/**
 * MeshBay Browser Crypto — AES-256-GCM private group decryption.
 * Uses WebCrypto SubtleCrypto API (available in all modern browsers).
 *
 * Handles groups with cipher="aes-256-gcm" (browser-accessible groups).
 * ChaCha20-Poly1305 groups (cipher="chacha20-poly1305") require the
 * native client (node) for decryption — not supported in browser.
 *
 * Usage:
 *   const gek = await importGEK(gekB64);
 *   const plaintext = await decryptChunk(gek, fileHashHex, chunkIndex, nonceB64, ctB64);
 */

const CIPHER_INFO_PREFIX = new TextEncoder().encode('file:');
const CIPHER_INFO_SUFFIX_AES = new TextEncoder().encode(':aes');


// ── Key derivation ────────────────────────────────────────────────────────────

/**
 * Import a raw GEK (base64) as a WebCrypto key for HKDF.
 * @param {string} gekB64 - base64-encoded GEK (32 bytes)
 * @returns {Promise<CryptoKey>}
 */
async function importGEK(gekB64) {
  const raw = b64decode(gekB64);
  return crypto.subtle.importKey('raw', raw, 'HKDF', false, ['deriveKey', 'deriveBits']);
}

/**
 * Derive a per-chunk AES-256-GCM key from the GEK.
 * Mirrors meshbay_common/webcrypto.py::chunk_key_aes().
 *
 * @param {CryptoKey} gek - HKDF key from importGEK()
 * @param {string} fileHashHex - blake3 hash of file (hex, 64 chars)
 * @param {number} chunkIndex
 * @returns {Promise<CryptoKey>}
 */
async function deriveChunkKey(gek, fileHashHex, chunkIndex) {
  // Build HKDF info: "file:" + file_hash_bytes + ":chunk:" + uint32be + ":aes"
  const fileHashBytes = hexToBytes(fileHashHex);
  const chunkIdxBytes = new Uint8Array(4);
  new DataView(chunkIdxBytes.buffer).setUint32(0, chunkIndex, false);   // big-endian

  const infoParts = [
    new TextEncoder().encode('file:'),
    fileHashBytes,
    new TextEncoder().encode(':chunk:'),
    chunkIdxBytes,
    new TextEncoder().encode(':aes'),
  ];
  const info = concatBuffers(infoParts);

  return crypto.subtle.deriveKey(
    { name: 'HKDF', hash: 'SHA-256', salt: new Uint8Array(0), info },
    gek,
    { name: 'AES-GCM', length: 256 },
    false,
    ['encrypt', 'decrypt'],
  );
}


// ── Decryption ────────────────────────────────────────────────────────────────

/**
 * Decrypt one chunk of a private group file.
 * @param {CryptoKey} gek - from importGEK()
 * @param {string} fileHashHex
 * @param {number} chunkIndex
 * @param {string} nonceB64 - 12-byte nonce, base64
 * @param {string} ctB64 - ciphertext + GCM tag, base64
 * @returns {Promise<Uint8Array>} plaintext
 */
async function decryptChunk(gek, fileHashHex, chunkIndex, nonceB64, ctB64) {
  const chunkKey = await deriveChunkKey(gek, fileHashHex, chunkIndex);
  const nonce = b64decode(nonceB64);
  const ct    = b64decode(ctB64);
  const plaintext = await crypto.subtle.decrypt(
    { name: 'AES-GCM', iv: nonce },
    chunkKey,
    ct,
  );
  return new Uint8Array(plaintext);
}

/**
 * Decrypt a full file by fetching and decrypting all chunks in order.
 * @param {CryptoKey} gek
 * @param {string} nodeUrl - base URL of node HTTP API
 * @param {string} fileId - blake3 hex hash (= file_id in index)
 * @param {string} fileHashHex - same as fileId (blake3 of file content)
 * @param {string} jwtToken
 * @returns {Promise<Blob>} decrypted file as Blob
 */
async function decryptFile(gek, nodeUrl, fileId, fileHashHex, jwtToken) {
  const chunks = [];
  let chunkIdx = 0;

  while (true) {
    const sep = nodeUrl.includes('?') ? '&' : '?';
    const url = `${nodeUrl}/file/${fileId}/${chunkIdx}${sep}token=${jwtToken}`;
    const resp = await fetch(url);
    if (!resp.ok) break;

    const data = await resp.json();
    if (data.encrypted === false) {
      // Public group: data_b64 is plaintext
      chunks.push(b64decode(data.data_b64));
    } else {
      // Private group with AES-256-GCM
      const plain = await decryptChunk(
        gek, fileHashHex, chunkIdx,
        data.nonce_b64, data.ct_b64,
      );
      chunks.push(plain);
    }

    if (data.plaintext_size < 1024 * 1024) break;   // last chunk (< 1MB)
    chunkIdx++;
  }

  return new Blob(chunks);
}


// ── Helpers ───────────────────────────────────────────────────────────────────

function b64decode(b64) {
  const binary = atob(b64);
  const bytes = new Uint8Array(binary.length);
  for (let i = 0; i < binary.length; i++) bytes[i] = binary.charCodeAt(i);
  return bytes;
}

