Feature: Publish Exchange Rates to Nostr (Censorship-Resistant Alternative to Yadio HTTP API)

[mostronatorcoder]

Problem

Currently, Mostro mobile clients rely on Yadio's HTTP API (https://api.yadio.io) for fetching Bitcoin/fiat exchange rates. This creates two issues:

1. Censorship vulnerability — The API is blocked in Venezuela (and potentially other countries), preventing users from creating/taking orders.
2. Scaling costs — HTTP APIs require infrastructure that scales with user count (more requests = higher costs for Yadio).

Proposed Solution

Publish exchange rates to Nostr relays using NIP-33 addressable events (kind 30078).

Benefits

• ✅ Censorship-resistant — Governments cannot block decentralized Nostr relays
• ✅ Zero scaling cost — Relays host events for free; Yadio (or Mostro) only publishes once every 5-10 minutes
• ✅ Better UX in censored regions — Venezuela, Cuba, and other restricted countries can access rates
• ✅ Backward compatible — HTTP API can remain as fallback

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Technical Specification

Event Structure

Kind: 30078 (Application-specific data)
d tag: "mostro-rates"
Content: JSON object with all currency rates

Example Event

{
  "kind": 30078,
  "pubkey": "<yadio_or_mostro_pubkey>",
  "created_at": 1732546800,
  "tags": [
    ["d", "mostro-rates"],
    ["updated_at", "1732546800"],
    ["source", "yadio"]
  ],
  "content": "{\"USD\": {\"BTC\": 0.000024}, \"EUR\": {\"BTC\": 0.000022}, \"VES\": {\"BTC\": 0.0000000012}, ...}",
  "sig": "..."
}

Client Integration (Mostro Mobile)

// Subscribe to exchange rates from Nostr
final subscription = nostrPool.subscribe([
  Filter(
    kinds: [30078],
    authors: [yadioNostrPubkey],
    tags: {'#d': ['mostro-rates']},
  ),
]);

subscription.stream.listen((event) {
  final rates = jsonDecode(event.content);
  // Update UI with rates['USD']['BTC'], etc.
});

Fallback: If Nostr fetch fails, fall back to HTTP API.

Update Frequency

• Recommended: Every 5-10 minutes (same as current HTTP API polling)
• Publisher: Yadio (or Mostro daemon if Yadio doesn't adopt)

Recommended Relays

• wss://relay.mostro.network (Mostro's relay, high availability)
• wss://relay.damus.io (public, widely used)
• wss://nos.lol (fast, good uptime)
• wss://relay.nostr.band (archives everything, good fallback)

---

Implementation Plan

Phase 1: Proof of Concept (Mostro Daemon)

If Yadio doesn't adopt Nostr publishing, Mostro daemon can:

1. Fetch rates from Yadio HTTP API every 5 minutes
2. Publish to relay.mostro.network + public relays
3. Sign events with Mostro's pubkey

Code location: src/exchange_rates.rs (new module)

Phase 2: Mobile Client Integration

1. Update ExchangeService to support Nostr as primary source
2. Add fallback to HTTP API if Nostr unavailable
3. Cache rates locally (5-10 min TTL)

Related spec: EXCHANGE_SERVICE.md

References

• NIP-33: Parameterized Replaceable Events (https://github.com/nostr-protocol/nips/blob/master/33.md)
• Yadio API docs: https://yadio.io/api
• Related issue: Censorship in Venezuela (blocked API access)

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Labels: enhancement, censorship-resistance, nostr, exchange-rates

[grunch]

#closing via #685