emp-wasm v0.2.1

emp-wasm

Wasm build of authenticated garbling from emp-toolkit/emp-agmpc.

(If you're not familiar with garbled circuits, you might find this video helpful.)

Usage

Take a look at mpc-framework for a nicer API which utilizes emp-wasm as a dependency. In particular, it allows you to write your circuits in a TypeScript-like language called summon.

Read on if you're more interested in what's going on under the hood and are prepared to use/write circuits like these directly.

npm install emp-wasm

import { secureMPC, BufferedIO, IO } from 'emp-wasm';

import circuit from './circuit.ts';

async function main() {
  // emp-wasm has no opinion about how you do your IO, websockets are just used
  // for this example
  const io = await makeWebSocketIO('wss://somehow-connect-to-bob');

  const output = await secureMPC({
    party: 0, // use 1 in the other client
    size: 2, // the number of participants
             // note: this example is a bit 2PC-specific, for a more general
             // example, see internalDemo3 in demo.ts
    circuit, // a string defining the circuit, see circuits/*.txt for examples
    inputBits: Uint8Array.from([/* 0s and 1s defining your input bits */]),
    inputBitsPerParty: [32, 32], // the number of bits contributed by each participant
    io,
    // mode: 'auto', // defaults to auto, but you can force '2pc' mode or 'mpc' mode
  });

  // the output bits from the circuit as a Uint8Array
  console.log(output);
}

async function makeWebSocketIO(url: string, otherParty: number) {
  const sock = new WebSocket(url);
  sock.binaryType = 'arraybuffer';

  await new Promise((resolve, reject) => {
    sock.onopen = resolve;
    sock.onerror = reject;
  });

  // You don't have to use BufferedIO, but it's a bit easier to use, otherwise
  // you need to implement io.recv(len) returning a promise to exactly len
  // bytes
  const io = new BufferedIO(
    otherParty,
    (toParty, channel, data) => {
      assert(toParty === otherParty, 'Unexpected party');
      const channelBuf = new Uint8Array(data.length + 1);

      // emp-wasm requires an a-channel and a b-channel between each pair of
      // participants. We implement this by actually using a single underlying
      // channel and prefixing each message with a byte indicating the channel.
      // BufferedIO assumes this too. If you're not using BufferedIO, you can
      // provide these multiple channels with a method of your choosing.
      channelBuf[0] = channel.charCodeAt(0);

      channelBuf.set(data, 1);
      sock.send(channelBuf);
    },
    () => sock.close(),
  );

  sock.onmessage = (event: MessageEvent) => {
    if (!(event.data instanceof ArrayBuffer)) {
      console.error('Unrecognized event.data');
      return;
    }

    const channelBuf = new Uint8Array(event.data);
    const channel = channelFromByte(channelBuf[0]);

    io.accept(channel, channelBuf.slice(1));
  };

  sock.onerror = (e) => {
    io.emit('error', new Error(`WebSocket error: ${e}`));
  };

  sock.onclose = () => io.close();

  return io;
}

function channelFromByte(byte: number): 'a' | 'b' {
  switch (byte) {
    case 'a'.charCodeAt(0):
      return 'a';
    case 'b'.charCodeAt(0):
      return 'b';
    default:
      throw new Error('Invalid channel');
  }
}

main().catch(console.error);

For a concrete example, see wsDemo in demo.ts (usage instructions further down in readme).

Demo

npm install
npm run build
npm run demo

Requirements:

• nodejs
• emscripten

internalDemo

If you don't want to juggle multiple pages, you can do await internalDemo(3, 5) in the console, which will run two instances in the same page communicating internally.

internalDemo3

This is the only included MPC (>2 parties) demo. MPC mode uses different underlying code, and you can also use MPC mode with the 2PC demos by appending , 'mpc' to your parameters. MPC Framework is the intended and higher level API, see that repo for the latest information and demos.

> await internalDemo3(1, 2, 3)
< { alice: 6, bob: 6, charlie: 6 }

consoleDemo

Open the url in the console in two tabs and run consoleDemo(0, 3) in one and consoleDemo(1, 5) in the other. This will begin a back-and-forth where each page prints write(...) to the console, which you can paste into the other console to send that data to the other instance (note: sometimes there are multiple writes, make sure to copy them over in order). After about 15 rounds you'll get an alert showing 8 (== 3 + 5).

wsDemo

Open the url in the console in two tabs and run await wsDemo(0, 3) in one and await wsDemo(1, 5) in the other. This uses a websocket relay included in npm run demo and defined in scripts/relayServer.ts.

rtcDemo

Open the url in the console in two tabs and run await rtcDemo('pair-id', 0, 3) in one and await rtcDemo('pair-id', 1, 5) in the other. This one uses WebRTC (via peerjs) and should work across networks, locating each other via pair-id.

Regular C++ Compile

This library started out as a stripped down version of the original C++ project. You can compile this for your local system and test it like this:

./scripts/build_mpc_test.sh
./scripts/mpc_test.sh

This will calculate sha1("")==da39a3ee5e6b4b0d3255bfef95601890afd80709. It proves to the three other participants that Alice (the first party) knows the preimage of this hash. Each side is run in a separate process and they communicate over a local socket.

Requirements:

• clang
• mbedtls (on macos: brew install mbedtls)
  • this version of mbedtls is actually not needed for the wasm version, since we need to compile a wasm-specific version ourselves

Uncertain Changes

For most of the changes I'm reasonably confident that I preserved behavior, but there some things I'm less confident about, including:

• send and recv swapped for bob in Fpre::generate (see TODO comment)
• after removing threading, I also moved everything to a single io channel
• rewrite of simd code to not use simd and updated aes usage written by chatgpt o1 preview
• used chatgpt o1 preview to migrate from openssl to mbedtls

Regardless, this version of emp-toolkit needs some checking.