diff --git a/js/nostr/encryptedSync.js b/js/nostr/encryptedSync.js
new file mode 100644
index 00000000..af3c3538
--- /dev/null
+++ b/js/nostr/encryptedSync.js
@@ -0,0 +1,291 @@
+// Opt-in encrypted cross-login sync of bitvid settings (storage credentials, NWC
+// wallet) published as NIP-78 app-data (kind 30078) *replaceable* events,
+// encrypted to the user's own key.
+//
+// Security model (see todo item #15 — keep this in sync with the opt-in UX copy):
+// - Content is NIP-44-encrypted to the user's OWN pubkey (NIP-04 fallback only if
+//   the signer lacks NIP-44, e.g. some NIP-46 remotes). The blob lives on PUBLIC
+//   relays: encryption hides the contents, NOT the fact that this pubkey stores
+//   bitvid settings.
+// - Anyone who controls the user's Nostr key can decrypt these — the same trust
+//   root as everything else they sign. The opt-in copy must say so.
+// - NWC URIs are bearer SPENDING secrets; callers must gate that sync behind an
+//   explicit warning.
+// - Never log decrypted payloads or ciphertext.
+// - Publish to WRITE relays so the update reaches everywhere; reads take the
+//   NEWEST event per d-tag so a stale device can't clobber newer settings.
+// - created_at is forced strictly-newer than any known prior event so a replace
+//   actually wins.
+//
+// This module is pure / dependency-injected so it can be unit-tested without a
+// live signer or relay pool.
+
+export const APP_DATA_KIND = 30078;
+export const ENCRYPTED_SYNC_VERSION = 1;
+
+function nowSeconds(now) {
+  const ms = typeof now === "function" ? now() : Date.now();
+  return Math.floor(ms / 1000);
+}
+
+function normalizeDTag(value) {
+  return typeof value === "string" ? value.trim() : "";
+}
+
+function normalizePubkey(value) {
+  return typeof value === "string" ? value.trim().toLowerCase() : "";
+}
+
+async function encryptToSelf(signer, pubkey, plaintext) {
+  const caps = signer.capabilities || { nip04: true, nip44: true };
+  if (caps.nip44 && typeof signer.nip44Encrypt === "function") {
+    try {
+      const ciphertext = await signer.nip44Encrypt(pubkey, plaintext);
+      return { alg: "nip44", ciphertext };
+    } catch (err) {
+      // Fall through to NIP-04 for signers whose NIP-44 path is unavailable.
+    }
+  }
+  if (caps.nip04 && typeof signer.nip04Encrypt === "function") {
+    const ciphertext = await signer.nip04Encrypt(pubkey, plaintext);
+    return { alg: "nip04", ciphertext };
+  }
+  throw new Error("Signer does not support encryption (NIP-04 or NIP-44).");
+}
+
+async function decryptFromSelf(signer, pubkey, alg, ciphertext) {
+  const caps = signer.capabilities || { nip04: true, nip44: true };
+  if (alg === "nip44" && caps.nip44 && typeof signer.nip44Decrypt === "function") {
+    return signer.nip44Decrypt(pubkey, ciphertext);
+  }
+  if ((alg === "nip04" || !alg) && caps.nip04 && typeof signer.nip04Decrypt === "function") {
+    return signer.nip04Decrypt(pubkey, ciphertext);
+  }
+  throw new Error(`Signer cannot decrypt ${alg || "nip04"} content.`);
+}
+
+// Replaceable semantics: of all events a relay returns for a (kind, author, d),
+// only the newest created_at is authoritative.
