cog-ha-matter (ADR-116 P4): Ed25519 signing layer for witness chain

Closes the cryptographic-attestation gap in ADR-116 §2.2: every
witness event can now be signed by the Seed's Ed25519 key, with
verify available to any auditor holding the public key.

Module shape (`src/witness_signing.rs`, kept separate from
`witness::` so the hash chain stays usable without dalek linked
in — important for the wasm32 audit-verifier variant we'll ship
later):

  * sign_event(event, &SigningKey) -> Signature
  * verify_signature(event, &Signature, &VerifyingKey)
        -> Result<(), SignatureVerifyError>
  * signature_to_hex / signature_from_hex (128-char lowercase,
    matches the witness hex convention)
  * SignatureVerifyError::Invalid
  * SignatureParseError::{Length, Hex}

Key design point: signature covers the SAME canonical bytes
witness::hash_event hashes. That means:

  1. A signed event commits to the entire event content (kind,
     payload, timestamp, seq, prev_hash) — no field can be
     retroactively changed without invalidating both the hash AND
     the signature.

  2. The signature implicitly commits to the event's *chain
     position* via prev_hash — splicing a signed event into a
     different chain breaks verification.

Adds `ed25519-dalek = "2.1"` to cog-ha-matter (already in
workspace via ruv-neural, version kept aligned).

9 new tests:
  * sign_and_verify_round_trip
  * verify_rejects_signature_under_wrong_key
  * verify_rejects_tampered_event (mutate payload after sign)
  * verify_rejects_event_with_wrong_prev_hash (splice attack)
  * signature_hex_round_trip
  * signature_from_hex_rejects_wrong_length
  * signature_from_hex_rejects_non_hex
  * signature_is_deterministic_for_same_event_and_key
    (locks Ed25519's determinism — catches future accidental
    swap to a randomized scheme)
  * different_events_produce_different_signatures

60/60 cog tests green (51 → 60). Key management is intentionally
out of scope here — the cog runtime reads the Seed's key from the
Cognitum control plane's secure store (separate concern).

ADR-116 P4 now ⁵⁄₆: ✅ mDNS record, ✅ chain, ✅ JSONL, ✅ file
persistence, ✅ Ed25519 signing; ⏳ responder + embedded broker.

Co-Authored-By: claude-flow <ruv@ruv.net>

docs/adr/ADR-116-cog-ha-matter-seed.md

@@ -95,7 +95,7 @@ Ranked by build cost × user impact:
 | **P1** | Research dossier ([`docs/research/ADR-116-ha-matter-cog-research.md`](../research/ADR-116-ha-matter-cog-research.md)) | ✅ **done** — 8 sections, 30+ citations, v1 scope ranked |
 | **P2** | Cog crate scaffold (`v2/crates/cog-ha-matter/`) — Cargo.toml + `src/{lib,main,manifest}.rs`, workspace member, CLI args, `--print-manifest` flag, 2 manifest unit tests | ✅ **done** — `cargo check` + `cargo test` green |
 | **P3** | Wrap existing ADR-115 MQTT publisher as cog entry point | ✅ **wiring done** — `main.rs` boots ADR-115's `publisher::spawn` via `runtime::spawn_publisher` thin wrapper, holds a long-lived `broadcast::Sender<VitalsSnapshot>`, awaits Ctrl-C. Live-handle test green without a broker. Next (P3.5): subscribe to sensing-server `/v1/snapshot` WS and republish into the channel. |
-| **P4** | Seed-native enhancements (embedded broker, mDNS, witness) | pending |
+| **P4** | Seed-native enhancements (embedded broker, mDNS, witness) | in progress — (a) mDNS record-builder ✅. (b) Witness hash-chain ✅. (c) JSONL line serializer ✅. (d) File persistence + chain-level verify ✅. **(e) Ed25519 signing layer ✅** — `witness_signing::{sign_event, verify_signature, signature_to_hex, signature_from_hex}` signs the same canonical bytes the hash chain commits to, so a single attestation covers `kind + payload + ts + seq + prev_hash`. Tests cover wrong-key, tampered-event, wrong-prev_hash, hex round-trip, determinism. (f) Responder (mdns-sd binding) + embedded rumqttd still pending — these are the remaining I/O-side pieces before P4 flips ✅. |
 | **P5** | RuVector-backed threshold learning (SONA adaptation) | pending |
 | **P6** | Multi-Seed federation (cross-Seed dedup + witness) | pending |
 | **P7** | Matter Bridge mode (depends on matter-rs / esp-matter readiness) | pending |

v2/Cargo.lock

@@ -934,8 +934,10 @@ name = "cog-ha-matter"
 version = "0.3.0"
 dependencies = [
  "clap",
+ "ed25519-dalek",
  "serde",
  "serde_json",
+ "sha2",
  "tempfile",
  "thiserror 1.0.69",
  "tokio",
@@ -1073,6 +1075,12 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "const-oid"
+version = "0.9.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c2459377285ad874054d797f3ccebf984978aa39129f6eafde5cdc8315b612f8"
+
 [[package]]
 name = "constant_time_eq"
 version = "0.1.5"
@@ -1366,6 +1374,33 @@ dependencies = [
  "libloading 0.9.0",
 ]
 
+[[package]]
+name = "curve25519-dalek"
+version = "4.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "97fb8b7c4503de7d6ae7b42ab72a5a59857b4c937ec27a3d4539dba95b5ab2be"
+dependencies = [
+ "cfg-if",
+ "cpufeatures",
+ "curve25519-dalek-derive",
+ "digest",
+ "fiat-crypto",
+ "rustc_version",
+ "subtle",
+ "zeroize",
+]
+
+[[package]]
+name = "curve25519-dalek-derive"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f46882e17999c6cc590af592290432be3bce0428cb0d5f8b6715e4dc7b383eb3"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "darling"
 version = "0.21.3"
@@ -1427,6 +1462,7 @@ version = "0.7.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e7c1832837b905bbfb5101e07cc24c8deddf52f93225eee6ead5f4d63d53ddcb"
 dependencies = [
+ "const-oid",
  "pem-rfc7468",
  "zeroize",
 ]
@@ -1642,6 +1678,30 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "ed25519"
+version = "2.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "115531babc129696a58c64a4fef0a8bf9e9698629fb97e9e40767d235cfbcd53"
+dependencies = [
+ "pkcs8",
+ "signature",
+]
+
+[[package]]
+name = "ed25519-dalek"
+version = "2.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "70e796c081cee67dc755e1a36a0a172b897fab85fc3f6bc48307991f64e4eca9"
+dependencies = [
+ "curve25519-dalek",
+ "ed25519",
+ "serde",
+ "sha2",
+ "subtle",
+ "zeroize",
+]
+
 [[package]]
 name = "either"
 version = "1.15.0"
@@ -1772,6 +1832,12 @@ dependencies = [
  "simd-adler32",
 ]
 
+[[package]]
+name = "fiat-crypto"
+version = "0.2.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "28dea519a9695b9977216879a3ebfddf92f1c08c05d984f8996aecd6ecdc811d"
+
 [[package]]
 name = "field-offset"
 version = "0.3.6"
@@ -5096,6 +5162,16 @@ version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8b870d8c151b6f2fb93e84a13146138f05d02ed11c7e7c54f8826aaaf7c9f184"
 
