feat: NIP-17 gift-wrapped check-in DM on confirmation (#5)

On settlement, send the guest their private check-in details as a NIP-59
gift-wrapped DM (nostr/giftwrap.py, built from lnbits core primitives — no
vendored crypto):

- rumor (kind 14) -> seal (kind 13, operator-encrypted + operator-signed via
  the signer abstraction) -> gift wrap (kind 1059, ephemeral-key encrypted +
  signed locally via core nip44_encrypt + sign_event). created_at randomised
  into the past per NIP-59.
- service.send_checkin_dm builds the message (room.checkin_instructions +
  settings times/policy) and publishes via nostrclient (_publish_signed,
  extracted from _sign_and_publish).
- tasks.on_invoice_paid calls it best-effort — a DM failure never undoes a
  confirmed, paid booking.

Encrypted layer (seal) soft-fails on a LocalSigner until bunker/server-
signing (lnbits#18), same as the reservation event; the ephemeral wrap layer
always works.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019VUQCfdqiLSsFS2jcGnaFD

nostr/giftwrap.py

@@ -0,0 +1,115 @@
+"""NIP-59 gift wrap for NIP-17 private DMs, built from LNbits core primitives
+(no vendored crypto).
+
+Three layers (https://github.com/nostr-protocol/nips/blob/master/59.md):
+
+  1. rumor (kind 14, unsigned) — the actual message; deniable if leaked.
+  2. seal  (kind 13) — NIP-44-encrypts the rumor to the recipient, signed by
+     the SENDER. Sender-identity crypto → routed through the operator's
+     `NostrSigner` so the nsec stays in the bunker. On a LocalSigner this
+     raises (bunker-forward), so `build_dm` returns None (soft-fail).
+  3. gift wrap (kind 1059) — NIP-44-encrypts the seal with a throwaway
+     EPHEMERAL key; only public metadata is the recipient p-tag. The
+     ephemeral key has no identity value, so it's generated + used locally
+     (core `nip44_encrypt` + `sign_event`), no bunker round-trip.
+
+Timestamps on seal + wrap are randomised into the past (NIP-59) so relays
+can't correlate by created_at.
+"""
+
+import hashlib
+import json
+import secrets
+import time
+
+from loguru import logger
+
+try:
+    import coincurve
+    from lnbits.core.services.nostr_transport.crypto import nip44_encrypt
+    from lnbits.core.signers.base import SignerUnavailableError
+    from lnbits.utils.nostr import json_dumps, sign_event
+
+    _GIFTWRAP_AVAILABLE = True
+except ImportError:  # pre-nostr-transport lnbits — check-in DMs soft-disable
+    _GIFTWRAP_AVAILABLE = False
+
+_TWO_DAYS = 2 * 24 * 60 * 60
+
+
+def _random_past() -> int:
+    return int(time.time()) - secrets.randbelow(_TWO_DAYS)
+
+
+def _event_id(pubkey: str, created_at: int, kind: int, tags: list, content: str) -> str:
+    # NIP-01 id — identical serialization to lnbits.utils.nostr.sign_event.
+    ser = json_dumps([0, pubkey, created_at, kind, tags, content])
+    return hashlib.sha256(ser.encode()).hexdigest()
+
+
+def _xonly_pubkey(privkey_hex: str) -> str:
+    sk = coincurve.PrivateKey(bytes.fromhex(privkey_hex))
+    return sk.public_key.format(compressed=True)[1:].hex()
+
+
+async def build_dm(
+    *,
+    signer,
+    sender_pubkey: str,
+    recipient_pubkey: str,
+    content: str,
+    inner_tags: list | None = None,
+) -> dict | None:
+    """Build a NIP-59 gift-wrapped NIP-17 DM (kind 1059) ready to publish.
+
+    Returns the signed gift-wrap event, or None if giftwrap primitives
+    aren't available or the operator signer can't encrypt the seal
+    (LocalSigner pre-bunker). Never raises for those soft-fail cases.
+    """
+    if not _GIFTWRAP_AVAILABLE:
+        return None
+
+    # 1. rumor (kind 14, unsigned) — sender is the operator.
+    tags = [["p", recipient_pubkey], *(inner_tags or [])]
+    created = int(time.time())
+    rumor = {
+        "pubkey": sender_pubkey,
+        "created_at": created,
+        "kind": 14,
+        "tags": tags,
+        "content": content,
+        "id": _event_id(sender_pubkey, created, 14, tags, content),
+    }
+
+    # 2. seal (kind 13) — operator-encrypted + operator-signed.
+    try:
+        sealed = await signer.nip44_encrypt(json.dumps(rumor), recipient_pubkey)
+    except SignerUnavailableError as exc:
+        logger.warning(
+            "chatelet: cannot seal check-in DM (operator signer can't NIP-44 "
+            f"encrypt — needs bunker/server-signing, lnbits#18): {exc}"
+        )
+        return None
+    seal = {
+        "pubkey": sender_pubkey,
+        "created_at": _random_past(),
+        "kind": 13,
+        "tags": [],
+        "content": sealed,
+    }
+    seal = await signer.sign_event(seal)  # operator fills id + sig
+    if not seal:
+        return None
+
+    # 3. gift wrap (kind 1059) — ephemeral key, local encrypt + sign.
+    eph_priv = secrets.token_bytes(32).hex()
+    eph_pub = _xonly_pubkey(eph_priv)
+    wrap_content = nip44_encrypt(json.dumps(seal), eph_priv, recipient_pubkey)
+    wrap = {
+        "pubkey": eph_pub,
+        "created_at": _random_past(),
+        "kind": 1059,
+        "tags": [["p", recipient_pubkey]],
+        "content": wrap_content,
+    }
+    return sign_event(wrap, eph_pub, coincurve.PrivateKey(bytes.fromhex(eph_priv)))

