/**
 * Noise NK initiator for Android / Capacitor.
 * Pattern: Noise_NK_25519_ChaChaPoly_BLAKE2s
 *
 * The Android client is a hand-port of this code and must derive byte-for-byte
 * the same handshake — interop holds by porting discipline, not by a shared
 * library. The earlier Rust `snow` host has been removed from this repo, so
 * there is no cross-implementation pin; `noise-js.test.ts` pins SELF-derived
 * vectors only (a regression guard against accidental changes on the JS side).
 *
 * PCH-01-002: this implementation derives `h` and HKDF over BLAKE2s to match the
 * named pattern. A stock `snow` stack would interoperate; the Android port must
 * mirror the same hash choices. Any change to the hash, HKDF, or transcript here
 * is a BREAKING wire change and requires a coordinated Android release.
 */

import { generateKeyPair, sharedKey } from "@stablelib/x25519";
import { ChaCha20Poly1305 } from "@stablelib/chacha20poly1305";
import { BLAKE2s } from "@stablelib/blake2s";
import { SHA256 } from "@stablelib/sha256";
import { HMAC } from "@stablelib/hmac";

const PROTOCOL_NAME = "Noise_NK_25519_ChaChaPoly_BLAKE2s";

export function initHash(): Uint8Array {
  const nameBytes = new TextEncoder().encode(PROTOCOL_NAME);
  if (nameBytes.length <= 32) {
    const h = new Uint8Array(32);
    h.set(nameBytes);
    return h;
  }
  // PCH-01-002: the named pattern hash is BLAKE2s; the protocol-name init hash
  // MUST use it too (Noise spec §5.2), not SHA-256.
  return new BLAKE2s(32).update(nameBytes).digest();
}

function mixHash(h: Uint8Array, data: Uint8Array): Uint8Array {
  const blake = new BLAKE2s(32);
  blake.update(h);
  blake.update(data);
  return blake.digest();
}

export function hkdf2(ck: Uint8Array, ikm: Uint8Array): [Uint8Array, Uint8Array] {
  // PCH-01-002: HKDF must run over the named hash (BLAKE2s), not SHA-256.
  // @stablelib's HKDF reuses one HMAC across blocks and BLAKE2s's restoreState()
  // never clears its `finished` flag, so `new HKDF(BLAKE2s, …)` throws. Run
  // standard HKDF-Expand (RFC 5869, empty info) with a FRESH HMAC-BLAKE2s per
  // block — byte-identical to BouncyCastle `HKDFBytesGenerator(Blake2sDigest)`
  // on peach-android. NOTE: the HMAC-SHA256 post-handshake auth proof is a
  // SEPARATE construction, intentionally unchanged (so the SHA256 import stays).
  const prk = new HMAC(BLAKE2s, ck).update(ikm).digest(); // Extract (salt=ck)
  const k1 = new HMAC(BLAKE2s, prk).update(Uint8Array.of(1)).digest();
  const k2 = new HMAC(BLAKE2s, prk).update(k1).update(Uint8Array.of(2)).digest();
  return [k1, k2];
}

export interface NoiseNKSession {
  encrypt(plaintext: Uint8Array): Uint8Array;
  decrypt(ciphertext: Uint8Array): Uint8Array;
  handshakeHash: Uint8Array;
}

/**
 * Run the initiator side of the Noise NK handshake. The caller provides
 * `send` / `receive` for the two transport-side handshake messages and gets
 * back a session object that encrypts/decrypts subsequent transport frames.
 */
export async function noiseNKInitiator(
  responderStaticPublicKey: Uint8Array,
  send: (msg: Uint8Array) => Promise<void>,
  receive: () => Promise<Uint8Array>,
): Promise<NoiseNKSession> {
  let h: Uint8Array = initHash();
  let ck: Uint8Array = h.slice();

  // NK pre-message: mix responder static public key.
  h = mixHash(h, responderStaticPublicKey);

  const ephemeral = generateKeyPair();

  // --- Message 1: e, es ---
  h = mixHash(h, ephemeral.publicKey);
  // PCH-02-009: reject all-zero DH outputs from low-order peer points before
  // they enter the Noise key schedule. Honest peers are unaffected.
  const es = sharedKey(ephemeral.secretKey, responderStaticPublicKey, true);
  const [ck1, k1] = hkdf2(ck, es);
  ck = ck1;

  const cipher1 = new ChaCha20Poly1305(k1);
  const nonce1 = new Uint8Array(12);
  const encrypted1 = cipher1.seal(nonce1, new Uint8Array(0), h);
  h = mixHash(h, encrypted1);

  const msg1 = new Uint8Array(32 + encrypted1.length);
  msg1.set(ephemeral.publicKey, 0);
  msg1.set(encrypted1, 32);
  await send(msg1);