function hexToBytes(hex) {
  const bytes = new Uint8Array(hex.length / 2);
  for (let i = 0; i < hex.length; i += 2)
    bytes[i / 2] = parseInt(hex.substring(i, i + 2), 16);
  return bytes;
}

function concatBuffers(arrays) {
  const total = arrays.reduce((s, a) => s + a.byteLength, 0);
  const result = new Uint8Array(total);
  let offset = 0;
  for (const arr of arrays) {
    result.set(new Uint8Array(arr.buffer || arr), offset);
    offset += arr.byteLength;
  }
  return result;
}

async function decryptChunkBin(gek, fileHashHex, chunkIndex, nonce, ct) {
  const chunkKey = await deriveChunkKey(gek, fileHashHex, chunkIndex);
  const plaintext = await crypto.subtle.decrypt(
    { name: 'AES-GCM', iv: nonce }, chunkKey, ct);
  return new Uint8Array(plaintext);
}

// ── GEK generation + ECIES wrapping ──────────────────────────────────────────

function generateGEK() {
  return crypto.getRandomValues(new Uint8Array(32));
}

async function wrapGEK(gek, pkXRaw) {
  const skEph = await crypto.subtle.generateKey({ name: 'X25519' }, true, ['deriveBits']);
  const pkEphRaw = new Uint8Array(await crypto.subtle.exportKey('raw', skEph.publicKey));

  const pkRecip = await crypto.subtle.importKey('raw', pkXRaw, { name: 'X25519' }, false, []);
  const sharedBits = await crypto.subtle.deriveBits(
    { name: 'X25519', public: pkRecip }, skEph.privateKey, 256);

  const sharedKey = await crypto.subtle.importKey(
    'raw', sharedBits, 'HKDF', false, ['deriveKey']);
  const wrapKey = await crypto.subtle.deriveKey(
    { name: 'HKDF', hash: 'SHA-256', salt: pkEphRaw,
      info: new TextEncoder().encode('meshbay:gek_wrap:v1:aes') },
    sharedKey,
    { name: 'AES-GCM', length: 256 }, false, ['encrypt']);

  const nonce = crypto.getRandomValues(new Uint8Array(12));
  const ct = await crypto.subtle.encrypt(
    { name: 'AES-GCM', iv: nonce, additionalData: pkXRaw }, wrapKey, gek);

  return {
    pk_eph_b64: btoa(String.fromCharCode(...pkEphRaw)),
    nonce_b64: btoa(String.fromCharCode(...nonce)),
    wrapped_b64: btoa(String.fromCharCode(...new Uint8Array(ct))),
  };
}

async function unwrapGEK(bundle, skXPkcs8, pkXRaw) {
  const pkEphRaw = b64decode(bundle.pk_eph_b64);
  const nonce = b64decode(bundle.nonce_b64);
  const wrapped = b64decode(bundle.wrapped_b64);

  const skX = await crypto.subtle.importKey(
    'pkcs8', skXPkcs8, { name: 'X25519' }, false, ['deriveBits']);
  const pkEph = await crypto.subtle.importKey(
    'raw', pkEphRaw, { name: 'X25519' }, false, []);
  const sharedBits = await crypto.subtle.deriveBits(
    { name: 'X25519', public: pkEph }, skX, 256);

  const sharedKey = await crypto.subtle.importKey(
    'raw', sharedBits, 'HKDF', false, ['deriveKey']);
  const wrapKey = await crypto.subtle.deriveKey(
    { name: 'HKDF', hash: 'SHA-256', salt: pkEphRaw,
      info: new TextEncoder().encode('meshbay:gek_wrap:v1:aes') },
    sharedKey,
    { name: 'AES-GCM', length: 256 }, false, ['decrypt']);

  const plain = await crypto.subtle.decrypt(
    { name: 'AES-GCM', iv: nonce, additionalData: pkXRaw }, wrapKey, wrapped);
  return new Uint8Array(plain);
}

// ── Chunk encryption (for upload) ────────────────────────────────────────────

async function encryptChunk(gek, fileHashHex, chunkIndex, plaintext) {
  const chunkKey = await deriveChunkKey(gek, fileHashHex, chunkIndex);
  const nonce = crypto.getRandomValues(new Uint8Array(12));
  const ct = await crypto.subtle.encrypt(
    { name: 'AES-GCM', iv: nonce }, chunkKey, plaintext);
  return { nonce, ct: new Uint8Array(ct) };
}

function b64encode(bytes) {
  return btoa(String.fromCharCode(...bytes));
}

// ── Admin operation transcript ───────────────────────────────────────────────
// Mirrors meshbay_common/adminop.py::admin_transcript(). Both sides build these
// bytes independently; they are never taken off the wire.
//
// Finding H5: the client used to sign 32 raw random bytes chosen by the node — a
// blind signing oracle. It now reconstructs a domain-separated, length-prefixed
// transcript naming the operation, subject, node and group, so the UI can show the
// user what they are authorizing and a signature cannot be reused elsewhere.

