spike(block2): de-risk NIP-17 messaging + validate shared-connection shape

Extends the Block 2 spike to attack the highest-risk unbuilt piece (private
messaging) and to prove the Q2 architecture recommendation with running code:

- NostrConnection: one shared socket+key+sign+REQ/EOSE lifecycle. Refactored
  NostrOfferTransport onto it; added NostrMessageTransport — two thin contracts
  on ONE connection (the 'one connection, three interfaces' shape).
- NIP-44 v2 encryption (secp256k1 ECDH -> ChaCha20 + HMAC, length-hiding pad).
- NIP-17/NIP-59 gift-wrap: rumor(14) -> seal(13) -> wrap(1059, ephemeral key).
  Alice->Bob DM round-trips through the relay: encrypted, sender authenticated,
  and metadata-private (relay/eavesdropper sees only ciphertext, an ephemeral
  author and the recipient p-tag — Alice stays hidden).

Findings updated: NIP-17 risk drops Medium-High -> Medium (hardening, not
feasibility); next de-risking target is the web of trust. NIP-44 not yet
vector-verified (noted). 20 tests green, analyzer clean, Block 1 untouched.

docs/design/spike-block2-findings.md

@@ -19,10 +19,11 @@ The three open decisions from [open-decisions.md](open-decisions.md) §D.3 and
 | # | Open question | Verdict |
 |---|---|---|
 | 1 | Can we derive a Nostr (secp256k1) key from the Ğ1 root seed, one-way & reproducible? | **Yes — low risk.** |
-| 2 | Does an `OfferTransport` abstraction hold, without leaking inventory/location? | **Yes — with a caveat on messaging/trust coupling.** |
+| 2 | Does an `OfferTransport` abstraction hold, without leaking inventory/location? | **Yes — and the "one connection, three interfaces" shape is now demonstrated in code.** |
 | 3 | Does publish→discover-by-geohash actually work on Nostr NIP-99? | **Yes mechanically; NIP *maturity* is the real risk, not the flow.** |
+| 3b | Does NIP-17 private, metadata-hiding messaging actually work? | **Yes — prototyped end to end. Risk drops from "unbuilt unknown" to "production hardening".** |
 
-All of it runs with tests. See "How to run" at the end.
+All of it runs with tests (20 passing). See "How to run" at the end.
 
 ---
 
@@ -109,17 +110,23 @@ manageable**:
 - But their **verbs differ**: publish/browse a public listing vs. send/receive a
   private encrypted DM vs. assert/read a signed certification. Forcing DMs and
   certifications behind `OfferTransport.publish/discover` would overload it.
-- **Recommendation:** in `commons_core`, model **one shared `NostrConnection`**
+- **Recommendation — now demonstrated in code, not just asserted:**
-  (socket, key, sign, req/sub lifecycle) with **three thin interfaces on top** —
+  [`NostrConnection`](../../spike/block2_spike/lib/src/nostr_connection.dart) is
-  `OfferTransport`, `MessageTransport`, `TrustTransport`. That matches how the
+  **one shared** socket + key + sign + REQ/EOSE lifecycle, and **two thin
-  code actually factored in the spike and keeps each contract honest, while
+  interfaces already sit on top of the same instance** —
-  admitting the reality that one protocol/key/relay underpins all three.
+  [`OfferTransport`](../../spike/block2_spike/lib/src/nostr_offer_transport.dart)
+  (NIP-99) and
+  [`MessageTransport`](../../spike/block2_spike/lib/src/nostr_message_transport.dart)
+  (NIP-17). A third, `TrustTransport` (WoT), would slot in the same way. This is
+  the shape to lift into `commons_core`: shared connection, per-concern
+  contract. Trying to make `OfferTransport` also carry DMs would have overloaded
+  it — the split is real.
 
-**Residual risk: medium.** The abstraction is right; the *scope* is the risk —
+**Residual risk: medium → low-medium.** The abstraction is right *and* two of
-"the social layer" is genuinely offers + messaging + trust + relays together
+the three contracts are now built. The remaining risk is **scope, not
-(the §D.1 "indivisible" point), and this spike only built the offers third end
+feasibility**: "the social layer" is offers + messaging + trust + relays
-to end. Messaging (NIP-17 gift-wrap, offline delivery, spam) and trust (Duniter-
+together (§D.1 "indivisible"), and trust/WoT plus production hardening of
-style WoT over Nostr) remain the large, unbuilt pieces.
+messaging (below) are still ahead.
 
 ---
 
@@ -147,9 +154,9 @@ not the mechanics.**
   gift/exchange/sale/wanted all fit; we add an `offer_type` tag for the
   reciprocity mode NIP-99 lacks. **Low risk.**
 - **NIP-17 (private DMs, gift-wrap):** newer, more moving parts (sealed +
-  gift-wrapped, per-message ephemeral keys). Viable but the **biggest unbuilt
+  gift-wrapped, per-message ephemeral keys). **The spike DID build it** — see §4
-  risk** — this is where Q2's "messaging is large" bites. **Medium-high risk;
+  below. Verdict moves from "biggest unbuilt risk" to **medium (production
-  spike did not implement it.**
+  hardening), not high (feasibility)**.
 - **NIP-85 / WoT over Nostr:** least settled. The Duniter-style certification
   model maps onto signed events, but there is no dominant standard. Likely a
   **custom event kind mapped to the Duniter model** (as g1-integration.md §"WoT
@@ -163,6 +170,45 @@ not the mechanics.**
 