  // --- Message 2: e, ee ---
  const msg2 = await receive();
  if (msg2.length < 48) throw new Error("Noise NK: msg2 too short");

  const remoteEphemeralPublic = msg2.slice(0, 32);
  const encrypted2 = msg2.slice(32);

  h = mixHash(h, remoteEphemeralPublic);
  const ee = sharedKey(ephemeral.secretKey, remoteEphemeralPublic, true);
  const [ck2, k2] = hkdf2(ck, ee);
  ck = ck2;

  const cipher2 = new ChaCha20Poly1305(k2);
  const nonce2 = new Uint8Array(12);
  const decrypted2 = cipher2.open(nonce2, encrypted2, h);
  if (!decrypted2) {
    throw new Error("Noise NK: msg2 auth failed — wrong responder key or MITM");
  }
  h = mixHash(h, encrypted2);

  // --- Transport phase ---
  const [sendKey, recvKey] = hkdf2(ck, new Uint8Array(0));
  const handshakeHash = h.slice();

  let sendNonce = 0n;
  let recvNonce = 0n;

  function nonceBytes(n: bigint): Uint8Array {
    const buf = new Uint8Array(12);
    new DataView(buf.buffer).setBigUint64(4, n, true);
    return buf;
  }

  const sendCipher = new ChaCha20Poly1305(sendKey);
  const recvCipher = new ChaCha20Poly1305(recvKey);

  return {
    encrypt(plaintext) {
      return sendCipher.seal(nonceBytes(sendNonce++), plaintext, new Uint8Array(0));
    },
    decrypt(ciphertext) {
      const result = recvCipher.open(nonceBytes(recvNonce++), ciphertext, new Uint8Array(0));
      if (!result) throw new Error("Noise NK: decryption failed");
      return result;
    },
    handshakeHash,
  };
}

/**
 * Compute the post-handshake auth proof.
 *
 * proof = HMAC-SHA256(
 *   key   = X25519(local_priv, remote_pub),  // 32-byte DH shared
 *   data  = "peach-sync-v1-initiator-auth"
 *        || initiator_public
 *        || responder_public
 *        || nonce_32
 * )
 *
 * Both sides pass the same DH peer key (their PEER's public key) and the same
 * initiator_public/responder_public ordering so the MAC matches. This helper
 * targets the initiator-side: `local_priv` is the initiator's private key,
 * `remote_pub` is the responder's public key.
 */
const AUTH_DOMAIN = new TextEncoder().encode("peach-sync-v1-initiator-auth");

export function computeAuthProof(
  initiatorPrivate: Uint8Array,
  initiatorPublic: Uint8Array,
  responderPublic: Uint8Array,
  nonce: Uint8Array,
  handshakeHash: Uint8Array,
): Uint8Array {
  if (nonce.length !== 32) throw new Error("auth nonce must be 32 bytes");
  if (handshakeHash.length !== 32) throw new Error("handshake hash must be 32 bytes");

  const dh = sharedKey(initiatorPrivate, responderPublic, true);
  const mac = new HMAC(SHA256, dh);
  // PCH-01-003b: bind the proof to THIS Noise session so it cannot be replayed
  // onto a different handshake. handshakeHash is prepended; both sides hold the
  // identical value and feed it in the same position.
  mac.update(handshakeHash);
  mac.update(AUTH_DOMAIN);
  mac.update(initiatorPublic);
  mac.update(responderPublic);
  mac.update(nonce);
  return mac.digest();
}

interface AuthChallengeFrame {
  type: "CHALLENGE";
  nonce_hex: string;
  responder_public_key_hex: string;
}

interface AuthProofFrame {
  type: "PROOF";
  initiator_public_key_hex: string;
  proof_hex: string;
}

function bytesToHex(bytes: Uint8Array): string {
  return Array.from(bytes, (b) => b.toString(16).padStart(2, "0")).join("");
}

function hexToBytes(hex: string): Uint8Array {
  if (hex.length % 2 !== 0) throw new Error("invalid hex");
  const bytes = new Uint8Array(hex.length / 2);
  for (let i = 0; i < hex.length; i += 2) {
    bytes[i / 2] = parseInt(hex.slice(i, i + 2), 16);
  }
  return bytes;
}