+function selectNewest(events) {
+  let newest = null;
+  for (const ev of Array.isArray(events) ? events : []) {
+    if (!ev || typeof ev !== "object") {
+      continue;
+    }
+    const createdAt = Number(ev.created_at);
+    if (!Number.isFinite(createdAt)) {
+      continue;
+    }
+    if (!newest || createdAt > Number(newest.created_at)) {
+      newest = ev;
+    }
+  }
+  return newest;
+}
+
+export function createEncryptedSyncManager(deps = {}) {
+  const {
+    getActivePubkey,
+    getSigner,
+    getWriteRelays,
+    getReadRelays,
+    getPool,
+    publishEventToRelays,
+    summarizePublishResults,
+    signEvent,
+    now,
+  } = deps;
+
+  function resolvePubkey() {
+    return normalizePubkey(
+      typeof getActivePubkey === "function" ? getActivePubkey() : "",
+    );
+  }
+
+  function resolveSigner() {
+    return typeof getSigner === "function" ? getSigner() : null;
+  }
+
+  function isAvailable() {
+    const pubkey = resolvePubkey();
+    const signer = resolveSigner();
+    if (!pubkey || !signer) {
+      return false;
+    }
+    const caps = signer.capabilities || { nip04: true, nip44: true };
+    return Boolean(
+      (caps.nip44 && typeof signer.nip44Encrypt === "function") ||
+        (caps.nip04 && typeof signer.nip04Encrypt === "function"),
+    );
+  }
+
+  function resolveCreatedAt(options) {
+    return Math.max(
+      nowSeconds(now),
+      (Number(options?.afterCreatedAt) || 0) + 1,
+    );
+  }
+
+  async function publishTemplate(template) {
+    let signed;
+    try {
+      signed = await signEvent(template);
+    } catch (err) {
+      return { ok: false, error: "sign-failed", cause: err };
+    }
+    const relays =
+      (typeof getWriteRelays === "function" ? getWriteRelays() : []) || [];
+    const pool = typeof getPool === "function" ? getPool() : null;
+    if (!pool || !relays.length) {
+      return { ok: false, error: "no-relays" };
+    }
+    const results = await publishEventToRelays(pool, relays, signed);
+    const { accepted, failed } = summarizePublishResults(results);
+    return {
+      ok: accepted.length > 0,
+      accepted: accepted.length,
+      total: accepted.length + failed.length,
+      event: signed,
+      createdAt: template.created_at,
+    };
+  }
+
+  async function push(dTag, payload, options = {}) {
+    const tag = normalizeDTag(dTag);
+    if (!tag) {
+      return { ok: false, error: "missing-d-tag" };
+    }
+    const pubkey = resolvePubkey();
+    const signer = resolveSigner();
+    if (!pubkey || !signer) {
+      return { ok: false, error: "not-authenticated" };
+    }
+
+    let plaintext;
+    try {
+      plaintext = JSON.stringify(payload ?? null);
+    } catch (err) {
+      return { ok: false, error: "payload-not-serializable" };
+    }
+
+    let envelope;
+    try {
+      envelope = await encryptToSelf(signer, pubkey, plaintext);
+    } catch (err) {
+      return { ok: false, error: "encrypt-failed", cause: err };
+    }
+
+    const template = {
+      kind: APP_DATA_KIND,
+      pubkey,
+      created_at: resolveCreatedAt(options),
+      tags: [
+        ["d", tag],
+        ["client", "bitvid"],
+      ],
+      content: JSON.stringify({
+        v: ENCRYPTED_SYNC_VERSION,
+        alg: envelope.alg,
+        data: envelope.ciphertext,
+      }),
+    };
+
+    return publishTemplate(template);
+  }
+
+  async function pull(dTag) {
+    const tag = normalizeDTag(dTag);
+    if (!tag) {
+      return { found: false, error: "missing-d-tag" };
+    }
+    const pubkey = resolvePubkey();
+    const signer = resolveSigner();
+    if (!pubkey || !signer) {
+      return { found: false, error: "not-authenticated" };
+    }
+
+    const relays =
+      (typeof getReadRelays === "function" ? getReadRelays() : []) || [];
+    const pool = typeof getPool === "function" ? getPool() : null;
+    if (!pool || !relays.length) {
+      return { found: false, error: "no-relays" };
+    }
+
+    let events = [];
+    try {
+      events = await pool.list(relays, [
+        { kinds: [APP_DATA_KIND], authors: [pubkey], "#d": [tag], limit: 20 },
+      ]);
+    } catch (err) {
+      return { found: false, error: "fetch-failed", cause: err };
+    }
+
+    const newest = selectNewest(Array.isArray(events) ? events.flat() : []);
+    if (!newest) {
+      return { found: false };
+    }
+
+    const rawContent =
+      typeof newest.content === "string" ? newest.content.trim() : "";
+    if (!rawContent) {
+      return { found: false, cleared: true, createdAt: newest.created_at };
+    }
+
+    let parsed;
+    try {
+      parsed = JSON.parse(rawContent);
+    } catch (err) {
+      return { found: false, error: "corrupt-content", createdAt: newest.created_at };
+    }
+    if (parsed && parsed.cleared === true) {
+      return { found: false, cleared: true, createdAt: newest.created_at };
+    }
+
+    let plaintext;
+    try {
+      plaintext = await decryptFromSelf(signer, pubkey, parsed?.alg, parsed?.data);
+    } catch (err) {
+      return { found: false, error: "decrypt-failed", cause: err };
+    }
+
+    let payload;
+    try {
+      payload = JSON.parse(plaintext);
+    } catch (err) {
+      return { found: false, error: "corrupt-payload" };
+    }
+
+    return { found: true, payload, createdAt: newest.created_at, event: newest };
+  }
+
+  async function clear(dTag, options = {}) {
+    const tag = normalizeDTag(dTag);
+    if (!tag) {
+      return { ok: false, error: "missing-d-tag" };
+    }
+    const pubkey = resolvePubkey();
+    const signer = resolveSigner();
+    if (!pubkey || !signer) {
+      return { ok: false, error: "not-authenticated" };
+    }
+
+    // Publish a cleared marker (NOT empty content — some relays reject empty) so
+    // the cleared state propagates and a stale older event can't win on read.
