freezmo/lib/sync.ts

"use client";

/**
 * Opt-in encrypted sync. The payload (decks, folders, homework — never the
 * API key) is encrypted in this browser with AES-GCM using a key derived from
 * the user's passphrase (PBKDF2-SHA256, 310k iterations). The server only
 * ever stores the opaque envelope under a random sync ID.
 *
 * Conflict policy: server-stamped `updatedAt` is the clock; when both sides
 * changed since the last sync the UI asks the user which one to keep.
 */

import { z } from "zod";

import { backupDataSchema } from "./backup";
import { ApiError } from "./client-api";
import { replaceFolders } from "./folders";
import { replaceHomework } from "./homework";
import { ensureLearningHydrated, learningStore, replaceLearning } from "./history";
import { localeHeader, t } from "./i18n";
import { authHeaders } from "./settings";
import { deckStore, replaceDecks } from "./storage";
import { homeworkStore } from "./homework";
import { folderStore } from "./folders";
import { clearDeviceKey, loadDeviceKey, saveDeviceKey } from "./sync-key";
import { newSyncId, setSyncRecord, syncStore, withoutDirtyTracking } from "./sync-state";

const KDF = "PBKDF2-SHA256";
const ITERATIONS = 310_000;

export type SyncEnvelope = {
  v: 1;
  kdf: "PBKDF2-SHA256";
  iterations: number;
  salt: string;
  iv: string;
  ciphertext: string;
};

export type SyncStatus = "pushed" | "pulled" | "up-to-date" | "conflict" | "locked" | "error" | "off";
export type SyncOutcome = { status: SyncStatus; message?: string };

const syncPayloadSchema = z.object({
  app: z.literal("freezmo"),
  kind: z.literal("sync"),
  version: z.literal(1),
  clientUpdatedAt: z.number(),
  data: backupDataSchema,
});

type SyncPayload = z.infer<typeof syncPayloadSchema>;

/* ------------------------------------------------------------------ */
/* Encoding helpers (chunked: payloads can be megabytes)               */
/* ------------------------------------------------------------------ */

function toBase64(bytes: Uint8Array): string {
  let binary = "";
  const chunk = 0x8000;
  for (let index = 0; index < bytes.length; index += chunk) {
    binary += String.fromCharCode(...bytes.subarray(index, index + chunk));
  }
  return btoa(binary);
}

function fromBase64(value: string): Uint8Array {
  const binary = atob(value);
  const bytes = new Uint8Array(binary.length);
  for (let index = 0; index < binary.length; index++) bytes[index] = binary.charCodeAt(index);
  return bytes;
}

/* ------------------------------------------------------------------ */
/* Crypto                                                              */
/* ------------------------------------------------------------------ */

export async function deriveSyncKey(passphrase: string, saltBase64: string): Promise<CryptoKey> {
  const material = await crypto.subtle.importKey(
    "raw",
    new TextEncoder().encode(passphrase),
    "PBKDF2",
    false,
    ["deriveKey"],
  );
  return crypto.subtle.deriveKey(
    { name: "PBKDF2", salt: fromBase64(saltBase64) as unknown as BufferSource, iterations: ITERATIONS, hash: "SHA-256" },
    material,
    { name: "AES-GCM", length: 256 },
    false,
    ["encrypt", "decrypt"],
  );
}

async function encryptWithKey(key: CryptoKey, payload: unknown): Promise<{ iv: string; ciphertext: string }> {
  const iv = crypto.getRandomValues(new Uint8Array(12));
  const encoded = new TextEncoder().encode(JSON.stringify(payload));
  const cipher = await crypto.subtle.encrypt({ name: "AES-GCM", iv }, key, encoded);
  return { iv: toBase64(iv), ciphertext: toBase64(new Uint8Array(cipher)) };
}

async function decryptWithKey(key: CryptoKey, ivBase64: string, ciphertextBase64: string): Promise<unknown> {
  const plain = await crypto.subtle.decrypt(
    { name: "AES-GCM", iv: fromBase64(ivBase64) as unknown as BufferSource },
    key,
    fromBase64(ciphertextBase64) as unknown as BufferSource,
  );
  return JSON.parse(new TextDecoder().decode(plain));
}