/**
 * Drive the initiator side of the post-handshake challenge/proof round. The
 * responder sends a `Challenge` JSON frame; we compute the HMAC and reply
 * with a `Proof` frame. Both frames are encrypted via the post-handshake
 * Noise transport (caller owns the `sendEncrypted` / `receiveEncrypted` pair).
 */
/**
 * Run the responder side of the Noise NK handshake. Symmetric with
 * `noiseNKInitiator` — the caller provides `send` / `receive` for the two
 * handshake messages and gets back a session for transport-phase frames.
 *
 * Symmetric with the Android hand-port; no shared Rust implementation remains.
 * An Android-as-host pairing session can be driven by the desktop initiator and
 * vice versa as long as both derive over BLAKE2s (PCH-01-002).
 */
export async function noiseNKResponder(
  ourStaticPrivateKey: Uint8Array,
  ourStaticPublicKey: Uint8Array,
  send: (msg: Uint8Array) => Promise<void>,
  receive: () => Promise<Uint8Array>,
): Promise<NoiseNKSession> {
  let h: Uint8Array = initHash();
  let ck: Uint8Array = h.slice();

  // NK pre-message: mix our (responder) static public key. The initiator
  // pre-mixed the same key on its side; both transcripts must match.
  h = mixHash(h, ourStaticPublicKey);

  // --- Message 1: e, es ---
  const msg1 = await receive();
  if (msg1.length < 48) throw new Error("Noise NK: msg1 too short");
  const remoteEphemeralPublic = msg1.slice(0, 32);
  const encrypted1 = msg1.slice(32);

  h = mixHash(h, remoteEphemeralPublic);
  const es = sharedKey(ourStaticPrivateKey, remoteEphemeralPublic, true);
  const [ck1, k1] = hkdf2(ck, es);
  ck = ck1;

  const cipher1 = new ChaCha20Poly1305(k1);
  const nonce1 = new Uint8Array(12);
  const decrypted1 = cipher1.open(nonce1, encrypted1, h);
  if (!decrypted1) {
    throw new Error("Noise NK: msg1 auth failed — wrong static key or MITM");
  }
  h = mixHash(h, encrypted1);

  // --- Message 2: e, ee ---
  const ephemeral = generateKeyPair();
  h = mixHash(h, ephemeral.publicKey);
  const ee = sharedKey(ephemeral.secretKey, remoteEphemeralPublic, true);
  const [ck2, k2] = hkdf2(ck, ee);
  ck = ck2;

  const cipher2 = new ChaCha20Poly1305(k2);
  const nonce2 = new Uint8Array(12);
  const encrypted2 = cipher2.seal(nonce2, new Uint8Array(0), h);
  h = mixHash(h, encrypted2);

  const msg2 = new Uint8Array(32 + encrypted2.length);
  msg2.set(ephemeral.publicKey, 0);
  msg2.set(encrypted2, 32);
  await send(msg2);

  // --- Transport phase ---
  // hkdf2 returns (k1, k2); the initiator uses k1 to send and k2 to receive,
  // so the responder swaps: k1 → recv, k2 → send.
  const [initiatorSendKey, responderSendKey] = hkdf2(ck, new Uint8Array(0));
  const handshakeHash = h.slice();

  let sendNonce = 0n;
  let recvNonce = 0n;

  function nonceBytes(n: bigint): Uint8Array {
    const buf = new Uint8Array(12);
    new DataView(buf.buffer).setBigUint64(4, n, true);
    return buf;
  }

  const sendCipher = new ChaCha20Poly1305(responderSendKey);
  const recvCipher = new ChaCha20Poly1305(initiatorSendKey);

  return {
    encrypt(plaintext) {
      return sendCipher.seal(nonceBytes(sendNonce++), plaintext, new Uint8Array(0));
    },
    decrypt(ciphertext) {
      const result = recvCipher.open(nonceBytes(recvNonce++), ciphertext, new Uint8Array(0));
      if (!result) throw new Error("Noise NK: decryption failed");
      return result;
    },
    handshakeHash,
  };
}

/**
 * Constant-time 32-byte equality. The MAC comparison in `verifyAuthProof`
 * goes through this so a same-LAN attacker can't time the HMAC compare.
 */
function constantTimeEq(a: Uint8Array, b: Uint8Array): boolean {
  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;
}

