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diff --git a/src/nostr.js b/src/nostr.js
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+// Encrypted cross-device state sync via Nostr.
+//
+// The wallet's Nostr identity is derived from the same seed (NIP-06,
+// m/44'/1237'/0'/0/0). Wallet state is encrypted to ourselves (NIP-44) and
+// published as a single parameterized-replaceable event (kind 30078, NIP-78)
+// to relay.coinos.io — so any device with the seed can pull the latest state
+// without re-scanning, and the relay only ever keeps the newest copy.
+
+import * as nip06 from 'nostr-tools/nip06';
+import * as nip44 from 'nostr-tools/nip44';
+import { getPublicKey, finalizeEvent } from 'nostr-tools/pure';
+import { Relay } from 'nostr-tools/relay';
+
+const RELAY = 'wss://relay.coinos.io';
+const DTAG = 'bitcoin-wallet';
+
+export class NostrSync {
+  constructor() {
+    this.sk = null;
+    this.pk = null;
+    this.ck = null; // self conversation key for NIP-44
+    this._relay = null;
+  }
+
+  load(mnemonic, passphrase = '') {
+    this.sk = nip06.privateKeyFromSeedWords(mnemonic, passphrase || undefined);
+    this.pk = getPublicKey(this.sk);
+    this.ck = nip44.getConversationKey(this.sk, this.pk);
+  }
+
+  unload() {
+    this.sk = this.pk = this.ck = null;
+    if (this._relay) {
+      try {
+        this._relay.close();
+      } catch {}
+      this._relay = null;
+    }
+  }
+
+  async _conn() {
+    if (this._relay && this._relay.connected) return this._relay;
+    this._relay = await Relay.connect(RELAY);
+    return this._relay;
+  }
+
+  // Encrypt + publish the latest state (best-effort; never throws).
+  async publish(stateObj) {
+    if (!this.sk) return;
+    try {
+      const content = nip44.encrypt(JSON.stringify(stateObj), this.ck);
+      const evt = finalizeEvent(
+        { kind: 30078, created_at: Math.floor(Date.now() / 1000), tags: [['d', DTAG]], content },
+        this.sk
+      );
+      const relay = await this._conn();
+      await relay.publish(evt);
+    } catch {
+      /* offline / relay down — localStorage still has the state */
+    }
+  }
+
+  // Fetch + decrypt the latest state, or null.
+  async fetch() {
+    if (!this.sk) return null;
+    try {
+      const relay = await this._conn();
+      return await new Promise((resolve) => {
+        let result = null;
+        const sub = relay.subscribe([{ kinds: [30078], authors: [this.pk], '#d': [DTAG], limit: 1 }], {
+          onevent: (e) => {
+            try {
+              result = JSON.parse(nip44.decrypt(e.content, this.ck));
+            } catch {}
+          },
+          oneose: () => {
+            try {
+              sub.close();
+            } catch {}
+            resolve(result);
+          },
+        });
+        setTimeout(() => {
+          try {
+            sub.close();
+          } catch {}
+          resolve(result);
+        }, 6000);
+      });
+    } catch {
+      return null;
+    }
+  }
+}