/* ------------------------------------------------------------------ */
/* Server API                                                          */
/* ------------------------------------------------------------------ */

async function apiGet(id: string): Promise<{ updatedAt: number; envelope: SyncEnvelope } | null> {
  let response: Response;
  try {
    response = await fetch(`/api/sync?id=${encodeURIComponent(id)}`, {
      cache: "no-store",
      headers: { ...authHeaders(), ...localeHeader() },
    });
  } catch {
    throw new ApiError(t("settings.syncError"), "network");
  }
  if (response.status === 404) return null;
  if (!response.ok) {
    const body = (await response.json().catch(() => null)) as { error?: string } | null;
    throw new ApiError(body?.error ?? t("settings.syncError"), "sync");
  }
  return (await response.json()) as { updatedAt: number; envelope: SyncEnvelope };
}

async function apiPut(id: string, envelope: SyncEnvelope): Promise<number> {
  let response: Response;
  try {
    response = await fetch("/api/sync", {
      method: "POST",
      headers: { "Content-Type": "application/json", ...authHeaders(), ...localeHeader() },
      body: JSON.stringify({ id, envelope }),
    });
  } catch {
    throw new ApiError(t("settings.syncError"), "network");
  }
  if (!response.ok) {
    const body = (await response.json().catch(() => null)) as { error?: string } | null;
    throw new ApiError(body?.error ?? t("settings.syncError"), "sync");
  }
  const body = (await response.json()) as { updatedAt: number };
  return body.updatedAt;
}

async function apiDelete(id: string): Promise<void> {
  const response = await fetch(`/api/sync?id=${encodeURIComponent(id)}`, {
    method: "DELETE",
    headers: { ...authHeaders(), ...localeHeader() },
  });
  if (!response.ok) throw new ApiError(t("settings.syncError"), "sync");
}

/* ------------------------------------------------------------------ */
/* Payload                                                             */
/* ------------------------------------------------------------------ */

async function buildPayload(): Promise<SyncPayload> {
  await ensureLearningHydrated();
  return {
    app: "freezmo",
    kind: "sync",
    version: 1,
    clientUpdatedAt: Date.now(),
    data: {
      decks: deckStore.getSnapshot(),
      folders: folderStore.getSnapshot(),
      homework: homeworkStore.getSnapshot(),
      history: learningStore.getAnswersSnapshot(),
      srs: learningStore.getSrsSnapshot(),
    },
  };
}

async function applyRemote(key: CryptoKey, remote: { updatedAt: number; envelope: SyncEnvelope }): Promise<void> {
  const parsed = syncPayloadSchema.parse(
    await decryptWithKey(key, remote.envelope.iv, remote.envelope.ciphertext),
  );
  withoutDirtyTracking(() => {
    replaceFolders(parsed.data.folders);
    replaceDecks(parsed.data.decks);
    replaceHomework(parsed.data.homework);
  });
  await replaceLearning(parsed.data.history ?? [], parsed.data.srs ?? []);
  setSyncRecord({ lastSyncedAt: remote.updatedAt, dirtyAt: null, conflict: false });
}

/* ------------------------------------------------------------------ */
/* Public API                                                          */
/* ------------------------------------------------------------------ */

export async function syncNow(strategy: "auto" | "push" | "pull" = "auto"): Promise<SyncOutcome> {
  const record = syncStore.getSnapshot();
  if (!record.enabled || !record.id) return { status: "off" };
  const key = await loadDeviceKey();
  if (!key) return { status: "locked" };

  try {
    const remote = await apiGet(record.id);
    const remoteIsNewer = Boolean(remote && remote.updatedAt > (record.lastSyncedAt ?? 0));

    if (strategy === "pull") {
      if (!remote) return { status: "error", message: t("server.syncNotFound") };
      await applyRemote(key, remote);
      return { status: "pulled" };
    }