/**
 * Drive the responder side of the post-handshake challenge/proof round. We
 * generate a random 32-byte nonce, send `Challenge`, receive `Proof`, and
 * verify. Returns the initiator's authenticated public key on success.
 *
 * The MAC key is X25519(our_priv, initiator_pub) and the data is the same
 * domain-separated transcript both sides feed into HMAC-SHA256.
 */
export async function performResponderAuth(
  session: NoiseNKSession,
  ourPrivateKey: Uint8Array,
  ourPublicKey: Uint8Array,
  sendEncryptedJson: (data: Uint8Array) => Promise<void>,
  receiveEncryptedJson: () => Promise<Uint8Array>,
): Promise<{ initiatorPublicKey: Uint8Array }> {
  // PCH-01-003b: bind the verifier's recomputed MAC to this Noise session.

  const nonce = new Uint8Array(32);
  crypto.getRandomValues(nonce);

  const challenge: AuthChallengeFrame = {
    type: "CHALLENGE",
    nonce_hex: bytesToHex(nonce),
    responder_public_key_hex: bytesToHex(ourPublicKey),
  };
  await sendEncryptedJson(new TextEncoder().encode(JSON.stringify(challenge)));

  const proofBytes = await receiveEncryptedJson();
  const proofFrame = JSON.parse(new TextDecoder().decode(proofBytes)) as AuthProofFrame;
  if (proofFrame.type !== "PROOF") {
    throw new Error("expected PROOF frame");
  }

  const initiatorPublicKey = hexToBytes(proofFrame.initiator_public_key_hex);
  const claimedProof = hexToBytes(proofFrame.proof_hex);
  if (initiatorPublicKey.length !== 32) {
    throw new Error("initiator public key must be 32 bytes");
  }
  if (claimedProof.length !== 32) {
    throw new Error("proof must be 32 bytes");
  }

  // mac_for(our_priv, initiator_pub_for_dh, initiator_pub, our_pub, nonce)
  const dh = sharedKey(ourPrivateKey, initiatorPublicKey, true);
  const mac = new HMAC(SHA256, dh);
  // PCH-01-003b: must mirror computeAuthProof — handshakeHash first, then the
  // existing domain/keys/nonce transcript. Same value, same position on both ends.
  mac.update(session.handshakeHash);
  mac.update(AUTH_DOMAIN);
  mac.update(initiatorPublicKey);
  mac.update(ourPublicKey);
  mac.update(nonce);
  const expected = mac.digest();

  if (!constantTimeEq(expected, claimedProof)) {
    throw new Error("auth proof did not verify");
  }
  return { initiatorPublicKey };
}

export async function performInitiatorAuth(
  session: NoiseNKSession,
  ourPrivateKey: Uint8Array,
  ourPublicKey: Uint8Array,
  responderPublicKey: Uint8Array,
  sendEncryptedJson: (data: Uint8Array) => Promise<void>,
  receiveEncryptedJson: () => Promise<Uint8Array>,
): Promise<void> {
  // PCH-01-003b: the Noise handshake hash is now mixed into the proof, binding
  // it to this exact session (was previously bound via the DH key only).

  const challengeBytes = await receiveEncryptedJson();
  const challenge = JSON.parse(new TextDecoder().decode(challengeBytes)) as AuthChallengeFrame;
  if (challenge.type !== "CHALLENGE") {
    throw new Error("expected CHALLENGE frame");
  }
  const nonce = hexToBytes(challenge.nonce_hex);
  const responderKeyFromChallenge = hexToBytes(challenge.responder_public_key_hex);
  // Defence against an off-key challenge: the responder's claimed key MUST be
  // the one we Noise-NK-handshook against, otherwise an active MITM that
  // managed to substitute a different key is rerouting the auth round.
  if (
    responderKeyFromChallenge.length !== responderPublicKey.length ||
    !responderKeyFromChallenge.every((b, i) => b === responderPublicKey[i])
  ) {
    throw new Error("responder key in challenge does not match Noise handshake");
  }

  const proof = computeAuthProof(
    ourPrivateKey,
    ourPublicKey,
    responderPublicKey,
    nonce,
    session.handshakeHash,
  );
  const proofFrame: AuthProofFrame = {
    type: "PROOF",
    initiator_public_key_hex: bytesToHex(ourPublicKey),
    proof_hex: bytesToHex(proof),
  };
  await sendEncryptedJson(new TextEncoder().encode(JSON.stringify(proofFrame)));
}

export { bytesToHex, hexToBytes };