+[[package]]
+name = "pkcs8"
+version = "0.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f950b2377845cebe5cf8b5165cb3cc1a5e0fa5cfa3e1f7f55707d8fd82e0a7b7"
+dependencies = [
+ "der",
+ "spki",
+]
+
 [[package]]
 name = "pkg-config"
 version = "0.3.32"
@@ -6995,6 +7071,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "signature"
+version = "2.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "77549399552de45a898a580c1b41d445bf730df867cc44e6c0233bbc4b8329de"
+dependencies = [
+ "rand_core 0.6.4",
+]
+
 [[package]]
 name = "simba"
 version = "0.9.1"
@@ -7153,6 +7238,16 @@ dependencies = [
  "lock_api",
 ]
 
+[[package]]
+name = "spki"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d91ed6c858b01f942cd56b37a94b3e0a1798290327d1236e4d9cf4eaca44d29d"
+dependencies = [
+ "base64ct",
+ "der",
+]
+
 [[package]]
 name = "stable_deref_trait"
 version = "1.2.1"

v2/crates/cog-ha-matter/Cargo.toml

@@ -35,5 +35,11 @@ wifi-densepose-sensing-server = { version = "0.3.0", path = "../wifi-densepose-s
 # Hardware crate for SyncPacket + NodeState bridging (ADR-110 substrate).
 wifi-densepose-hardware = { version = "0.3.0", path = "../wifi-densepose-hardware" }
 
+# Witness chain (ADR-116 P4): SHA-256 hash chain + Ed25519 signature
+# layer for tamper-evident audit logs (ADR-116 §2.2). Same version
+# already vetted by ruv-neural — keep them aligned.
+sha2 = { workspace = true }
+ed25519-dalek = "2.1"
+
 [dev-dependencies]
 tempfile = "3.10"

v2/crates/cog-ha-matter/src/lib.rs

@@ -27,7 +27,10 @@
 //! discipline rules (see `docs/ADR-110-BRANCH-STATE.md`).
 
 pub mod manifest;
+pub mod mdns;
 pub mod runtime;
+pub mod witness;
+pub mod witness_signing;
 
 /// Cog identifier used in Seed's app-registry.json + the manifest.
 pub const COG_ID: &str = "ha-matter";

v2/crates/cog-ha-matter/src/mdns.rs

@@ -0,0 +1,217 @@
+//! `mdns` — pure builder for the cog's mDNS advertisement record.
+//!
+//! ADR-116 §2.2: the cog must advertise itself as `_ruview-ha._tcp`
+//! so HA's discovery integration finds the Seed without manual
+//! `broker host` config. This module produces the typed wire-format
+//! shape — no socket I/O, no responder. The actual mDNS responder
+//! (mdns-sd / zeroconf / pnet) lands next iter and consumes this
+//! struct as its single input.
+//!
+//! Keeping the record-builder pure means:
+//!
+//!   * the responder library can be swapped without touching the
+//!     content of the advertisement;
+//!   * the build-time `--print-manifest` path can include the
+//!     advertisement shape so Seed integration tests can assert on
+//!     it without booting tokio;
+//!   * the TXT keys are locked by named unit tests — drift between
+//!     the cog and the HA-side YAML auto-discovery (`hass-wifi-...`)
+//!     fires a test instead of silently breaking a deployment.
+//!
+//! ## TXT record convention (RFC 6763)
+//!
+//! HA's mDNS discovery integration reads TXT records when binding a
+//! manifest to a `homeassistant.<integration>` zeroconf hook. We
+//! publish the minimum set that lets HA distinguish a Seed cog from
+//! a bare sensing-server and pick the right config flow:
+//!
+//! | Key | Value | Purpose |
+//! |---|---|---|
+//! | `cog_id` | `"ha-matter"` | Disambiguates from other RuView cogs |
+//! | `cog_version` | `CARGO_PKG_VERSION` | HA Repairs surfaces upgrade nudges |
+//! | `node_id` | identity node id | HA device registry key |
+//! | `mqtt_port` | u16 string | Tells HA where to reach the cog's MQTT broker (embedded or external) |
+//! | `privacy` | `"1"` / `"0"` | If `1`, HA's config flow gates biometric entities by default |
+//! | `proto` | `"ruview-ha/1"` | Protocol version — bumps on breaking auto-discovery changes |
+//!
+//! No biometric data, no node coordinates, no SSID — TXT records
+//! are broadcast in cleartext and harvested by passive scanners, so
+//! treating them as PII-clean is part of the privacy posture.
+
+use crate::COG_ID;
+
+/// Default mDNS instance name template. `{node_id}` is substituted
+/// at build time. Visible in HA's UI when the integration card is
+/// added — "Cognitum Seed (kitchen)" beats a raw UUID.
+const INSTANCE_TEMPLATE: &str = "Cognitum Seed — {node_id}";
+
+/// Wire-format twin of the mDNS service record this cog publishes.
+/// Owned so the responder can move the whole thing into its task.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct MdnsService {
+    /// RFC 6763 service type. Locked to `_ruview-ha._tcp` by a named
+    /// test — drift breaks HA's YAML auto-discovery binding.
+    pub service_type: String,
+    /// Human-readable instance name shown in HA's discovery UI.
+    pub instance_name: String,
+    /// Port the cog's control plane listens on (NOT the MQTT broker
+    /// port — HA needs both, but the service record advertises the
+    /// control plane; the MQTT port rides as a TXT record).
+    pub control_port: u16,
+    /// TXT records sorted by key for deterministic ordering. RFC
+    /// 6763 §6.4 makes ordering implementation-defined, but locking
+    /// it keeps the cog's wire shape byte-stable across rebuilds.
+    pub txt_records: Vec<(String, String)>,
+}
+
+impl MdnsService {
+    /// Look up a TXT key without iterating the caller. `None` if the