nostr/service.py

@@ -23,7 +23,7 @@ from loguru import logger
 
 from .. import crud, services
 from ..models import Booking, Room
-from . import events
+from . import events, giftwrap
 from .kinds import KIND_AVAILABILITY_QUERY
 
 try:
@@ -104,7 +104,12 @@ async def _sign_and_publish(unsigned: dict, *, encrypt_to: str | None = None) ->
         return None
     if not signed:
         return None
+    return _publish_signed(signed)
 
+
+def _publish_signed(signed: dict) -> str | None:
+    """Publish an already-signed event via nostrclient. Returns the event id,
+    or None if nostrclient isn't installed."""
     try:
         _, nostr_client = _nostrclient()
     except _NostrclientUnavailable:
@@ -120,6 +125,40 @@ async def _sign_and_publish(unsigned: dict, *, encrypt_to: str | None = None) ->
     return signed.get("id")
 
 
+async def send_checkin_dm(booking, room, settings) -> str | None:
+    """On confirmation, send the guest a NIP-17 gift-wrapped DM with the
+    private check-in details. Encrypted end-to-end to the guest; soft-fails
+    (returns None) if the operator signer can't encrypt (LocalSigner pre-
+    bunker) or nostrclient isn't installed."""
+    account, signer = await _operator_signer()
+    if not signer:
+        return None
+    wrap = await giftwrap.build_dm(
+        signer=signer,
+        sender_pubkey=account.pubkey,
+        recipient_pubkey=booking.guest_pubkey,
+        content=_checkin_message(booking, room, settings),
+    )
+    if not wrap:
+        return None
+    return _publish_signed(wrap)
+
+
+def _checkin_message(booking, room, settings) -> str:
+    lines = [
+        f"Your booking at {room.title} is confirmed! 🏰",
+        "",
+        f"Check-in:  {booking.check_in} from {settings.checkin_time}",
+        f"Check-out: {booking.check_out} by {settings.checkout_time}",
+        f"Guests:    {booking.num_guests}",
+    ]
+    if room.checkin_instructions:
+        lines += ["", room.checkin_instructions]
+    if settings.cancellation_policy:
+        lines += ["", f"Cancellation policy: {settings.cancellation_policy}"]
+    return "\n".join(lines)
+
+
 async def publish_listing(room: Room) -> str | None:
     """Publish/refresh a room's NIP-99 kind:30402 listing (public)."""
     return await _sign_and_publish(events.build_listing_event(room))

tasks.py

@@ -42,8 +42,17 @@ async def on_invoice_paid(payment: Payment):
     await crud.update_booking(booking)
     booking.reservation_event_id = await nostr.publish_reservation(booking)
     await crud.update_booking(booking)
-    # TODO(checkin): DM check-in details (address, gate code, times) to the
-    # guest via NIP-17 giftwrap once relay plumbing lands.
+
+    # Send the guest their private check-in details (NIP-17 gift-wrapped DM).
+    # Best-effort: a publish failure must not undo a confirmed, paid booking.
+    try:
+        room = await crud.get_room(booking.room_id)
+        settings = await crud.get_or_create_settings()
+        if room:
+            await nostr.send_checkin_dm(booking, room, settings)
+    except Exception as exc:  # noqa: BLE001
+        logger.warning(f"chatelet: check-in DM failed for {booking.id} (continuing): {exc}")
+
     logger.info(f"chatelet: booking {booking.id} confirmed")