sigil (1.1.0)

Published 2026-08-23 13:15:28 +00:00 by faicel

Setup this registry in the Cargo configuration file (for example ~/.cargo/config.toml):

[registries.forgejo]
index = "sparse+" # Sparse index
# index = "" # Git

[net]
git-fetch-with-cli = true

To install the package using Cargo, run the following command:

cargo add sigil@1.1.0 --registry forgejo

For more information on the Cargo registry, see the documentation.

no_std secure session engine for embedded sensor/gateway communication

sigil

no_std Rust library for secure cryptographic communication over LoRa networks on embedded IoT devices. It provides device identification, authentication, and encrypted messaging primitives for LoRa-based sensor/gateway communication.

What is a Sigil?

Sigil (from Latin sigillum, “seal”) traditionally denotes a symbol or mark that represents an identity, intent, or authority—like a seal on a document. In this project, sigil is the “seal” of your device stack: the layer that defines how devices identify, authenticate, and exchange encrypted messages over LoRa.

Architecture

sigil is a radio-non-blocking, hardware-agnostic session engine. The library never waits for radio traffic or sleeps; the host firmware owns I/O and timing. Handshake calls use synchronous host SecureElement methods and occupy the calling task for the duration of those hardware operations.

Every frame carries an outer header [sender_mac:17] [protocol_version:1 = 0x03] [frame_type:1] where sender_mac is the transmitting device's durable identity (SessionManager::local_mac). The protocol_version byte at position 17 and FrameType at position 18 let the core reject foreign protocols and classify incoming frames without decryption, returning a typed SigilEvent:

FrameType::Data → fully decrypted, returned as SigilEvent::Data
FrameType::TimeSync → authenticated gateway clock sample, returned as SigilEvent::TimeSync; it shares the session AEAD replay counter
FrameType::TimeSyncRequest → authenticated sensor request for a fresh clock sample; the gateway host decides whether to answer with TimeSync
FrameType::Event / EventAck → reliable sensor event with an authenticated, replay-protected application ACK generated only after durable gateway processing
Handshake frames → per-step events, e.g. InstallChallenge ≠ InstallRequest, paired with SigilAction::NeedsHostDispatch (not Success):
- InstallRequest / InstallChallenge / InstallConfirm / InstallCommitConfirmed
- RenewRequest / RenewChallenge / RenewConfirm / RenewCommitConfirmed
- ResumeRequest / ResumeChallenge / ResumeConfirm / ResumeCommitConfirmed

The host routes *Request to handle_* and challenge/confirm/ack/CommitConfirmed to continue_*.

Features

Role-scoped builds — sensor is the safe, smallest default. Gateway dependencies must use default-features = false, features = ["gateway"]; pairwise development uses --all-features.
Non-blocking core — Host-owned radio + secure element via the SecureElement / RandomSource traits. Provides SessionManager<N>, a single ingress (process_incoming_packet), egress (encrypt_data), and three host-driven phases: install, sensor-initiated renew, and fast resume.
N must be a power of two and >= 2 (constraint from heapless::FnvIndexMap); use <2> for single-peer sensors, <128> for gateways.
Host-owned footprint budgets — final flash, static RAM, and stack budgets are measured and enforced by the host firmware on its own final linked image. The crate guards its dominant fixed-capacity types with internal type-size regression bounds (see the crate-level rustdoc); those recorded sizes are planning inputs for host budgeting, not a substitute for target-level measurement.

Handshake flows

Install (first pairing)

Clear-text identity exchange plus AEAD CommitConfirmed, with static ECDH + ECDSA and mutual factory attestation (same manufacturing root on both devices):

InstallRequest → InstallChallenge → InstallConfirm (Final) → InstallCommitConfirmed
→ factory_cert verifies peer pubkey under SessionManager factory root
→ DirectionalMaterial = HKDF-Expand(ECDH(static↔static), …)  // s2g ‖ g2s
→ Sensor stages keys until CommitConfirmed; gateway commits on Final

Construction: SessionManager::new(factory_pubkey, local_mac)? — host loads the factory root and this device's MAC once at boot (ATECC/NVRAM).

Gateway discovery is mandatory for first pairing: the sensor does not know the gateway MAC a priori.

