SkateSpots/src/lib/cordn/client.ts

/**
 * Cordn transport layer.
 * ─────────────────────────────────────────────────────────────────────────────
 * Publishes and fetches crew group messages through the crew's coordinator,
 * on the coordinator's ContextVM relay. Messages are addressed to the group by
 * the crew coordinate (`a` tag) and carry a per-group monotonic cursor for
 * bounded catch-up + live delivery (fetch-then-poll).
 *
 * Every payload is already sealed by the crypto layer — this file never sees
 * plaintext. The relay/coordinator only ever routes ciphertext.
 */

import type { NostrEvent } from '@nostrify/nostrify';
import { finalizeEvent, generateSecretKey } from 'nostr-tools';
import { CORDN_APP_TAG, CORDN_MESSAGE_KIND, type CordnEndpoint, type CordnMessage } from './types';
import { openGroupMessage, parseSealed, type Nip44Signer } from './crypto';

/** Minimal relay handle (the object returned by `nostr.relay(url)`). */
export interface CordnRelay {
  query: (filters: unknown[], opts: { signal: AbortSignal }) => Promise<NostrEvent[]>;
  event: (event: NostrEvent, opts?: { signal: AbortSignal }) => Promise<void>;
}

/**
 * Build the sealed envelope event for a group message.
 *
 * The envelope is signed with a **fresh ephemeral key** (NIP-59 style), never
 * the sender's real key — so the coordinator/relay never learns who is talking.
 * Authorship is proven from *inside* the ciphertext (v2 signed author proof),
 * so hiding the outer identity costs nothing in trust.
 *
 * The `cursor` is set to the current time (ms) so ordering is stable across
 * coordinators (per-group monotonic; ties broken by event id).
 */
export async function buildEnvelope(
  endpoint: CordnEndpoint,
  sealedContent: string,
): Promise<NostrEvent> {
  const cursor = Date.now();
  const ephemeralSk = generateSecretKey();
  const tmpl = {
    kind: CORDN_MESSAGE_KIND,
    content: sealedContent,
    created_at: Math.floor(Date.now() / 1000),
    tags: [
      ['a', endpoint.groupId], // group = crew coordinate 35006:pubkey:d
      ['t', CORDN_APP_TAG],
      ['cursor', String(cursor)],
      ...(endpoint.coordinatorPubkey ? [['p', endpoint.coordinatorPubkey]] : []),
      ['alt', 'Encrypted crew chat message (Cordn)'],
    ] as string[][],
  };
  return finalizeEvent(tmpl, ephemeralSk) as NostrEvent;
}

/** Publish a pre-built, signed envelope to the coordinator's relay. */
export async function publishEnvelope(
  relay: CordnRelay,
  envelope: NostrEvent,
  signal: AbortSignal,
): Promise<void> {
  await relay.event(envelope, { signal });
}

/**
 * Fetch the group's message backlog from the coordinator, newest-capped by
 * `limit`, and decrypt everything this viewer can open. Returns messages sorted
 * oldest→newest by cursor.
 */
export async function fetchGroupMessages(params: {
  relay: CordnRelay;
  endpoint: CordnEndpoint;
  signer: Nip44Signer;
  viewerPk: string;
  limit?: number;
  /** Only fetch events created at/after this unix-seconds bound (catch-up window). */
  sinceSeconds?: number;
  signal: AbortSignal;
}): Promise<CordnMessage[]> {
  const { relay, endpoint, signer, viewerPk, limit = 200, sinceSeconds, signal } = params;

  const filter: Record<string, unknown> = {
    kinds: [CORDN_MESSAGE_KIND],
    '#a': [endpoint.groupId],
    '#t': [CORDN_APP_TAG],
    limit,
  };
  if (sinceSeconds) filter.since = sinceSeconds;

  const events = await relay.query([filter], { signal });

  const out: CordnMessage[] = [];
  for (const ev of events) {
    const sealed = parseSealed(ev.content);
    if (!sealed) continue;
    const opened = await openGroupMessage(signer, viewerPk, sealed, endpoint.groupId);
    if (!opened) continue;

    // Establish the true sender without trusting the coordinator:
    //  - v2 (`verified`): the crypto layer already checked a signed author proof
    //    inside the ciphertext, so the outer envelope key is an anonymous
    //    ephemeral and we simply trust `opened.sender`.
    //  - v1 (legacy): authorship was only bound to the outer envelope pubkey, so
    //    we keep the original guard and reject mismatches.
    if (!opened.verified && opened.sender !== ev.pubkey.toLowerCase()) continue;

    const cursorTag = ev.tags.find(([t]) => t === 'cursor')?.[1];
    const cursor = cursorTag ? Number(cursorTag) : ev.created_at * 1000;
    out.push({
      id: ev.id,
      pubkey: opened.sender, // real sender (proven), not the ephemeral envelope key
      content: opened.content,
      createdAt: opened.ts || ev.created_at,
      cursor: Number.isFinite(cursor) ? cursor : ev.created_at * 1000,
    });
  }

  // Oldest → newest by cursor, id as tiebreaker for determinism.
  out.sort((a, b) => (a.cursor - b.cursor) || a.id.localeCompare(b.id));
  return out;
}