path: root/webAO/ext_packet.ts

import { client } from "./client";

const VLI32_MAXBYTES = 5;

function encodeVli32(u: number): Uint8Array {
  if (u >> 7 === 0) {
    return new Uint8Array([(u << 1) | 0x1]);
  }
  let shift = u >> 7;
  for (let bytes = 2; bytes < 5; ++bytes) {
    shift >>= 7;
    if (shift === 0) {
      const result = new Uint8Array(bytes);
      const encoded = ((u << 1) | 0x1) << (bytes - 1);
      for (let i = 0; i < bytes; ++i) {
        result[i] = (encoded >> (i * 8)) & 0xff;
      }
      return result;
    }
  }
  const result = new Uint8Array(5);
  result[0] = 0;
  for (let i = 1; i < 5; ++i) {
    result[i] = (u >> (i * 8)) & 0xff;
  }
}

function decodeVli32(p: Uint8Array): [number, number] {
  const u = p[0];
  if (u & 1) {
    return [u >> 1, 1];
  }
  let result = 0;
  if ((u & 0xf) === 0) {
    for (let i = 1; i < 5; ++i) {
      result |= p[i] << (i * 8);
    }
    return [result, 5];
  }
  // ctz in terms of clz
  const bytes = 31 - Math.clz32(u & -u);
  result = u >> (bytes + 1);
  for (let i = 1; i < bytes + 1; ++i) {
    result |= p[i] << (i * 8);
  }
  return [result, 1 + bytes];
}

// Wrappers around buffers for parsing and serialization.

class Cursor {
  buf: Uint8Array;
  offset: number;

  constructor(buf: Uint8Array, offset = 0) {
    this.buf = buf;
    this.offset = offset;
  }

  remaining(): number {
    return this.buf.length - this.offset;
  }

  skip(i: number) {
    if (this.remaining() < i) {
      throw new Error();
    }
    this.offset += i;
  }

  readExact(i: number): Uint8Array {
    if (this.remaining() < i) {
      throw new Error();
    }
    const out = this.buf.subarray(this.offset, this.offset + i);
    this.offset += i;
    return out;
  }

  readVli32(): number {
    const result = decodeVli32(this.buf.subarray(this.offset));
    if (result === null) {
      throw new Error();
    }
    const [u, bytes] = result;
    this.offset += bytes;
    return u;
  }

  readVli32s(): number {
    const result = this.readVli32();
    return (result >> 1) ^ -(result & 1);
  }

  readArrayVli32s(): Int32Array {
    const count = this.readVli32();
    const result = new Int32Array(count);
    for (let i = 0; i < count; ++i) {
      result[i] = this.readVli32s();
    }
    return result;
  }

  readBytes(): Uint8Array {
    const len = this.readVli32();
    return this.readExact(len);
  }

  readText(): string {
    const len = this.readVli32();
    const utf8bytes = this.readExact(len);
    return new TextDecoder().decode(utf8bytes);
  }
}

class Writer {
  buf: Uint8Array;
  offset: number;

  constructor(buf: Uint8Array) {
    this.buf = buf;
    this.offset = 0;
  }

  writeByte(b: number) {
    this.buf[this.offset] = b;
    this.offset += 1;
  }

  writeVli32(u: number) {
    const result = encodeVli32(u);
    this.buf.set(result, this.offset);
    this.offset += result.length;
  }

  writeFixed(bs: Uint8Array) {
    this.buf.set(bs, this.offset);
    this.offset += bs.length;
  }

  writeBytes(bs: Uint8Array) {
    this.writeVli32(bs.length);
    this.writeFixed(bs);
  }

  out(): Uint8Array {
    return this.buf.subarray(0, this.offset);
  }
}

export enum ExMsgType {
  AuthRequest = 2,
  RegisterCredential = 5,
  RegistrationFinish = 6,
  AssertCredential = 7,
  AssertionFinish = 8,
}

function serializeMessage(type: ExMsgType, body: Uint8Array): Uint8Array {
  const buf = new Uint8Array(1 + body.length);
  buf[0] = type;
  buf.set(body, 1);
  return buf;
}

const CHALLENGE_BYTES = 32;
const USERID_BYTES = 16;

const enum CredentialType {
  Passkey = 1,
}

interface PasskeyCredential {
  challenge: Uint8Array;
  rpId: string;
  userId: Uint8Array;
  userName: string;
  algorithms: Int32Array;
}

function parsePasskeyCredential(buf: Uint8Array): PasskeyCredential | null {
  const cur = new Cursor(buf);
  try {
    // Skip the presence bitmap.
    cur.skip(1);
    const challenge = cur.readExact(CHALLENGE_BYTES);
    const rpId = cur.readText();
    const userId = cur.readExact(USERID_BYTES);
    const userName = cur.readText();
    const algorithms = cur.readArrayVli32s();
    return { challenge, rpId, userId, userName, algorithms };
  } catch {
    return null;
  }
}

