(payload, separators=(",", ":"), sort_keys=True, ensure_ascii=False) guarantees a byte-identical preimage for semantically equivalent payloads regardless of key insertion order. ensure_ascii=False forces raw UTF-8 output for all characters, eliminating the \uXXXX-vs-raw-UTF-8 split-brain divergence that would cause cross-platform HMAC verification to fail silently on any payload containing characters outside U+007F.

4. UTF-8 byte-count-prefixed preimage assembly — node_id, timestamp, and the algorithm identifier are each encoded to UTF-8 independently; the prefix for each field is the UTF-8 byte count (not the Unicode character count). The preimage is assembled from raw byte slices:

   1|{len(node_bytes)}:{node_id}|{len(time_bytes)}:{timestamp}|{seq}|{len(algo_bytes)}:{algo}|{canonical}
   

   Byte-count prefixes close all variable-length field injection surfaces: delimiter injection (any two inputs that differ only in where a | character falls produce identical naive pipe-joined preimage bytes without prefixes) and multi-byte encoding ambiguity (a receiver using character-count semantics parses field boundaries at the wrong byte offset for any non-ASCII field value).

5. Driver signing — raw preimage bytes traverse driver.sign(), returning raw binary output from the underlying cryptographic primitive.

6. Hex encoding — binascii.hexlify() maps all byte values 0x00–0xFF to the lowercase alphanumeric characters 0–9, a–f, preventing UnicodeDecodeError on constrained MicroPython silicon.

7. Wire frame serialization — all seven envelope fields (v, n, t, q, alg, d, s) are packed into a dict and serialized with json.dumps(..., separators=(",", ":"), sort_keys=True, ensure_ascii=False), freezing the alphabetical key sequence and enforcing raw UTF-8 wire encoding independently of MicroPython allocator-driven insertion order.

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Quick Start

from sovereign_sensor import bootstrap_sensor_node

# Auto-selects ESP32HardwareDriver or SoftwareFallbackDriver at runtime.
# Falls back to SoftwareFallbackDriver with a warning if hardware crypto
# is not yet implemented on the target.
envelope = bootstrap_sensor_node(
    node_id="node-temperature-01",
    private_key_path="/flash/keys/node.key",
    sequence_file="/flash/.sovereign_sequence",
)

observation = {"sensor": "temperature", "value": 21.4, "unit": "C"}
wire_bytes = envelope.seal("2026-06-16T12:00:00Z", observation)
# → b'{"alg":"hmac-sha256","d":{"sensor":"temperature","unit":"C","value":21.4},'
#     '"n":"node-temperature-01","q":1,"s":"<64-char hex>","t":"2026-06-16T12:00:00Z","v":1}'

Desktop / CI (mock key sentinel)

from sovereign_sensor import bootstrap_sensor_node
from sovereign_sensor.drivers.software_fallback import SoftwareFallbackDriver

envelope = bootstrap_sensor_node(
    node_id="ci-node-001",
    private_key_path=SoftwareFallbackDriver.MOCK_KEY_SENTINEL,
)
wire = envelope.seal("2026-06-16T00:00:00Z", {"ping": True})

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Invariants

Property	Guarantee
Replay protection	Monotonic q counter persisted to VFS; resumes across reboots.
Sequence atomicity	Counter advanced only after sign() succeeds; a signing failure leaves the on-disk counter unchanged with no gap.
Key material safety	Missing or empty key file raises immediately; no silent substitution.
Preimage determinism	sort_keys=True and ensure_ascii=False on payload; byte-count length prefixes on all three variable-length fields.
Wire frame determinism	sort_keys=True and ensure_ascii=False on the outer frame; byte-identical UTF-8 output across CPython and MicroPython builds.
Encoding safety	binascii.hexlify prevents UnicodeDecodeError on raw binary digest bytes.
Zero dependencies	No network calls, no PyTorch, no external packages at runtime.