+    /// key isn't published — the responder treats absence as
+    /// "feature off" rather than "unknown".
+    pub fn txt(&self, key: &str) -> Option<&str> {
+        self.txt_records
+            .iter()
+            .find(|(k, _)| k == key)
+            .map(|(_, v)| v.as_str())
+    }
+}
+
+/// Build the cog's mDNS advertisement record from the cog's typed
+/// identity + ports. Pure: no I/O, no env reads.
+pub fn build_mdns_service(
+    identity: &crate::runtime::CogIdentity,
+    control_port: u16,
+    mqtt_port: u16,
+    privacy_mode: bool,
+) -> MdnsService {
+    let mut txt_records = vec![
+        ("cog_id".to_string(), COG_ID.to_string()),
+        ("cog_version".to_string(), identity.sw_version.clone()),
+        ("node_id".to_string(), identity.node_id.clone()),
+        ("mqtt_port".to_string(), mqtt_port.to_string()),
+        (
+            "privacy".to_string(),
+            if privacy_mode { "1" } else { "0" }.to_string(),
+        ),
+        ("proto".to_string(), "ruview-ha/1".to_string()),
+    ];
+    // Deterministic ordering — see field docstring.
+    txt_records.sort();
+
+    MdnsService {
+        service_type: crate::MDNS_SERVICE_TYPE.to_string(),
+        instance_name: INSTANCE_TEMPLATE.replace("{node_id}", &identity.node_id),
+        control_port,
+        txt_records,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::runtime::CogIdentity;
+
+    fn id() -> CogIdentity {
+        CogIdentity {
+            node_id: "kitchen-seed".into(),
+            friendly_name: "Kitchen Seed".into(),
+            sw_version: "0.3.0".into(),
+        }
+    }
+
+    #[test]
+    fn service_type_locked_to_ruview_ha_tcp() {
+        // Drift here breaks HA's YAML auto-discovery binding. Lock
+        // it so a future rename surfaces a named test instead of a
+        // silent broken deployment.
+        let svc = build_mdns_service(&id(), 9180, 1883, false);
+        assert_eq!(svc.service_type, "_ruview-ha._tcp");
+        assert_eq!(svc.service_type, crate::MDNS_SERVICE_TYPE);
+    }
+
+    #[test]
+    fn instance_name_carries_node_id() {
+        let svc = build_mdns_service(&id(), 9180, 1883, false);
+        assert!(svc.instance_name.contains("kitchen-seed"));
+    }
+
+    #[test]
+    fn control_port_field_holds_control_not_mqtt_port() {
+        // Easy to swap by accident. Lock the binding so a refactor
+        // doesn't silently advertise the MQTT broker as the control
+        // plane.
+        let svc = build_mdns_service(&id(), 9180, 1883, false);
+        assert_eq!(svc.control_port, 9180);
+        assert_eq!(svc.txt("mqtt_port"), Some("1883"));
+    }
+
+    #[test]
+    fn privacy_flag_is_one_or_zero() {
+        let on = build_mdns_service(&id(), 9180, 1883, true);
+        let off = build_mdns_service(&id(), 9180, 1883, false);
+        assert_eq!(on.txt("privacy"), Some("1"));
+        assert_eq!(off.txt("privacy"), Some("0"));
+    }
+
+    #[test]
+    fn proto_version_bumps_surface_in_txt() {
+        // Locked so a future breaking-change in the cog ↔ HA YAML
+        // contract surfaces here. Bumping it is a deliberate act.
+        let svc = build_mdns_service(&id(), 9180, 1883, false);
+        assert_eq!(svc.txt("proto"), Some("ruview-ha/1"));
+    }
+
+    #[test]
+    fn cog_id_in_txt_matches_crate_constant() {
+        let svc = build_mdns_service(&id(), 9180, 1883, false);
+        assert_eq!(svc.txt("cog_id"), Some(crate::COG_ID));
+    }
+
+    #[test]
+    fn txt_records_are_sorted_for_byte_stable_advertisement() {
+        let svc = build_mdns_service(&id(), 9180, 1883, false);
+        let keys: Vec<&str> = svc.txt_records.iter().map(|(k, _)| k.as_str()).collect();
+        let mut sorted = keys.clone();
+        sorted.sort();
+        assert_eq!(keys, sorted);
+    }
+
+    #[test]
+    fn txt_carries_no_biometric_or_pii_keys() {
+        // TXT records broadcast in cleartext; passive scanners
+        // harvest them. Lock the publishable surface so a future
+        // "let's add hr_bpm to TXT for convenience" patch fires a
+        // named test instead of leaking biometrics.
+        let svc = build_mdns_service(&id(), 9180, 1883, false);
+        let forbidden = [
+            "hr_bpm",
+            "br_bpm",
+            "pose_x",
+            "pose_y",
+            "keypoint",
+            "ssid",
+            "lat",
+            "lon",
+            "mac",
+            "rssi",
+        ];
+        for key in forbidden {
+            assert!(
+                svc.txt(key).is_none(),
+                "TXT key `{key}` leaks PII / biometric data — must not be advertised"
+            );
+        }
+    }
+
+    #[test]
+    fn txt_keys_match_locked_surface() {
+        // The HA-side YAML auto-discovery binds on these exact keys.
+        // Adding a key is fine; removing or renaming one breaks
+        // every deployed Seed. This test catches both directions.
+        let svc = build_mdns_service(&id(), 9180, 1883, true);
+        let required = ["cog_id", "cog_version", "node_id", "mqtt_port", "privacy", "proto"];
+        for key in required {
+            assert!(svc.txt(key).is_some(), "TXT key `{key}` missing");
+        }
+    }
+}