// The default RPID is the origin's domain name, but the server also provides
// one.
// From spec: "Relying Parties SHOULD set [requireResidentKey] to true if, and
// only if, residentKey is set to required."
// We require a discoverable (resident) credential for a single-factor auth.
function extractCreationOptions(
  cred: PasskeyCredential,
): PublicKeyCredentialCreationOptions {
  return {
    authenticatorSelection: {
      residentKey: "required",
      requireResidentKey: true,
      userVerification: "required",
    },
    challenge: cred.challenge as Uint8Array<ArrayBuffer>,
    pubKeyCredParams: Array.from(cred.algorithms, (alg) => ({
      alg,
      type: "public-key",
    })),
    rp: {
      id: cred.rpId,
      name: "",
    },
    user: {
      displayName: "",
      id: cred.userId as Uint8Array<ArrayBuffer>,
      name: cred.userName,
    },
    timeout: 300000,
  };
}

// The previous one was for registration, this one is for authentication.
// Lots of duplication. One is a subset of another.
interface PasskeyCredentialRequest {
  challenge: Uint8Array;
  rpId: string;
}

function parsePasskeyCredentialRequest(
  buf: Uint8Array,
): PasskeyCredentialRequest | null {
  const cur = new Cursor(buf);
  try {
    cur.skip(1);
    const challenge = cur.readExact(CHALLENGE_BYTES);
    const rpId = cur.readText();
    return { challenge, rpId };
  } catch {
    return null;
  }
}

function extractRequestOptions(
  cred: PasskeyCredentialRequest,
): PublicKeyCredentialRequestOptions {
  return {
    challenge: cred.challenge as Uint8Array<ArrayBuffer>,
    rpId: cred.rpId,
    timeout: 300000,
    userVerification: "required",
  };
}

// clientData happens to be a UTF-8 encoded JSON string, but we explicitly treat
// it as opaque bytes because it's canonically signed as such. Avoid
// unnecessary reencodings, let the server validate.
interface PasskeyRecord {
  publicKey: Uint8Array;
  authData: Uint8Array;
  clientData: Uint8Array;
}

function serializePasskeyRecord(rec: PasskeyRecord): Uint8Array {
  const capacity =
    VLI32_MAXBYTES +
    rec.publicKey.length +
    VLI32_MAXBYTES +
    rec.authData.length +
    VLI32_MAXBYTES +
    rec.clientData.length;
  const buf = new Uint8Array(capacity);
  const w = new Writer(buf);
  w.writeByte(0);
  w.writeBytes(rec.publicKey);
  w.writeBytes(rec.authData);
  w.writeBytes(rec.clientData);
  return w.out();
}

// userHandle must always be present for discoverable credentials.
interface PasskeyAssertion {
  credId: Uint8Array;
  authData: Uint8Array;
  clientData: Uint8Array;
  signature: Uint8Array;
  userHandle: Uint8Array;
}

function serializePasskeyAssertion(rec: PasskeyAssertion): Uint8Array {
  const capacity =
    VLI32_MAXBYTES +
    rec.credId.length +
    VLI32_MAXBYTES +
    rec.authData.length +
    VLI32_MAXBYTES +
    rec.clientData.length +
    VLI32_MAXBYTES +
    rec.signature.length +
    rec.userHandle.length;
  const buf = new Uint8Array(capacity);
  const w = new Writer(buf);
  w.writeByte(0);
  w.writeBytes(rec.credId);
  w.writeBytes(rec.authData);
  w.writeBytes(rec.clientData);
  w.writeBytes(rec.signature);
  w.writeFixed(rec.userHandle);
  return w.out();
}

export enum AuthMethod {
  Certificate = 1,
  Envelope = 2,
  Passkey = 3,
}

type AuthRequest =
  | { method: AuthMethod.Certificate; username: string }
  | { method: AuthMethod.Envelope; username: string; record: Uint8Array }
  | { method: AuthMethod.Passkey };

function authRequestMaxSize(req: AuthRequest): number {
  // Presence bitmap.
  let cap = 1;
  switch (req.method) {
    case AuthMethod.Certificate:
      cap += new TextEncoder().encode(req.username).length;
      break;
    case AuthMethod.Passkey:
      // Passkeys don't involve usernames or other credentials, but because I
      // was stupid, I made the username field mandatory in all AuthRequest
      // messages. So, send an empty string, which is always encoded as a
      // one-byte VLI 0.
      cap += 1;
      break;
    case AuthMethod.Envelope:
      cap += new TextEncoder().encode(req.username).length;
      cap += req.record.length;
      break;
  }
  cap += VLI32_MAXBYTES;
  return cap;
}