const ADMIN_TRANSCRIPT_PREFIX = new TextEncoder().encode('meshbay:admin:v1');

function adminTranscript(op, nodePkB64, groupId, subject, nonceB64, ts) {
  const enc = new TextEncoder();
  const fields = [
    enc.encode(op),
    enc.encode(nodePkB64),
    enc.encode(groupId),
    enc.encode(subject),
    b64decode(nonceB64),
    enc.encode(String(ts)),
  ];
  let total = ADMIN_TRANSCRIPT_PREFIX.length;
  for (const f of fields) total += 4 + f.length;

  const out = new Uint8Array(total);
  out.set(ADMIN_TRANSCRIPT_PREFIX, 0);
  let off = ADMIN_TRANSCRIPT_PREFIX.length;
  for (const f of fields) {
    new DataView(out.buffer).setUint32(off, f.length, false);
    off += 4;
    out.set(f, off);
    off += f.length;
  }
  return out;
}

// ── GEK proof (HMAC-SHA256 for handshake challenge) ─────────────────────────

// Mirrors meshbay_common/handshake.py. Every field length-prefixed and the role
// bound in, so a client proof can never be replayed as a node proof and a missing
// fingerprint cannot silently degrade the proof to nonce-only (L4).
const HANDSHAKE_PREFIX = new TextEncoder().encode('meshbay:mnp:handshake:v1');

function _lenPrefixed(parts) {
  let total = 0;
  for (const p of parts) total += 4 + p.length;
  const out = new Uint8Array(total);
  const view = new DataView(out.buffer);
  let off = 0;
  for (const p of parts) {
    view.setUint32(off, p.length, false);
    off += 4;
    out.set(p, off);
    off += p.length;
  }
  return out;
}

function webrtcBinding(offerFp, answerFp) {
  if (!offerFp || !offerFp.length || !answerFp || !answerFp.length) {
    throw new Error('Channel binding unavailable — refusing to handshake');
  }
  return _lenPrefixed([offerFp, answerFp]);
}

function handshakeTranscript(role, groupId, nonceClient, nonceNode, binding) {
  const enc = new TextEncoder();
  const body = _lenPrefixed([
    enc.encode(role), enc.encode(groupId), nonceClient, nonceNode, binding,
  ]);
  const out = new Uint8Array(HANDSHAKE_PREFIX.length + body.length);
  out.set(HANDSHAKE_PREFIX, 0);
  out.set(body, HANDSHAKE_PREFIX.length);
  return out;
}

async function handshakeProof(gekRaw, role, groupId, nonceClient, nonceNode, binding) {
  const transcript = handshakeTranscript(role, groupId, nonceClient, nonceNode, binding);
  const key = await crypto.subtle.importKey(
    'raw', gekRaw, { name: 'HMAC', hash: 'SHA-256' }, false, ['sign']);
  const sig = await crypto.subtle.sign('HMAC', key, transcript);
  return new Uint8Array(sig);
}

// ── Join / pairing transcript ───────────────────────────────────────────────

// Mirrors meshbay_common/join.py. Signing both of our public keys together binds
// the X25519 key to the Ed25519 identity the node pins, so the node can safely
// wrap the group key for a key that came over the wire instead of one fetched
// from the hub's directory (H3). nonce_node ties it to this connection.
const JOIN_PREFIX = new TextEncoder().encode('meshbay:join:v1');

function joinTranscript(nodePkB64, groupId, userId, pkEdB64, pkXB64, nonceNode, ts) {
  const enc = new TextEncoder();
  const body = _lenPrefixed([
    enc.encode(nodePkB64),
    enc.encode(groupId),
    enc.encode(userId),
    enc.encode(pkEdB64),
    enc.encode(pkXB64),
    nonceNode,
    enc.encode(String(ts)),
  ]);
  const out = new Uint8Array(JOIN_PREFIX.length + body.length);
  out.set(JOIN_PREFIX, 0);
  out.set(body, JOIN_PREFIX.length);
  return out;
}

function constantTimeEqual(a, b) {
  if (a.length !== b.length) return false;
  let diff = 0;
  for (let i = 0; i < a.length; i++) diff |= a[i] ^ b[i];
  return diff === 0;
}

/** Verify the node's Ed25519 signature over the handshake transcript (C3). */
async function verifyNodeSignature(nodePkB64, sigB64, transcript) {
  const raw = b64decode(nodePkB64);
  const key = await crypto.subtle.importKey('raw', raw, { name: 'Ed25519' }, false, ['verify']);
  return crypto.subtle.verify('Ed25519', key, b64decode(sigB64), transcript);
}

// Export for use in app.js
window.MeshBayCrypto = {
  importGEK, deriveChunkKey, decryptChunk, decryptChunkBin, decryptFile,
  generateGEK, wrapGEK, unwrapGEK, encryptChunk, b64encode, b64decode,
  adminTranscript, handshakeTranscript, handshakeProof, webrtcBinding,
  joinTranscript, verifyNodeSignature, constantTimeEqual,
};