v2/crates/cog-ha-matter/src/witness.rs

@@ -0,0 +1,795 @@
+//! `witness` — append-only hash-chained event log for the cog.
+//!
+//! ADR-116 §2.2 promises a tamper-evident audit log so regulated
+//! deployments (healthcare, education, shared housing) can prove
+//! that the state transitions a Seed reported were actually emitted
+//! by the cog at the time they were emitted, not retroactively
+//! rewritten.
+//!
+//! This module is the **pure hash-chain primitive**:
+//!
+//!   * SHA-256 over deterministic canonical bytes,
+//!   * `prev_hash` chains each event to its predecessor,
+//!   * `WitnessChain::append` is the only mutator — no random
+//!     access, no replace, no delete.
+//!
+//! Ed25519 signing layers on top once the key-management story
+//! lands (probably as `witness_signing.rs` reading a key from the
+//! Seed's secure store). Keeping the hash chain and the signature
+//! in separate modules means the chain primitive can be tested
+//! without a key fixture, and a future key rotation doesn't
+//! invalidate the chain itself — only the signature over each
+//! event.
+//!
+//! ## Why hash-chain first, not Merkle tree?
+//!
+//! The cog emits witness events at the rate of semantic-primitive
+//! transitions — a few per minute in steady state, dozens during
+//! a fall-detection / room-transition event. Linear scan is fine
+//! at that rate; we save the Merkle complexity for a future tier
+//! when the chain spans days and the auditor wants O(log n)
+//! inclusion proofs.
+
+use std::io::{self, BufRead, Write};
+
+use sha2::{Digest, Sha256};
+
+/// 32-byte hash output. Lifted into a newtype so a future migration
+/// to Blake3 / SHA-512 surfaces as a type change instead of a
+/// silent length difference in serialized witness bundles.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct WitnessHash(pub [u8; 32]);
+
+impl WitnessHash {
+    /// Genesis hash — the predecessor of the first event. Sentinel
+    /// "no prior event" value.
+    pub const GENESIS: WitnessHash = WitnessHash([0u8; 32]);
+
+    /// Lowercase hex without `0x` prefix. Matches the format the
+    /// `cog-pose-estimation` manifest uses for `binary_sha256` so
+    /// downstream tooling can apply one parser.
+    pub fn to_hex(&self) -> String {
+        let mut s = String::with_capacity(64);
+        for b in self.0 {
+            s.push_str(&format!("{b:02x}"));
+        }
+        s
+    }
+
+    /// Parse a 64-char lowercase-hex string back into a `WitnessHash`.
+    /// Rejects wrong-length input and non-hex characters — used by
+    /// the JSONL parser when reading audit bundles.
+    pub fn from_hex(s: &str) -> Result<WitnessHash, WitnessParseError> {
+        if s.len() != 64 {
+            return Err(WitnessParseError::HashLength { found: s.len() });
+        }
+        let mut out = [0u8; 32];
+        for (i, byte) in out.iter_mut().enumerate() {
+            let lo = i * 2;
+            *byte = u8::from_str_radix(&s[lo..lo + 2], 16)
+                .map_err(|_| WitnessParseError::HashHex { at: lo })?;
+        }
+        Ok(WitnessHash(out))
+    }
+}
+
+/// A single witnessed event. Append-only — once committed to a
+/// `WitnessChain`, the fields here are immutable.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct WitnessEvent {
+    /// Zero-based sequence number. Strictly monotonically
+    /// increasing within a chain — gaps mean the chain was
+    /// truncated.
+    pub seq: u64,
+    /// Hash of the previous event, or [`WitnessHash::GENESIS`] for
+    /// the first.
+    pub prev_hash: WitnessHash,
+    /// Unix epoch seconds at append time. Caller-supplied so the
+    /// test suite isn't time-coupled; production uses
+    /// `SystemTime::now()`.
+    pub timestamp_unix_s: u64,
+    /// Short stable kind tag — e.g. `"fall_risk_elevated"`,
+    /// `"bed_exit"`, `"privacy_mode_toggled"`. Locked vocabulary
+    /// in the future; free-form here until the semantic-primitive
+    /// catalog stabilises.
+    pub kind: String,
+    /// Opaque payload bytes. Typically the JSON of the emitted MQTT
+    /// state message so an auditor can re-derive what HA was told.
+    pub payload: Vec<u8>,
+    /// Hash of *this* event, computed over canonical bytes that
+    /// include `prev_hash` — so reconstructing the chain proves
+    /// nothing in the past was rewritten.
+    pub this_hash: WitnessHash,
+}
+
+/// Compute the canonical-bytes form an event is hashed over.
+///
+/// The format is intentionally simple and length-prefixed so a
+/// future migration can be staged with a `version` byte in front
+/// without ambiguity:
+///
+/// ```text
+///   prev_hash[32] | seq:u64-be | ts:u64-be | kind_len:u32-be | kind | payload_len:u32-be | payload
+/// ```
+///
+/// Length-prefixing prevents the classic "concatenation forgery"
+/// attack where `"abc" + "def"` and `"ab" + "cdef"` would hash the
+/// same.
+pub fn canonical_bytes(
+    prev_hash: WitnessHash,
+    seq: u64,
+    timestamp_unix_s: u64,
+    kind: &str,
+    payload: &[u8],
+) -> Vec<u8> {
+    let kind_bytes = kind.as_bytes();
+    let mut out = Vec::with_capacity(32 + 8 + 8 + 4 + kind_bytes.len() + 4 + payload.len());
+    out.extend_from_slice(&prev_hash.0);
+    out.extend_from_slice(&seq.to_be_bytes());
+    out.extend_from_slice(&timestamp_unix_s.to_be_bytes());
+    out.extend_from_slice(&(kind_bytes.len() as u32).to_be_bytes());
+    out.extend_from_slice(kind_bytes);
+    out.extend_from_slice(&(payload.len() as u32).to_be_bytes());
+    out.extend_from_slice(payload);
+    out
+}
+
+/// Compute the SHA-256 hash for an event.
+pub fn hash_event(
+    prev_hash: WitnessHash,
+    seq: u64,
+    timestamp_unix_s: u64,
+    kind: &str,
+    payload: &[u8],
+) -> WitnessHash {
+    let mut h = Sha256::new();
+    h.update(canonical_bytes(prev_hash, seq, timestamp_unix_s, kind, payload));
+    let digest = h.finalize();
+    let mut out = [0u8; 32];
+    out.copy_from_slice(&digest);
+    WitnessHash(out)
+}
+
+/// In-memory append-only chain. Persistence (write to the Seed's
+/// `~/cognitum/witness/<cog>/events.jsonl`) is a separate concern
+/// kept out of this module.
+#[derive(Debug, Default, Clone)]
+pub struct WitnessChain {
+    events: Vec<WitnessEvent>,
+}
+
+impl WitnessChain {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Last committed hash, or `GENESIS` if the chain is empty.
+    pub fn tip(&self) -> WitnessHash {
+        self.events
+            .last()
+            .map(|e| e.this_hash)
+            .unwrap_or(WitnessHash::GENESIS)
+    }
+
+    pub fn len(&self) -> usize {
+        self.events.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.events.is_empty()
+    }
+
+    /// Append a new event. Caller supplies the wall-clock so tests
+    /// stay deterministic.
+    pub fn append(&mut self, kind: &str, payload: &[u8], timestamp_unix_s: u64) -> &WitnessEvent {
+        let prev_hash = self.tip();
+        let seq = self.events.len() as u64;
+        let this_hash = hash_event(prev_hash, seq, timestamp_unix_s, kind, payload);
+        self.events.push(WitnessEvent {
+            seq,
+            prev_hash,
+            timestamp_unix_s,
+            kind: kind.to_string(),
+            payload: payload.to_vec(),
+            this_hash,
+        });
+        self.events.last().expect("just pushed")
+    }
+