+    const template = {
+      kind: APP_DATA_KIND,
+      pubkey,
+      created_at: resolveCreatedAt(options),
+      tags: [
+        ["d", tag],
+        ["client", "bitvid"],
+      ],
+      content: JSON.stringify({ v: ENCRYPTED_SYNC_VERSION, cleared: true }),
+    };
+
+    return publishTemplate(template);
+  }
+
+  return { isAvailable, push, pull, clear, APP_DATA_KIND };
+}
+
+export default createEncryptedSyncManager;
diff --git a/tests/encrypted-sync.test.mjs b/tests/encrypted-sync.test.mjs
new file mode 100644
index 00000000..75ad25be
--- /dev/null
+++ b/tests/encrypted-sync.test.mjs
@@ -0,0 +1,175 @@
+// Opt-in encrypted settings sync (todo #15): bitvid settings are published as
+// NIP-78 (kind 30078) replaceable events, encrypted to the user's own key, read
+// back newest-per-d-tag. These scenarios assert the observable boundary outcomes:
+//   - a pushed secret round-trips through encrypt -> relay -> decrypt
+//   - the secret is NEVER stored in cleartext on the relay (encryption is real)
+//   - the NEWEST event wins (a stale device cannot clobber newer settings)
+//   - clear() makes a subsequent pull report "no data" (cleared)
+//   - NIP-04 is used when the signer lacks NIP-44
+//
+// Uses the REAL publishEventToRelays/summarizePublishResults so the publish
+// boundary is exercised, with a faithful in-memory relay + a reversible fake
+// signer (so "encryption" is observable but not identity passthrough).
+
+import "./test-helpers/setup-localstorage.mjs";
+import assert from "node:assert/strict";
+import test from "node:test";
+import { createEncryptedSyncManager } from "../js/nostr/encryptedSync.js";
+import {
+  publishEventToRelays,
+  summarizePublishResults,
+} from "../js/nostrPublish.js";
+
+const PUBKEY = "d".repeat(64);
+const D_TAG = "bitvid:storage-connections";
+
+// Reversible, non-identity "encryption": base64 so the plaintext secret is not a
+// literal substring of the ciphertext. Tagged by scheme so we can prove which
+// path (nip44 vs nip04) was used.