    if (strategy === "auto" && remoteIsNewer) {
      if (record.dirtyAt) {
        setSyncRecord({ conflict: true });
        return { status: "conflict" };
      }
      await applyRemote(key, remote!);
      return { status: "pulled" };
    }

    if (strategy === "auto" && !remoteIsNewer && !record.dirtyAt) {
      return { status: "up-to-date" };
    }

    const { iv, ciphertext } = await encryptWithKey(key, await buildPayload());
    const envelope: SyncEnvelope = {
      v: 1,
      kdf: KDF,
      iterations: ITERATIONS,
      salt: record.salt,
      iv,
      ciphertext,
    };
    const updatedAt = await apiPut(record.id, envelope);
    setSyncRecord({ lastSyncedAt: updatedAt, dirtyAt: null, conflict: false });
    return { status: "pushed" };
  } catch (error) {
    return {
      status: "error",
      message: error instanceof ApiError ? error.message : t("settings.syncError"),
    };
  }
}

/** First device: brand-new id and salt, uploads straight away. */
export async function enableSync(passphrase: string): Promise<SyncOutcome> {
  const id = newSyncId();
  const salt = toBase64(crypto.getRandomValues(new Uint8Array(16)));
  const key = await deriveSyncKey(passphrase, salt);
  await saveDeviceKey(key);
  setSyncRecord({ enabled: true, id, salt, lastSyncedAt: null, dirtyAt: Date.now(), conflict: false });
  return syncNow("push");
}

/** Same device after the browser dropped the stored key. */
export async function unlockSync(passphrase: string): Promise<SyncOutcome> {
  const record = syncStore.getSnapshot();
  if (!record.enabled || !record.id) return { status: "off" };
  try {
    const remote = await apiGet(record.id);
    const salt = remote?.envelope.salt || record.salt;
    if (!salt) return { status: "error", message: t("settings.syncError") };
    const key = await deriveSyncKey(passphrase, salt);
    if (remote) {
      try {
        syncPayloadSchema.parse(await decryptWithKey(key, remote.envelope.iv, remote.envelope.ciphertext));
      } catch {
        return { status: "error", message: t("settings.syncWrongPass") };
      }
    }
    await saveDeviceKey(key);
    setSyncRecord({ salt });
    return syncNow("auto");
  } catch (error) {
    return {
      status: "error",
      message: error instanceof ApiError ? error.message : t("settings.syncError"),
    };
  }
}

/** New device: the sync ID plus the passphrase from the other device. */
export async function joinSync(id: string, passphrase: string): Promise<SyncOutcome> {
  try {
    const remote = await apiGet(id.trim());
    if (!remote) return { status: "error", message: t("server.syncNotFound") };
    const key = await deriveSyncKey(passphrase, remote.envelope.salt);
    try {
      syncPayloadSchema.parse(await decryptWithKey(key, remote.envelope.iv, remote.envelope.ciphertext));
    } catch {
      return { status: "error", message: t("settings.syncWrongPass") };
    }
    await saveDeviceKey(key);
    setSyncRecord({
      enabled: true,
      id: id.trim(),
      salt: remote.envelope.salt,
      lastSyncedAt: null,
      dirtyAt: null,
      conflict: false,
    });
    await applyRemote(key, remote);
    return { status: "pulled" };
  } catch (error) {
    return {
      status: "error",
      message: error instanceof ApiError ? error.message : t("settings.syncError"),
    };
  }
}

export async function deleteSyncedData(): Promise<SyncOutcome> {
  const record = syncStore.getSnapshot();
  try {
    if (record.id) await apiDelete(record.id);
  } catch (error) {
    return {
      status: "error",
      message: error instanceof ApiError ? error.message : t("settings.syncError"),
    };
  }
  await clearDeviceKey();
  setSyncRecord({ enabled: false, id: "", salt: "", lastSyncedAt: null, dirtyAt: null, conflict: false });
  return { status: "up-to-date" };
}

export async function forgetDeviceKey(): Promise<void> {
  await clearDeviceKey();
}

export async function hasDeviceKey(): Promise<boolean> {
  return Boolean(await loadDeviceKey());
}