Sensor: start_install(INSTALL_DISCOVERY_DEST) (or host broadcast address) — outer header = sensor local_mac; host must TX as discovery/broadcast. continue_install learns the real gateway MAC from InstallChallenge and rebinds the pending entry. Gateway host: on SigilEvent::InstallRequest (not Challenge/Confirm), check pairing DB for peer (sender / sensor MAC from the outer header), then handle_install_request(..., now, key_lifetime_secs) — cheap DoS guards (pending / cooldown 15 s / capacity) run first. After the factory certificate and dynamic device signature authenticate the sensor, the cooldown is stamped before ECDH and challenge construction. Sensor hosts route InstallChallenge / InstallConfirm / InstallCommitConfirmed via continue_install. On gateway Final, PersistPeerStaticKey.transmit carries CommitConfirmed (persist then TX). Host allowlist remains policy for who may pair; the library rate-limits SE amplification. Host supplies an authenticated key lifetime in seconds.

Renew (session key rotation, sensor-initiated)

Request, Challenge and Final (RenewConfirm) are encrypted under the old session keys. CommitConfirmed uses the new gateway→sensor key. Renew uses ephemeral P-256 ECDH with a sensor private key confined to ATECC TempKey. Only SessionRole::Sensor may call start_renew:

RenewRequest(empty) → RenewChallenge(gateway_pub, lifetime)
→ RenewConfirm(sensor_pub, proof) → RenewCommitConfirmed
→ DirectionalMaterial_new = HKDF-Expand(ECDH(P-256 eph↔eph), transcript-v4)
→ Sensor keeps old key until CommitConfirmed verifies

Sensor: start_renew(peer, now), then continue_renew(frame, secure_element, now). The ATECC operation must chain GenKey TempKey and ECDH atomically, with no EEPROM write in between. Gateway: handle_renew_request(frame, secure_element, random, ...), then handle_renew_confirm(frame, secure_element, now). The host implements the gateway P-256 methods of SecureElement with the backend of its choice (RustCrypto, OpenSSL, TPM, HSM...) and supplies its own RandomSource; there is no gateway initiation API.

Session resume (fast re-pair after reboot)

Request carries a MAC keyed by the durable resume_psk (derived at install); Challenge is clear text with a MAC over both nonces and the lifetime; Final (ResumeConfirm) and CommitConfirmed are AEAD under the new keys. Crypto uses static↔static ECDH only (no ECDSA, no ephemeral keys). Only SessionRole::Sensor may call start_resume:

ResumeRequest (nonce ‖ req_mac) → ResumeChallenge → ResumeConfirm (Final) → ResumeCommitConfirmed
→ DirectionalMaterial = HKDF-Expand(ECDH(static↔static), …)  // s2g ‖ g2s
→ Sensor stages keys until CommitConfirmed verifies

Gateway verifies req_mac before SE-ECDH. Host must persist resume_psk with the peer static pubkey after install and reload via Session::new_enrolled(peer, peer_pubkey, resume_psk, SessionRole)? after reboot.

Directional AEAD: each peer holds independent TX/RX key material (SessionRole::{Sensor,Gateway} maps shared directional halves). Both TX counters may start at 0 without nonce reuse across directions.

NVRAM: persist enrollment only (peer MAC, static pubkey, resume_psk, role). Never persist AEAD keys / nonce prefixes / counters — Session::new_active is not part of the host API.

Sensor: start_resume(..., now) — MACs the request with resume_psk; an old active/expired key is retained until commit so a gateway reboot is recoverable; ECDH at challenge Gateway: handle_resume_request(..., now, key_lifetime_secs) — verify req MAC and gateway role → pending/cooldown → static↔static ECDH; retains any old key until Final, then emits CommitConfirmed

See diagramme/install.mermaid, diagramme/renew.mermaid, and diagramme/session_resume.mermaid.

Host timebase (seconds)

Product contract for all hosts (sensor SAMD21 and gateway):

Value	Unit	Role
`now`	seconds	Local monotonic time passed into pipeline / handshake APIs
`key_lifetime_secs`	seconds	Authenticated duration; each peer computes `local_now + lifetime`
`RESUME_REQUEST_COOLDOWN`	15 seconds	Min gap between accepted resume ECDH per peer
`INSTALL_REQUEST_COOLDOWN`	15 seconds	Min gap between accepted install SE work per peer
`MAX_SESSION_LIFETIME_SECS`	2 days (172 800 s)	Maximum accepted wire lifetime
`*_TIMEOUT_MS` / `wait_timeout_ms`	milliseconds	Advisory radio waits only — never mixed with `now`

Sensor and gateway may use different clock epochs. Do not feed millisecond counters into now while leaving cooldown constants at 15. Hosts must pass 0 < key_lifetime_secs <= MAX_SESSION_LIFETIME_SECS (for example 86_400).