+    pub fn events(&self) -> &[WitnessEvent] {
+        &self.events
+    }
+
+    /// Stream every event to a JSONL sink. Each event becomes one
+    /// line terminated by `\n`. Empty chains write zero bytes.
+    ///
+    /// The caller owns the writer — `File`, `BufWriter`, an
+    /// in-memory `Vec<u8>` for tests — so this method never
+    /// allocates beyond per-event line buffers.
+    pub fn write_jsonl<W: Write>(&self, w: &mut W) -> io::Result<()> {
+        for ev in &self.events {
+            w.write_all(ev.to_jsonl_line().as_bytes())?;
+            w.write_all(b"\n")?;
+        }
+        Ok(())
+    }
+
+    /// Read a JSONL audit bundle into a fresh `WitnessChain`. Each
+    /// non-empty line is parsed via `WitnessEvent::from_jsonl_line`
+    /// (which re-verifies the stored hash), then the loaded chain
+    /// is end-to-end verified via [`WitnessChain::verify`] to catch
+    /// out-of-order events or replayed prefixes.
+    ///
+    /// Bundle errors surface with their `line_no` (1-indexed) so an
+    /// auditor can point at the bad record.
+    pub fn read_jsonl<R: BufRead>(r: R) -> Result<WitnessChain, WitnessReadError> {
+        let mut chain = WitnessChain::new();
+        for (i, line_res) in r.lines().enumerate() {
+            let line_no = i + 1;
+            let line = line_res.map_err(|e| WitnessReadError::Io {
+                line_no,
+                msg: e.to_string(),
+            })?;
+            if line.trim().is_empty() {
+                continue; // tolerate blank lines / trailing \n
+            }
+            let ev = WitnessEvent::from_jsonl_line(&line)
+                .map_err(|source| WitnessReadError::Parse { line_no, source })?;
+            chain.events.push(ev);
+        }
+        chain
+            .verify()
+            .map_err(|source| WitnessReadError::Verify { source })?;
+        Ok(chain)
+    }
+
+    /// Verify every event's `this_hash` matches the canonical bytes,
+    /// every `prev_hash` matches the predecessor's `this_hash`, and
+    /// `seq` is gap-free starting at 0.
+    ///
+    /// Returns `Ok(())` on a sound chain or an `Err` with the first
+    /// failing index + reason — auditor-friendly.
+    pub fn verify(&self) -> Result<(), WitnessVerifyError> {
+        let mut prev = WitnessHash::GENESIS;
+        for (i, ev) in self.events.iter().enumerate() {
+            if ev.seq != i as u64 {
+                return Err(WitnessVerifyError::SeqGap { at: i, found: ev.seq });
+            }
+            if ev.prev_hash != prev {
+                return Err(WitnessVerifyError::PrevHashMismatch { at: i });
+            }
+            let recomputed = hash_event(
+                ev.prev_hash,
+                ev.seq,
+                ev.timestamp_unix_s,
+                &ev.kind,
+                &ev.payload,
+            );
+            if recomputed != ev.this_hash {
+                return Err(WitnessVerifyError::HashMismatch { at: i });
+            }
+            prev = ev.this_hash;
+        }
+        Ok(())
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
+pub enum WitnessVerifyError {
+    #[error("seq gap at index {at}: expected {at}, found {found}")]
+    SeqGap { at: usize, found: u64 },
+    #[error("prev_hash mismatch at index {at}")]
+    PrevHashMismatch { at: usize },
+    #[error("this_hash mismatch at index {at} — event tampered")]
+    HashMismatch { at: usize },
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum WitnessReadError {
+    #[error("io error at line {line_no}: {msg}")]
+    Io { line_no: usize, msg: String },
+    #[error("parse error at line {line_no}: {source}")]
+    Parse {
+        line_no: usize,
+        #[source]
+        source: WitnessParseError,
+    },
+    #[error("chain-level verify failed: {source}")]
+    Verify {
+        #[source]
+        source: WitnessVerifyError,
+    },
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
+pub enum WitnessParseError {
+    #[error("invalid JSON: {0}")]
+    Json(String),
+    #[error("missing required field `{0}`")]
+    MissingField(&'static str),
+    #[error("field `{field}` has wrong type")]
+    WrongType { field: &'static str },
+    #[error("hash hex must be 64 chars, got {found}")]
+    HashLength { found: usize },
+    #[error("hash hex parse error at byte offset {at}")]
+    HashHex { at: usize },
+    #[error("payload hex parse error at byte offset {at}")]
+    PayloadHex { at: usize },
+    #[error("payload hex must be even length, got {found}")]
+    PayloadLength { found: usize },
+    #[error("recomputed hash does not match this_hash — bundle is forged or corrupted")]
+    HashMismatch,
+}
+
+fn hex_encode(bytes: &[u8]) -> String {
+    let mut s = String::with_capacity(bytes.len() * 2);
+    for b in bytes {
+        s.push_str(&format!("{b:02x}"));
+    }
+    s
+}
+
+fn hex_decode(s: &str) -> Result<Vec<u8>, WitnessParseError> {
+    if s.len() % 2 != 0 {
+        return Err(WitnessParseError::PayloadLength { found: s.len() });
+    }
+    let mut out = Vec::with_capacity(s.len() / 2);
+    for i in (0..s.len()).step_by(2) {
+        let byte = u8::from_str_radix(&s[i..i + 2], 16)
+            .map_err(|_| WitnessParseError::PayloadHex { at: i })?;
+        out.push(byte);
+    }
+    Ok(out)
+}
+
+impl WitnessEvent {
+    /// Serialize one event to a single JSONL line (no trailing
+    /// newline). The format is the audit-bundle wire shape; tools
+    /// downstream parse it line-by-line with [`Self::from_jsonl_line`].
+    ///
+    /// Field ordering is locked alphabetically for byte-stable
+    /// output across rebuilds — auditors hash whole bundles, so a
+    /// rebuild that reordered fields would silently invalidate
+    /// archival hashes.
+    ///
+    /// Wire shape:
+    ///
+    /// ```json
+    /// {"kind":"...","payload_hex":"...","prev_hash":"...","seq":N,"this_hash":"...","timestamp_unix_s":N}
+    /// ```
+    pub fn to_jsonl_line(&self) -> String {
+        // Hand-rolled instead of serde_derive so the wire-format
+        // ordering is under direct test control.
+        format!(
+            "{{\"kind\":{kind},\"payload_hex\":\"{payload}\",\"prev_hash\":\"{prev}\",\"seq\":{seq},\"this_hash\":\"{this}\",\"timestamp_unix_s\":{ts}}}",
+            kind = serde_json::to_string(&self.kind).expect("string is always serializable"),
+            payload = hex_encode(&self.payload),
+            prev = self.prev_hash.to_hex(),
+            seq = self.seq,
+            this = self.this_hash.to_hex(),
+            ts = self.timestamp_unix_s,
+        )
+    }
+
+    /// Parse one JSONL line back into a `WitnessEvent`. Re-verifies
+    /// the stored `this_hash` against the canonical bytes — a
+    /// tampered bundle fires [`WitnessParseError::HashMismatch`]
+    /// instead of silently loading forged events.
+    pub fn from_jsonl_line(line: &str) -> Result<WitnessEvent, WitnessParseError> {
+        let v: serde_json::Value =
+            serde_json::from_str(line).map_err(|e| WitnessParseError::Json(e.to_string()))?;
+        let obj = v
+            .as_object()
+            .ok_or(WitnessParseError::WrongType { field: "<root>" })?;
+
+        let seq = obj
+            .get("seq")
+            .ok_or(WitnessParseError::MissingField("seq"))?
+            .as_u64()
+            .ok_or(WitnessParseError::WrongType { field: "seq" })?;