// Top-level serialization functions

function serializeAuthRequest(req: AuthRequest): Uint8Array {
  const buf = new Uint8Array(authRequestMaxSize(req));
  const w = new Writer(buf);
  w.writeByte(0);
  switch (req.method) {
    // Empty username. Other methods not implemented.
    case AuthMethod.Passkey:
      w.writeVli32(0);
  }
  w.writeVli32(req.method);
  return w.out();
}

// Only expect the passkey credential.
export function parseRegisterCredential(
  buf: Uint8Array,
): PasskeyCredential | null {
  // At least the presence bitmap and the enumerant.
  if (buf.length < 2) {
    return null;
  }
  const choice = decodeVli32(buf.subarray(1));
  if (choice === null) {
    return null;
  }
  const [credType, credTypeN] = choice;
  switch (credType) {
    case CredentialType.Passkey:
      // We only care about the passkey now, so break out and continue in the
      // function body.
      break;
    default:
      // This one shall be a different error.
      return null;
  }
  const cred = parsePasskeyCredential(buf.subarray(1 + credTypeN));
  return cred;
}

// Similar to parsing registration data, only serialize the passkey as nothing
// else exists at the moment.
function serializeRegistrationFinish(msg: PasskeyRecord): Uint8Array {
  const record = serializePasskeyRecord(msg);
  const capacity = 1 + VLI32_MAXBYTES + record.length;
  const buf = new Uint8Array(capacity);
  const w = new Writer(buf);
  w.writeByte(0);
  w.writeVli32(CredentialType.Passkey);
  w.writeFixed(record);
  return w.out();
}

export function parseAssertCredential(
  buf: Uint8Array,
): PasskeyCredentialRequest | null {
  if (buf.length < 2) {
    return null;
  }
  const choice = decodeVli32(buf.subarray(1));
  if (!choice) {
    return null;
  }
  const [credType, credTypeN] = choice;
  switch (credType) {
    case CredentialType.Passkey:
      break;
    default:
      return null;
  }
  const cred = parsePasskeyCredentialRequest(buf.subarray(1 + credTypeN));
  return cred;
}

function serializeAssertionFinish(msg: PasskeyAssertion): Uint8Array {
  const record = serializePasskeyAssertion(msg);
  const capacity = 1 + VLI32_MAXBYTES + record.length;
  const buf = new Uint8Array(capacity);
  const w = new Writer(buf);
  w.writeByte(0);
  w.writeVli32(CredentialType.Passkey);
  w.writeFixed(record);
  return w.out();
}

// Handlers

export enum AuthState {
  None,
  Pending,
}

export async function handleRegisterCredential(buf: Uint8Array) {
  const registerCred = parseRegisterCredential(buf);
  const publicKey = extractCreationOptions(registerCred);
  let credential;
  try {
    credential = (await navigator.credentials.create({
      publicKey,
    })) as PublicKeyCredential;
  } catch {
    alert("Registration failed.");
    return;
  }
  const response = credential.response;
  if (!(response instanceof AuthenticatorAttestationResponse)) {
    alert("Invalid attestation response.");
    return;
  }
  const reply = serializeRegistrationFinish({
    publicKey: new Uint8Array(response.getPublicKey()),
    authData: new Uint8Array(response.getAuthenticatorData()),
    clientData: new Uint8Array(response.clientDataJSON),
  });
  const msg = serializeMessage(ExMsgType.RegistrationFinish, reply);
  client.serv.send(msg);
}

export function sendPasskeyLoginRequest() {
  const reply = serializeAuthRequest({
    method: AuthMethod.Passkey,
  });
  const msg = serializeMessage(ExMsgType.AuthRequest, reply);
  client.authState = AuthState.Pending;
  client.serv.send(msg);
}

export async function handleAssertCredential(buf: Uint8Array) {
  if (client.authState !== AuthState.Pending) {
    return;
  }
  const credOptions = parseAssertCredential(buf);
  const publicKey = extractRequestOptions(credOptions);
  let credential;
  try {
    credential = (await navigator.credentials.get({
      publicKey,
    })) as PublicKeyCredential;
  } catch {
    alert("Authentication failed.");
    return;
  }
  const response = credential.response;
  if (!(response instanceof AuthenticatorAssertionResponse)) {
    alert("Invalid assertion response.");
    return;
  }
  const reply = serializeAssertionFinish({
    credId: new Uint8Array(credential.rawId),
    authData: new Uint8Array(response.authenticatorData),
    clientData: new Uint8Array(response.clientDataJSON),
    signature: new Uint8Array(response.signature),
    userHandle: new Uint8Array(response.userHandle),
  });
  const msg = serializeMessage(ExMsgType.AssertionFinish, reply);
  client.authState = AuthState.None;
  client.serv.send(msg);
}