The sensor may call request_time_sync(peer, now). The gateway receives SigilEvent::TimeSyncRequest and may answer with encrypt_time_sync(peer, gateway_time, sequence, uncertainty_secs, now). Both messages are AEAD and replay-protected. Requests are limited to one accepted request per peer every TIME_SYNC_REQUEST_COOLDOWN_SECS (15 seconds); the gateway host still owns the policy deciding whether to transmit a response. Only SessionRole::Sensor can request and only SessionRole::Gateway can emit the clock sample. The sensor host decides how to discipline its RTC.

Reliable events and authenticated ACK

Use send_event(peer, event_id, payload, now) for sensor events that must not be silently lost. This is separate from encrypt_data, which remains fire-and-forget.

Sensor                         Gateway host
send_event(id, payload)  ───▶  Event { id, payload, duplicate: false }
                               persist/deduplicate + perform durable work
                         ◀───  ack_event(id) → authenticated EventAck
process EventAck → clear pending

Security and reliability contract:

Only a sensor-role session can send Event; only a gateway-role session can create its EventAck. Both frames use ChaCha20-Poly1305, bind the outer header/type as AAD, and share the session replay counters.
The gateway must not call ack_event before durable processing succeeds. A construction failure does not commit the receive state. A retry arriving while processing is still pending is rejected with EventDeliveryPending. If durable processing fails, call release_event_delivery; it emits no ACK and permits the same id to be delivered again on the next sensor retry.
If the ACK is lost, the sensor calls send_event again after EVENT_ACK_TIMEOUT_MS with the exact same id and semantic payload. Sigil re-encrypts it with a fresh AEAD counter; never replay cached wire bytes.
A committed duplicate is returned with duplicate: true: do not repeat its side effect, but call ack_event again to recover from ACK loss.
One outbound Event and one uncommitted inbound Event are allowed per session. A wrong authenticated ACK cannot clear a different pending id.
The sensor host must durably allocate strictly increasing u64 ids across reboot. The gateway application/DB must also deduplicate (peer, event_id) durably across reboot; Sigil's session-level cache is RAM-only.
After the host retry budget is exhausted, cancel_pending_event abandons the event but its id stays consumed. tick() manages handshake state only; it does not retry or cancel application Events.
With process_incoming_packet::<DATA, MSG>, the maximum Event application payload is DATA - 8 bytes. An EventAck is 55 bytes on wire.

u32 wrap: unix-epoch seconds wrap ~2106; a monotonic counter from 0 wraps after ~136 years of uptime. Out of product horizon — no u64 migration in v1.

Host global rate-limit

Library DoS guards are per-peer (INSTALL_REQUEST_COOLDOWN / RESUME_REQUEST_COOLDOWN / PendingAlreadyExists). The pending map is a separate table of capacity N (same N as sessions): a flood of distinct peers can fill every pending slot and block further handshakes while existing sessions still work.

Gateway firmware must enforce a global admission policy before calling handle_install_request / handle_resume_request / renew handlers (after the install allowlist), e.g.:

max concurrent pending handshakes across all peers; and/or
a global token bucket (new handshake starts per minute).

On pending/admission saturation (PendingTableFull / AdmissionTableFull), drop or defer RF work and cancel_pending stale peers when host radio timers expire. Sigil does not provide a global rate limiter.

Post-commit recovery entries are intentionally not removed by tick, because silently discarding staged keys or a cached CommitConfirmed can split the two peers across key epochs. Hosts must nevertheless bound recovery operationally: retry retransmit_pending_cached, observe pending_len / commit_recovery_peers, then call abandon_commit_recovery after the product retry budget is exhausted. That explicit terminal operation preserves the current session/enrollment while zeroizing and freeing the recovery slot.

Sensor install vs door / vibration

Install chains multiple secure-element ops in one call. Product priority: door-open / vibration IRQs > install. Keep those IRQs enabled during SE I/O; defer install if a critical sensing window needs the main loop. See CLAUDE.md and SecureElement rustdoc.

Security and threat model

Formally accepted limitation — Install/Resume epochs have no forward secrecy. Install and Resume session epochs are derived from static↔static ECDH. If a device's static private key is extracted after traffic recording, those recorded epochs become decryptable. This limitation is formally accepted by the crate owner (2026-08-22).