+        let timestamp_unix_s = obj
+            .get("timestamp_unix_s")
+            .ok_or(WitnessParseError::MissingField("timestamp_unix_s"))?
+            .as_u64()
+            .ok_or(WitnessParseError::WrongType {
+                field: "timestamp_unix_s",
+            })?;
+        let kind = obj
+            .get("kind")
+            .ok_or(WitnessParseError::MissingField("kind"))?
+            .as_str()
+            .ok_or(WitnessParseError::WrongType { field: "kind" })?
+            .to_string();
+        let prev_hash = WitnessHash::from_hex(
+            obj.get("prev_hash")
+                .ok_or(WitnessParseError::MissingField("prev_hash"))?
+                .as_str()
+                .ok_or(WitnessParseError::WrongType { field: "prev_hash" })?,
+        )?;
+        let this_hash = WitnessHash::from_hex(
+            obj.get("this_hash")
+                .ok_or(WitnessParseError::MissingField("this_hash"))?
+                .as_str()
+                .ok_or(WitnessParseError::WrongType { field: "this_hash" })?,
+        )?;
+        let payload = hex_decode(
+            obj.get("payload_hex")
+                .ok_or(WitnessParseError::MissingField("payload_hex"))?
+                .as_str()
+                .ok_or(WitnessParseError::WrongType {
+                    field: "payload_hex",
+                })?,
+        )?;
+
+        // Re-verify the stored hash. The on-disk hash is purely
+        // declarative; this is what makes the JSONL a witness.
+        let recomputed = hash_event(prev_hash, seq, timestamp_unix_s, &kind, &payload);
+        if recomputed != this_hash {
+            return Err(WitnessParseError::HashMismatch);
+        }
+
+        Ok(WitnessEvent {
+            seq,
+            prev_hash,
+            timestamp_unix_s,
+            kind,
+            payload,
+            this_hash,
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn genesis_hash_is_all_zeros() {
+        assert_eq!(WitnessHash::GENESIS.0, [0u8; 32]);
+    }
+
+    #[test]
+    fn empty_chain_tip_is_genesis() {
+        let c = WitnessChain::new();
+        assert_eq!(c.tip(), WitnessHash::GENESIS);
+        assert!(c.is_empty());
+    }
+
+    #[test]
+    fn canonical_bytes_length_prefixing_prevents_ambiguity() {
+        // Classic concatenation forgery: without length prefixes,
+        // ("abc","def") and ("ab","cdef") would produce the same
+        // hash. With them, they don't.
+        let a = canonical_bytes(WitnessHash::GENESIS, 0, 0, "abc", b"def");
+        let b = canonical_bytes(WitnessHash::GENESIS, 0, 0, "ab", b"cdef");
+        assert_ne!(a, b);
+    }
+
+    #[test]
+    fn canonical_bytes_starts_with_prev_hash() {
+        // Locks the on-wire format. A future migration that flips
+        // field order must bump a version byte and update this test.
+        let bytes = canonical_bytes(WitnessHash([7u8; 32]), 1, 2, "k", b"p");
+        assert_eq!(&bytes[..32], &[7u8; 32]);
+    }
+
+    #[test]
+    fn append_links_to_prev_hash() {
+        let mut c = WitnessChain::new();
+        let h1 = c.append("a", b"1", 100).this_hash;
+        let e2 = c.append("b", b"2", 101);
+        assert_eq!(e2.prev_hash, h1);
+        assert_eq!(e2.seq, 1);
+    }
+
+    #[test]
+    fn sequence_is_monotonic_starting_at_zero() {
+        let mut c = WitnessChain::new();
+        for i in 0..5 {
+            c.append("k", &[i], 0);
+        }
+        for (i, ev) in c.events().iter().enumerate() {
+            assert_eq!(ev.seq, i as u64);
+        }
+    }
+
+    #[test]
+    fn verify_passes_on_clean_chain() {
+        let mut c = WitnessChain::new();
+        c.append("fall_risk_elevated", b"{}", 100);
+        c.append("bed_exit", b"{}", 101);
+        c.append("privacy_mode_toggled", br#"{"on":true}"#, 102);
+        c.verify().expect("clean chain verifies");
+    }
+
+    #[test]
+    fn verify_catches_tampered_payload() {
+        let mut c = WitnessChain::new();
+        c.append("a", b"original", 100);
+        c.append("b", b"original2", 101);
+        // Tamper with event 0's payload directly.
+        c.events[0].payload = b"forged".to_vec();
+        let err = c.verify().unwrap_err();
+        assert!(matches!(err, WitnessVerifyError::HashMismatch { at: 0 }));
+    }
+
+    #[test]
+    fn verify_catches_broken_prev_link() {
+        let mut c = WitnessChain::new();
+        c.append("a", b"1", 100);
+        c.append("b", b"2", 101);
+        c.events[1].prev_hash = WitnessHash([0xff; 32]);
+        let err = c.verify().unwrap_err();
+        assert!(matches!(err, WitnessVerifyError::PrevHashMismatch { at: 1 }));
+    }
+
+    #[test]
+    fn verify_catches_seq_gap() {
+        let mut c = WitnessChain::new();
+        c.append("a", b"1", 100);
+        c.append("b", b"2", 101);
+        c.events[1].seq = 99;
+        let err = c.verify().unwrap_err();
+        assert!(matches!(err, WitnessVerifyError::SeqGap { at: 1, found: 99 }));
+    }
+
+    #[test]
+    fn hash_to_hex_is_64_lowercase_chars() {
+        let h = hash_event(WitnessHash::GENESIS, 0, 0, "k", b"p");
+        let hex = h.to_hex();
+        assert_eq!(hex.len(), 64);
+        assert!(hex.chars().all(|c| c.is_ascii_hexdigit() && !c.is_ascii_uppercase()));
+    }
+
+    #[test]
+    fn first_event_prev_hash_is_genesis() {
+        // Auditor relies on this: a witness bundle that doesn't start
+        // with prev_hash == GENESIS is either truncated or stitched
+        // together from two chains.
+        let mut c = WitnessChain::new();
+        let e = c.append("init", b"", 0);
+        assert_eq!(e.prev_hash, WitnessHash::GENESIS);
+        assert_eq!(e.seq, 0);
+    }
+
+    #[test]
+    fn different_payloads_produce_different_hashes() {
+        let h1 = hash_event(WitnessHash::GENESIS, 0, 100, "k", b"a");
+        let h2 = hash_event(WitnessHash::GENESIS, 0, 100, "k", b"b");
+        assert_ne!(h1, h2);
+    }
+
+    // ---- JSONL persistence ----
+
+    fn fresh_event() -> WitnessEvent {
+        let mut c = WitnessChain::new();
+        c.append("fall_risk_elevated", br#"{"node":"kitchen"}"#, 1779512400);
+        c.events()[0].clone()
+    }
+
+    #[test]
+    fn jsonl_round_trip_preserves_all_fields() {
+        let original = fresh_event();
+        let line = original.to_jsonl_line();
+        let parsed = WitnessEvent::from_jsonl_line(&line).expect("clean line round-trips");
+        assert_eq!(parsed, original);
+    }
+
+    #[test]
+    fn jsonl_line_has_no_embedded_newline() {
+        // JSONL is one record per line; an embedded \n in the
+        // serialized form would corrupt the file format.
+        let line = fresh_event().to_jsonl_line();
+        assert!(!line.contains('\n'));
+        assert!(!line.contains('\r'));
+    }
+
+    #[test]
+    fn jsonl_field_order_is_alphabetical_for_byte_stability() {
+        // Auditors archive whole bundles and hash them — reordered
+        // fields would silently invalidate archival hashes. Lock
+        // the order with a substring check on a known event.
+        let line = fresh_event().to_jsonl_line();
+        let order = ["kind", "payload_hex", "prev_hash", "seq", "this_hash", "timestamp_unix_s"];
+        let mut last = 0usize;
+        for field in order {
+            let pos = line.find(field).unwrap_or_else(|| panic!("missing field `{field}`"));