 ---
 
+## Q3b — NIP-17 private messaging (the piece that was flagged highest-risk)
+
+**Result: the metadata-private DM flow works end to end. Built, tested, and it
+fits the shared-connection architecture.**
+
+The plan ([network-trust.md](network-trust.md) §4) calls messaging "grande" and
+[open-decisions.md](open-decisions.md) §D.3 singled it out as the tightest
+coupling to Nostr. So the spike built it rather than hand-waving:
+
+- **NIP-44 v2 encryption** ([`Nip44`](../../spike/block2_spike/lib/src/nip44.dart)):
+  secp256k1 ECDH → HKDF conversation key → ChaCha20 + HMAC-SHA256 with
+  length-hiding padding. Conversation key is symmetric (A→B == B→A, tested); a
+  wrong key cannot decrypt (throws, tested); short messages pad to the same
+  bucket so ciphertext length doesn't leak plaintext length (tested).
+- **NIP-17 / NIP-59 gift-wrap onion**
+  ([`NostrMessageTransport`](../../spike/block2_spike/lib/src/nostr_message_transport.dart)):
+  rumor (kind 14, unsigned) → seal (kind 13, signed by sender, NIP-44 to
+  recipient) → gift wrap (kind 1059, signed by a **throwaway ephemeral key**,
+  NIP-44 to recipient).
+- **End-to-end over the relay (tested):**
+  - Alice → Bob: the message arrives and Bob **authenticates the sender as
+    Alice** — even though the wrap was signed by an ephemeral key.
+  - **The relay/eavesdropper sees neither the plaintext nor the real sender:**
+    snooping stored kind-1059 events yields only ciphertext, an ephemeral
+    author, and a `p` tag for the recipient. Alice's identity is hidden.
+  - A third party cannot read a message not addressed to them.
+
+**What this de-risks:** the hard, novel part (metadata-private messaging on the
+*same derived key and relay* as offers) is feasible and architecturally clean.
+
+**What remains (why it's still medium, not solved):**
+- The NIP-44 implementation here is **structurally faithful but NOT verified
+  against the reference test vectors** — cross-client interop must be proven
+  before shipping (a real client library should be used, not this spike code).
+- **Offline delivery, retries, and multi-device** (a recipient who's offline
+  when the wrap is published) are untouched — this is the real "messaging is
+  large" weight.
+- **Spam/abuse** on DMs leans on the (unbuilt) web of trust.
+
 ## Overall recommendation
 
 1. **Q1 is done enough to lock.** Adopt seed→secp256k1 derivation as specified;
@@ -171,15 +217,17 @@ not the mechanics.**
 2. **Q2: adopt the "one `NostrConnection`, three interfaces" shape.** Don't try
    to make `OfferTransport` also carry messaging and trust. Keep `Offer`
    agnostic and the geohash-coarsening seam exactly as prototyped.
-3. **Q3: Nostr-first is the right bet, but scope honestly.** Offers are the easy
+3. **Q3: Nostr-first is the right bet, and now two of three contracts are
-   third and they work. **Messaging (NIP-17) and WoT (NIP-85/custom) are the
+   proven.** Offers (NIP-99) and private messaging (NIP-17) both work on the
-   real weight and remain unproven here** — they, not the offer flow, should
+   shared connection. The remaining unproven weight is **WoT/trust
-   drive the Phase-3 estimate and the funding ask ([open-decisions.md](open-decisions.md)
+   (NIP-85/custom)** plus **production hardening of messaging** (interop-exact
-   §D.1, §D.6).
+   NIP-44, offline delivery, multi-device) — that, not the happy-path flow,
+   should drive the Phase-3 estimate and the funding ask
+   ([open-decisions.md](open-decisions.md) §D.1, §D.6).
 4. **Do not start Block 2 from this spike.** Delete `spike/block2_spike/` once
-   these findings are absorbed. The next concrete step is a *funded* social
+   these findings are absorbed. The next concrete de-risking target is the **web
-   round that starts by building NIP-17 messaging as its own de-risking target,
+   of trust** (Duniter-style certifications mapped to Nostr events) — the last
-   since that is where the residual risk concentrates.
+   large unknown — followed by hardening messaging for real-world delivery.
 
 ## Risk summary
 
@@ -188,16 +236,16 @@ not the mechanics.**
 | Seed→Nostr derivation | **Low** | Works, one-way, reproducible, tested. Only standardisation cosmetics left. |
 | Offer transport + privacy seam | **Low** | Abstraction holds; leak-proofing is structural and tested. |
 | Offer publish/discover (NIP-99) | **Low** | Mechanically proven; mature-ish NIP with real ecosystem. |
-| Messaging (NIP-17) | **Medium-High** | Newer, complex, unbuilt; the big Phase-3 cost. |
+| Messaging (NIP-17) | **Medium** | Flow + metadata-privacy prototyped & tested. Left: interop-exact NIP-44, offline delivery, multi-device. |
-| Web of trust (NIP-85/custom) | **High** | No settled standard; genuine design work. |
+| Web of trust (NIP-85/custom) | **High** | No settled standard; genuine design work. The last big unknown. |
-| Overall scope (§D.1) | **High** | Social layer is indivisible & large; only 1/3 proven here. |
+| Overall scope (§D.1) | **Medium-High** | Social layer is indivisible & large; 2 of 3 transport contracts now proven, WoT + hardening remain. |
 
 ## How to run
 
 ```sh
 cd spike/block2_spike
 dart pub get
-dart test            # derivation_test.dart, privacy_test.dart, roundtrip_test.dart
+dart test   # derivation · privacy · roundtrip · messaging  (20 tests)
 ```
 
 Nothing here is wired into `app_seeds` or `commons_core`'s production graph; the