+function makeSigner({ nip44 = true } = {}) {
+  const b64 = (s) => Buffer.from(s, "utf8").toString("base64");
+  const unb64 = (s) => Buffer.from(s, "base64").toString("utf8");
+  const enc = (scheme) => async (_pk, text) => `${scheme}|${b64(text)}`;
+  const dec = (scheme) => async (_pk, cipher) => {
+    const [usedScheme, payload] = String(cipher).split("|");
+    assert.equal(usedScheme, scheme, "decrypt scheme must match encrypt scheme");
+    return unb64(payload);
+  };
+  const signer = {
+    capabilities: { nip04: true, nip44 },
+    nip04Encrypt: enc("nip04"),
+    nip04Decrypt: dec("nip04"),
+  };
+  if (nip44) {
+    signer.nip44Encrypt = enc("nip44");
+    signer.nip44Decrypt = dec("nip44");
+  }
+  return signer;
+}
+
+function makeRelay() {
+  const events = [];
+  return {
+    events,
+    publish(_urls, event) {
+      events.push(event);
+      return Promise.resolve(); // thenable -> publishEventToRelay success path
+    },
+    async list(_relays, filters) {
+      const f = filters[0];
+      return events.filter(
+        (e) =>
+          f.kinds.includes(e.kind) &&
+          f.authors.includes(e.pubkey) &&
+          Array.isArray(e.tags) &&
+          e.tags.some((t) => t[0] === "d" && f["#d"].includes(t[1])),
+      );
+    },
+  };
+}
+
+function makeManager({ nip44 = true } = {}) {
+  const relay = makeRelay();
+  const clock = { ms: 1_700_000_000_000 };
+  const signer = makeSigner({ nip44 });
+  const manager = createEncryptedSyncManager({
+    getActivePubkey: () => PUBKEY,
+    getSigner: () => signer,
+    getWriteRelays: () => ["wss://write.relay"],
+    getReadRelays: () => ["wss://read.relay"],
+    getPool: () => relay,
+    publishEventToRelays,
+    summarizePublishResults,
+    signEvent: async (tpl) => ({
+      ...tpl,
+      id: `id-${tpl.created_at}-${relay.events.length}`,
+      sig: "sig",
+    }),
+    now: () => clock.ms,
+  });
+  return { manager, relay, clock };
+}
+
+test("push -> pull round-trips a secret without ever storing it in cleartext", async () => {
+  const { manager, relay } = makeManager();
+  const secret = "SUPER-SECRET-R2-ACCESS-KEY-xyz";
+  const payload = { accessKeyId: secret, bucket: "my-bucket" };
+
+  const pushed = await manager.push(D_TAG, payload);
+  assert.equal(pushed.ok, true, "push must succeed when a relay accepts");
+  assert.ok(pushed.accepted >= 1);
+
+  // Anti-cheat: the secret must NOT appear in cleartext anywhere on the relay.
+  const stored = relay.events[0];
+  assert.equal(stored.kind, 30078);
+  assert.ok(
+    !stored.content.includes(secret),
+    "the raw secret must never be stored in cleartext on the relay",
+  );
+  assert.ok(
+    !stored.content.includes("my-bucket"),
+    "plaintext metadata (bucket) must be encrypted too",
+  );
+
+  const pulled = await manager.pull(D_TAG);
+  assert.equal(pulled.found, true, "pull must find the pushed note");
+  assert.deepEqual(
+    pulled.payload,
+    payload,
+    "decrypted payload must match what was pushed",
+  );
+});
+
+test("the NEWEST event wins — a stale copy cannot clobber newer settings", async () => {
+  const { manager, relay, clock } = makeManager();
+
+  await manager.push(D_TAG, { generation: "old", bucket: "old-bucket" });
+  clock.ms += 120_000; // 2 minutes later
+  await manager.push(D_TAG, { generation: "new", bucket: "new-bucket" });
+
+  // Both events exist on the relay (replaceable selection happens on read).
+  assert.equal(relay.events.length, 2, "both copies are present on the relay");
+
+  const pulled = await manager.pull(D_TAG);
+  assert.equal(pulled.found, true);
+  assert.equal(
+    pulled.payload.generation,
+    "new",
+    "pull must return the newest event's payload, not a stale one",
+  );
+});
+
+test("clear() makes a subsequent pull report cleared / no data", async () => {
+  const { manager, clock } = makeManager();
+  await manager.push(D_TAG, { secret: "to-be-wiped" });
+
+  clock.ms += 60_000;
+  const cleared = await manager.clear(D_TAG);
+  assert.equal(cleared.ok, true, "clear must publish successfully");
+
+  const pulled = await manager.pull(D_TAG);
+  assert.equal(pulled.found, false, "cleared settings must not be found");
+  assert.equal(pulled.cleared, true, "pull must report the cleared marker");
+});
+
+test("falls back to NIP-04 when the signer lacks NIP-44", async () => {
+  const { manager, relay } = makeManager({ nip44: false });
+  const payload = { nwcUri: "nostr+walletconnect://deadbeef" };
+
+  const pushed = await manager.push("bitvid:nwc", payload);
+  assert.equal(pushed.ok, true);
+
+  const envelope = JSON.parse(relay.events[0].content);
+  assert.equal(envelope.alg, "nip04", "must record the nip04 scheme actually used");
+
+  const pulled = await manager.pull("bitvid:nwc");
+  assert.deepEqual(pulled.payload, payload);
+});
+
+test("isAvailable() is false without a signer or pubkey", async () => {
+  const manager = createEncryptedSyncManager({
+    getActivePubkey: () => "",
+    getSigner: () => null,
+  });
+  assert.equal(manager.isAvailable(), false);
+});