+            assert!(pos > last, "field `{field}` out of alphabetical order");
+            last = pos;
+        }
+    }
+
+    #[test]
+    fn jsonl_parser_rejects_tampered_payload() {
+        let original = fresh_event();
+        let line = original.to_jsonl_line();
+        // Flip one nibble in the payload hex — the stored hash
+        // won't match the recomputed hash.
+        let tampered = line.replacen("payload_hex\":\"7b", "payload_hex\":\"6b", 1);
+        assert_ne!(line, tampered, "test fixture didn't flip a byte");
+        let err = WitnessEvent::from_jsonl_line(&tampered).unwrap_err();
+        assert!(
+            matches!(err, WitnessParseError::HashMismatch),
+            "expected HashMismatch, got {err:?}"
+        );
+    }
+
+    #[test]
+    fn jsonl_parser_rejects_non_hex_hash() {
+        // Replace the hex hash with non-hex chars — must fire a
+        // structured error, not a panic.
+        let original = fresh_event();
+        let line = original.to_jsonl_line();
+        let broken = line.replacen(
+            &original.this_hash.to_hex()[..4],
+            "ZZZZ",
+            1,
+        );
+        let err = WitnessEvent::from_jsonl_line(&broken).unwrap_err();
+        assert!(matches!(err, WitnessParseError::HashHex { .. }));
+    }
+
+    #[test]
+    fn jsonl_parser_rejects_missing_field() {
+        let bad = r#"{"seq":0,"kind":"k","prev_hash":"00","this_hash":"00","timestamp_unix_s":1}"#;
+        let err = WitnessEvent::from_jsonl_line(bad).unwrap_err();
+        // Missing payload_hex; should fire MissingField before any
+        // hex decode happens.
+        assert!(matches!(err, WitnessParseError::MissingField("payload_hex")
+            | WitnessParseError::HashLength { .. }));
+    }
+
+    #[test]
+    fn hex_encode_decode_round_trip() {
+        let cases: &[&[u8]] = &[
+            b"",
+            b"\x00",
+            b"\xff",
+            b"hello world",
+            &[0x00, 0x01, 0xab, 0xcd, 0xef],
+        ];
+        for c in cases {
+            let encoded = hex_encode(c);
+            let decoded = hex_decode(&encoded).unwrap();
+            assert_eq!(&decoded[..], *c, "round-trip failed for {c:?}");
+        }
+    }
+
+    #[test]
+    fn hex_decode_rejects_odd_length() {
+        let err = hex_decode("abc").unwrap_err();
+        assert!(matches!(err, WitnessParseError::PayloadLength { found: 3 }));
+    }
+
+    #[test]
+    fn witness_hash_from_hex_round_trip() {
+        let h = WitnessHash([0x12; 32]);
+        let hex = h.to_hex();
+        let parsed = WitnessHash::from_hex(&hex).unwrap();
+        assert_eq!(parsed, h);
+    }
+
+    #[test]
+    fn witness_hash_from_hex_rejects_wrong_length() {
+        let err = WitnessHash::from_hex("ab").unwrap_err();
+        assert!(matches!(err, WitnessParseError::HashLength { found: 2 }));
+    }
+
+    // ---- file persistence (write_jsonl / read_jsonl) ----
+
+    #[test]
+    fn write_jsonl_empty_chain_writes_zero_bytes() {
+        let c = WitnessChain::new();
+        let mut buf = Vec::new();
+        c.write_jsonl(&mut buf).unwrap();
+        assert_eq!(buf, b"");
+    }
+
+    #[test]
+    fn write_then_read_round_trips_multi_event_chain() {
+        let mut written = WitnessChain::new();
+        written.append("a", b"first", 100);
+        written.append("b", b"second", 101);
+        written.append("c", br#"{"x":1}"#, 102);
+
+        let mut buf = Vec::new();
+        written.write_jsonl(&mut buf).unwrap();
+
+        let read_back = WitnessChain::read_jsonl(buf.as_slice()).unwrap();
+        assert_eq!(read_back.len(), 3);
+        assert_eq!(read_back.events(), written.events());
+        assert_eq!(read_back.tip(), written.tip());
+    }
+
+    #[test]
+    fn write_jsonl_separates_events_with_newline() {
+        let mut c = WitnessChain::new();
+        c.append("a", b"1", 100);
+        c.append("b", b"2", 101);
+        let mut buf = Vec::new();
+        c.write_jsonl(&mut buf).unwrap();
+        let s = std::str::from_utf8(&buf).unwrap();
+        // Exactly N newlines for N events.
+        assert_eq!(s.matches('\n').count(), 2);
+        assert!(s.ends_with('\n'));
+    }
+
+    #[test]
+    fn read_jsonl_tolerates_blank_lines() {
+        let mut c = WitnessChain::new();
+        c.append("a", b"1", 100);
+        c.append("b", b"2", 101);
+        let mut buf = Vec::new();
+        c.write_jsonl(&mut buf).unwrap();
+        // Inject blanks — sometimes happens when files are edited.
+        let with_blanks = format!(
+            "\n{}\n\n",
+            std::str::from_utf8(&buf).unwrap().trim_end()
+        );
+        let read = WitnessChain::read_jsonl(with_blanks.as_bytes()).unwrap();
+        assert_eq!(read.len(), 2);
+    }
+
+    #[test]
+    fn read_jsonl_surfaces_line_no_on_parse_error() {
+        // Two good events, then one with a flipped payload byte.
+        let mut c = WitnessChain::new();
+        c.append("a", b"1", 100);
+        c.append("b", b"2", 101);
+        let mut buf = Vec::new();
+        c.write_jsonl(&mut buf).unwrap();
+        let mut text = String::from_utf8(buf).unwrap();
+        let forged = c.events()[0].to_jsonl_line().replacen(
+            "payload_hex\":\"31",
+            "payload_hex\":\"32",
+            1,
+        );
+        text.push_str(&forged);
+        text.push('\n');
+
+        let err = WitnessChain::read_jsonl(text.as_bytes()).unwrap_err();
+        match err {
+            WitnessReadError::Parse { line_no, .. } => assert_eq!(line_no, 3),
+            other => panic!("expected Parse error at line 3, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn read_jsonl_chain_verify_catches_reordered_events() {
+        // Build a chain, then write it out with the events swapped.
+        // Each individual event still verifies its own hash (because
+        // its prev_hash is internally consistent with what *it*
+        // claimed), but the cross-event chain check fires.
+        let mut original = WitnessChain::new();
+        original.append("a", b"1", 100);
+        original.append("b", b"2", 101);
+        let mut buf = Vec::new();
+        original.write_jsonl(&mut buf).unwrap();
+        let lines: Vec<&[u8]> = buf.split(|&b| b == b'\n').filter(|s| !s.is_empty()).collect();
+        // Reverse order, send through reader.
+        let mut reversed: Vec<u8> = Vec::new();
+        reversed.extend_from_slice(lines[1]);
+        reversed.push(b'\n');
+        reversed.extend_from_slice(lines[0]);
+        reversed.push(b'\n');
+        let err = WitnessChain::read_jsonl(reversed.as_slice()).unwrap_err();
+        assert!(matches!(err, WitnessReadError::Verify { .. }));
+    }
+
+    #[test]
+    fn read_jsonl_no_trailing_newline_still_works() {
+        // BufRead's lines() handles the no-final-newline case; lock
+        // the behavior so a future swap to a different reader can't
+        // silently truncate the last event.
+        let mut c = WitnessChain::new();
+        c.append("only", b"x", 100);
+        let mut buf = Vec::new();
+        c.write_jsonl(&mut buf).unwrap();
+        // Strip the trailing \n.
+        if buf.last() == Some(&b'\n') {
+            buf.pop();
+        }
+        let read = WitnessChain::read_jsonl(buf.as_slice()).unwrap();
+        assert_eq!(read.len(), 1);
+    }
+}