Context:

Renew is forward-secret. Renew derives each epoch from ephemeral P-256 ECDH (TempKey on the sensor side), so recorded renew epochs stay confidential even against a later static-key compromise.
Secure elements raise the attack cost. ATECC608-class secure elements keep static private keys non-exportable, which pushes extraction into the physical/side-channel attack class rather than remote or firmware-level compromise.
Re-install freshness does not restore forward secrecy. Each install mixes fresh nonces into the HKDF (new keys per pairing), but the underlying static↔static ECDH input is unchanged — epochs recorded before a static-key extraction remain decryptable afterwards.

Integrator guidance:

Do not rely on Install/Resume for long-epoch confidentiality.
Keep key_lifetime_secs well under MAX_SESSION_LIFETIME_SECS (2 days) and drive Renew regularly so most live epochs are forward-secret.
Prefer Renew rotation over re-install/resume cycles to refresh ongoing sessions.

Usage

Choose the product role explicitly in its Cargo.toml. This is important on the SAMD21: default-features = false prevents the gateway implementation and gateway-only API/events/state from being compiled into the sensor firmware.

[dependencies]
# SAMD21 sensor firmware
sigil = { version = "1.0.1", registry = "forgejo", default-features = false, features = ["sensor"] }

# Raspberry Pi (or equivalent) gateway software
# sigil = { version = "1.0.1", registry = "forgejo", default-features = false, features = ["gateway"] }

The default feature is sensor, the safe smallest build for sensor firmware. Supported configurations are wire (manufacturing contract), sensor, gateway, and the explicit two-role composition sensor,gateway; pairwise development uses --all-features. A build without any supported feature set (no role and no wire) is rejected at compile time with an explicit diagnostic:

sigil: no supported feature configuration selected; enable `wire` (manufacturing contract) or one of the session roles `sensor` / `gateway`

SigilEvent intentionally keeps the complete wire-classification enum in both role builds so shared host adapters can remain exhaustive. Role-disabled frames are rejected by the ingress pipeline and their handlers/API/state are not compiled into the product.

Main loop sketch:

use sigil::core::{SessionManager, SigilEvent, SigilError};

// Host loads factory root + local MAC once at boot (from ATECC / NVRAM).
let factory_pubkey = [0x02u8; 33];
let local_mac = [0x01u8; 17];
let mut mgr: SessionManager<2> = SessionManager::new(factory_pubkey, local_mac)?;

loop {
    // Radio RX (host-owned)
    if let Some(raw) = radio.try_recv() {
        match mgr.process_incoming_packet::<200, 256>(&raw, now) {
            // DATA=200 = max decrypted app payload; MSG=256 fits InstallChallenge (219).
 // Handshake frames use MSG, not DATA .