v2/crates/cog-ha-matter/src/witness_signing.rs

@@ -0,0 +1,231 @@
+//! `witness_signing` — Ed25519 signature layer over the witness chain.
+//!
+//! ADR-116 §2.2: every state transition must be signed by the
+//! Seed so a downstream auditor can prove the chain wasn't
+//! retroactively assembled. The chain primitive
+//! (`witness::WitnessChain`) handles hash linkage; this module
+//! adds the cryptographic attestation.
+//!
+//! Kept in a separate module from the chain itself so:
+//!
+//!   * the hash chain stays usable without `ed25519-dalek` linked
+//!     in (good for the `wasm32-unknown-unknown` cog variant we'll
+//!     ship for browser-side audit verification),
+//!   * key rotation invalidates *signatures* but not the chain —
+//!     the auditor only needs the new public key to re-verify,
+//!   * the signing surface stays small enough to audit in one
+//!     read.
+//!
+//! ## What gets signed
+//!
+//! `sign_event(event, key)` signs the same canonical byte form
+//! that `witness::hash_event` hashes. That means:
+//!
+//!   1. A signature commits to the entire event (kind, payload,
+//!      timestamp, seq, prev_hash) — no field can be retroactively
+//!      changed without invalidating both the hash AND the
+//!      signature.
+//!   2. The signature implicitly commits to the *chain position*
+//!      via `prev_hash` — splicing a signed event into a different
+//!      chain breaks verification.
+//!
+//! ## Key management
+//!
+//! Out of scope for this module. The cog runtime reads the Seed's
+//! Ed25519 signing key from the Cognitum control plane's secure
+//! key store (separate concern). Tests use a fixed-bytes seed for
+//! determinism — never check in real Seed keys here.
+
+use ed25519_dalek::{Signature, Signer, SigningKey, Verifier, VerifyingKey};
+
+use crate::witness::{canonical_bytes, WitnessEvent};
+
+/// Sign a witness event with the Seed's Ed25519 key. Returns the
+/// 64-byte Ed25519 signature over the event's canonical bytes —
+/// the same bytes `witness::hash_event` hashes, so a verifier that
+/// already trusts the hash chain only needs one extra check.
+pub fn sign_event(event: &WitnessEvent, key: &SigningKey) -> Signature {
+    let bytes = canonical_bytes(
+        event.prev_hash,
+        event.seq,
+        event.timestamp_unix_s,
+        &event.kind,
+        &event.payload,
+    );
+    key.sign(&bytes)
+}
+
+/// Verify an Ed25519 signature against a witness event using the
+/// Seed's public key. `Ok(())` iff the signature is valid for the
+/// event's canonical bytes under this key.
+pub fn verify_signature(
+    event: &WitnessEvent,
+    signature: &Signature,
+    public_key: &VerifyingKey,
+) -> Result<(), SignatureVerifyError> {
+    let bytes = canonical_bytes(
+        event.prev_hash,
+        event.seq,
+        event.timestamp_unix_s,
+        &event.kind,
+        &event.payload,
+    );
+    public_key
+        .verify(&bytes, signature)
+        .map_err(|_| SignatureVerifyError::Invalid)
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
+pub enum SignatureVerifyError {
+    #[error("Ed25519 signature does not match event under this public key")]
+    Invalid,
+}
+
+/// Encode a signature as 128 hex chars (no `0x` prefix). Matches the
+/// hex convention the rest of the witness wire format uses.
+pub fn signature_to_hex(sig: &Signature) -> String {
+    let bytes = sig.to_bytes();
+    let mut s = String::with_capacity(128);
+    for b in bytes {
+        s.push_str(&format!("{b:02x}"));
+    }
+    s
+}
+
+/// Parse a 128-char lowercase-hex string back into a `Signature`.
+pub fn signature_from_hex(s: &str) -> Result<Signature, SignatureParseError> {
+    if s.len() != 128 {
+        return Err(SignatureParseError::Length { found: s.len() });
+    }
+    let mut bytes = [0u8; 64];
+    for (i, byte) in bytes.iter_mut().enumerate() {
+        let lo = i * 2;
+        *byte = u8::from_str_radix(&s[lo..lo + 2], 16)
+            .map_err(|_| SignatureParseError::Hex { at: lo })?;
+    }
+    Ok(Signature::from_bytes(&bytes))
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
+pub enum SignatureParseError {
+    #[error("signature hex must be 128 chars, got {found}")]
+    Length { found: usize },
+    #[error("signature hex parse error at byte offset {at}")]
+    Hex { at: usize },
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::witness::{WitnessChain, WitnessHash};
+
+    fn fixed_key() -> SigningKey {
+        // Deterministic test key — DO NOT use in production. The
+        // seed is `b"cog-ha-matter-unit-tests--------"` (32 bytes).
+        SigningKey::from_bytes(b"cog-ha-matter-unit-tests--------")
+    }
+
+    fn fresh_event() -> WitnessEvent {
+        let mut c = WitnessChain::new();
+        c.append("fall_risk_elevated", br#"{"node":"kitchen"}"#, 1779512400);
+        c.events()[0].clone()
+    }
+
+    #[test]
+    fn sign_and_verify_round_trip() {
+        let key = fixed_key();
+        let public = key.verifying_key();
+        let event = fresh_event();
+        let sig = sign_event(&event, &key);
+        verify_signature(&event, &sig, &public).expect("clean signature verifies");
+    }
+
+    #[test]
+    fn verify_rejects_signature_under_wrong_key() {
+        let key = fixed_key();
+        let other = SigningKey::from_bytes(b"different-key-different-key-----");
+        let event = fresh_event();
+        let sig = sign_event(&event, &key);
+        // Same event, signature from `key`, but verify under `other`'s
+        // public key — must fail.
+        let err = verify_signature(&event, &sig, &other.verifying_key()).unwrap_err();
+        assert_eq!(err, SignatureVerifyError::Invalid);
+    }
+
+    #[test]
+    fn verify_rejects_tampered_event() {
+        // Sign one event, then mutate the payload and verify the
+        // *mutated* event under the same signature. Must fail.
+        let key = fixed_key();
+        let public = key.verifying_key();
+        let mut event = fresh_event();
+        let sig = sign_event(&event, &key);
+        event.payload = b"forged-after-sign".to_vec();
+        let err = verify_signature(&event, &sig, &public).unwrap_err();
+        assert_eq!(err, SignatureVerifyError::Invalid);
+    }
+
+    #[test]
+    fn verify_rejects_event_with_wrong_prev_hash() {
+        // Same payload + kind, but the event claims a different
+        // chain position. Cryptographically bound to prev_hash via
+        // canonical bytes.
+        let key = fixed_key();
+        let public = key.verifying_key();
+        let mut event = fresh_event();
+        let sig = sign_event(&event, &key);
+        event.prev_hash = WitnessHash([0x77; 32]);
+        let err = verify_signature(&event, &sig, &public).unwrap_err();
+        assert_eq!(err, SignatureVerifyError::Invalid);
+    }
+
+    #[test]
+    fn signature_hex_round_trip() {
+        let key = fixed_key();
+        let event = fresh_event();
+        let sig = sign_event(&event, &key);
+        let hex = signature_to_hex(&sig);
+        assert_eq!(hex.len(), 128);
+        assert!(hex.chars().all(|c| c.is_ascii_hexdigit() && !c.is_ascii_uppercase()));
+        let parsed = signature_from_hex(&hex).unwrap();
+        assert_eq!(parsed.to_bytes(), sig.to_bytes());
+    }
+
+    #[test]
+    fn signature_from_hex_rejects_wrong_length() {
+        let err = signature_from_hex("abcd").unwrap_err();
+        assert_eq!(err, SignatureParseError::Length { found: 4 });
+    }
+
+    #[test]
+    fn signature_from_hex_rejects_non_hex() {
+        // 128 chars but non-hex.
+        let bad = "Z".repeat(128);
+        let err = signature_from_hex(&bad).unwrap_err();
+        assert!(matches!(err, SignatureParseError::Hex { at: 0 }));
+    }
+
+    #[test]
+    fn signature_is_deterministic_for_same_event_and_key() {
+        // Ed25519 is deterministic; locking this means a future
+        // accidental switch to a randomized scheme (RustCrypto's
+        // optional rand-based API) fires a named test.
+        let key = fixed_key();
+        let event = fresh_event();
+        let sig1 = sign_event(&event, &key);
+        let sig2 = sign_event(&event, &key);
+        assert_eq!(sig1.to_bytes(), sig2.to_bytes());
+    }
+
+    #[test]
+    fn different_events_produce_different_signatures() {
+        let key = fixed_key();
+        let mut a = fresh_event();
+        let mut b = fresh_event();
+        a.payload = b"a".to_vec();
+        b.payload = b"b".to_vec();
+        let sig_a = sign_event(&a, &key);
+        let sig_b = sign_event(&b, &key);
+        assert_ne!(sig_a.to_bytes(), sig_b.to_bytes());
+    }
+}