            Ok((SigilEvent::Data { peer, payload }, _)) => {
                // handle decrypted application data
            }
            Ok((SigilEvent::TimeSync { gateway_time, sync_sequence,
                                      uncertainty_secs, .. }, _)) => {
                // Sensor host applies its RTC discipline / anti-rollback policy.
            }
            Ok((SigilEvent::TimeSyncRequest { peer }, _)) => {
                // Gateway host applies reply policy/rate limits, then sends TimeSync.
                if let Ok(frame) = mgr.encrypt_time_sync::<256>(
                    &peer, gateway_time, next_sync_sequence(), uncertainty, now)
                {
                    radio.send(peer, &frame);
                }
            }
            Ok((SigilEvent::Event { peer, event_id, payload, duplicate }, _)) => {
                // Gateway: transactionally dedupe/persist before ACK.
                if duplicate || host_db_commit_event(peer, event_id, &payload) {
                    if let Ok(SigilAction::Transmit { payload: ack, .. }) =
                        mgr.ack_event::<256>(&peer, event_id, now)
                    {
                        radio.send(peer, &ack);
                    }
                }
            }
            Ok((SigilEvent::EventAck { event_id, .. }, _)) => {
                // Sensor: matching pending Event was authenticated and cleared.
                mark_event_delivered(event_id);
            }
            Ok((SigilEvent::InstallRequest { peer, frame }, _)) => {
                // Host policy: allowlist + **global** handshake admission .
                // Library cooldowns are per-peer only; pending table can fill with
                // many distinct MACs and block renew/resume — rate-limit globally here.
                if host_db_allows_install(&peer) && host_global_handshake_admit() {
                    mgr.handle_install_request::<256>(&frame, &mut se, &mut rng, now, key_lifetime_secs).ok();
                }
            }
            Ok((SigilEvent::InstallChallenge { frame, .. }
            | SigilEvent::InstallConfirm { frame, .. }
            | SigilEvent::InstallCommitConfirmed { frame, .. }, _)) => {
                // PersistPeerStaticKey may carry CommitConfirmed in `.transmit`.
                mgr.continue_install::<256>(&frame, &mut se, now).ok();
            }
            Ok((SigilEvent::RenewRequest { peer: _, frame }, _)) => {
                if host_global_handshake_admit() {
                    mgr.handle_renew_request::<256>(&frame, &mut se, &mut rng, now, key_lifetime_secs).ok();
                }
            }
            Ok((SigilEvent::RenewChallenge { frame, .. }
            | SigilEvent::RenewCommitConfirmed { frame, .. }, _)) => {
                mgr.continue_renew::<256>(&frame, &mut se, now).ok();
            }
            Ok((SigilEvent::RenewConfirm { frame, .. }, _)) => {
                mgr.handle_renew_confirm::<256>(&frame, &mut se, now).ok();
            }
            Ok((SigilEvent::ResumeRequest { peer: _, frame }, _)) => {
                if host_global_handshake_admit() {
                    mgr.handle_resume_request::<256>(&frame, &mut se, &mut rng, now, key_lifetime_secs).ok();
                }
            }
            Ok((SigilEvent::ResumeChallenge { frame, .. }
            | SigilEvent::ResumeConfirm { frame, .. }
            | SigilEvent::ResumeCommitConfirmed { frame, .. }, _)) => {
                mgr.continue_resume::<256>(&frame, &mut se, now).ok();
            }
            Err(SigilError::SessionExpired { peer }) => {
                // Sensor: call start_renew(...) — library does NOT auto-renew
            }
            Err(e) => { /* log error */ }
        }
    }

    // Host owns handshake pending lifetime — pick one or both:
    // A) Transmit { wait_timeout_ms: Some(t) } → host timer → cancel_pending
    // B) mgr.tick(now) each loop → purge expired deadline_at (TimeoutElapsed semantics)
    // Event timers instead retry send_event(same id + payload), then optionally
    // cancel_pending_event after the product retry budget.
}

The authoritative integration contract is this document together with the crate rustdoc (cargo doc --open); every public API carries its host-facing contract inline.

Project structure

sigil/
├── diagramme/           # Mermaid sequence diagrams (install, renew, session_resume)
├── src/
│   └── core/                  # Non-blocking session engine
│       ├── frame/             # FrameType discriminator (byte 18) + outer header (protocol 0x03)
│       ├── factory_cert/      # Factory certificate parsing / verification types
│       ├── install/           # Install handshake (clear-text + ECDSA + static↔static ECDH)
│       ├── renew/             # Renew handshake (P-256 ephemeral ECDH via host SecureElement)
│       ├── resume/            # Fast resume (req MAC + static↔static ECDH)
│       ├── manager/           # SessionManager<N> facade over crate-private session/pending/admission stores
│       ├── pipeline/          # Single authenticated ingress + data/event/timesync handlers
│       ├── aead/              # ChaCha20-Poly1305 encrypt_append/decrypt_into (1 buffer)
│       ├── derive/            # HKDF session key derivation
│       ├── pending/           # PendingHandshake with tagged PendingState variants (zeroizing secrets)
│       ├── secret/            # Zeroizing fixed-size secret buffers
│       ├── session/           # Session struct (keys, counters, expiry)
│       ├── event/             # SigilEvent + SigilAction enums
│       ├── error/             # SigilError enum (one host-policy category per variant)
│       └── traits/            # SecureElement + RandomSource traits
├── scripts/test.sh
└── Cargo.toml

Status

Active development. The non-blocking core is the single supported API as of v1.0.0.

License

Personal Use Only — Non-Commercial. See LICENSE for full terms and disclaimers.

ID	Version
chacha20poly1305	^0.10.1
heapless	^0.8
hkdf	^0.12
sha2	^0.10
subtle	^2.6
zeroize	^1.8
p256	^0.13

Details

Cargo

2026-08-23 13:15:28 +00:00

Repository website

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sigil-1.1.0.crate 254 KiB

Versions (13) View all

1.2.2

2026-09-04

1.2.1