Copay

Installation:

git clone https://github.com/bitpay/copay.git
cd copay

Install bower and grunt if you haven't already:

npm install -g bower
npm install -g grunt-cli

Build Copay:

npm install
bower install
grunt shell --target=dev

Open Copay:

npm start

Then visit localhost:3000 in your browser.

Running copay

To run on a different port:

PORT=3001 npm start

To open up five different instances to test 3-of-5 multisig with yourself, then run this in 5 different terminals:

PORT=3001 npm start
PORT=3002 npm start
PORT=3003 npm start
PORT=3004 npm start
PORT=3005 npm start

To open n different instances more easily, just run:

n=5
node launch.js $n &

To require Copay as a module for use within you application:

require('copay').start(3000, function(location) {
  console.log('Copay server running at: ' + location);
});

Configuration

Default configuration can be found in the config.js file. See config.js for more info on configuration options.

About Copay

General

Copay implements a multisig wallet using p2sh addresses. It supports multiple wallet configurations, such as 3-of-5 (3 required signatures from 5 participant peers) or 2-of-3. To create a multisig wallet shared between multiple participants, Copay needs the public keys of all the wallet participants. Those public keys are incorporated into the wallet configuration and are combined to generate a payment address with which funds can be sent into the wallet.

To unlock the payment and spend the wallet's funds, a quorum of participant signatures must be collected and assembled in the transaction. The funds cannot be spent without at least the minimum number of signatures required by the wallet configuration (2 of 3, 3 of 5, 6 of 6, etc). Each participant manages their own private key, and that private key is never transmitted anywhere. Once a transaction proposal is created, the proposal is distributed among the wallet participants for each participant to sign the transaction locally. Once the transaction is signed, the last signing participant will broadcast the transaction to the Bitcoin network using a public API (defaults to the Insight API).

Copay also implements BIP32 to generate new addresses for the peers. The public key each participant contributes to the wallet is a BIP32 extended public key. As additional public keys are needed for wallet operations (to produce new addresses to receive payments into the wallet, for example) new public keys can be derived from the participants' original extended public keys. Each participant keeps their own private keys locally. Private keys are not shared. Private keys are used to sign transaction proposals to make a payment from the shared wallet.

Serverless web

Copay software does not need an application server to run. All the software is implemented in client-side JavaScript. For persistent storage, the client browser's localStorage is used. Locally stored data is encrypted using a password provided by the local user. Data kept in browser local storage should be backed up for safekeeping using one of the methods provided by Copay, such as downloading the data into a file.
Without a proper backup of the user's private key data, all funds stored in the wallet may be lost or inaccessible if the browser's localStorage is deleted, the browser uninstalled, the local hard disk fails, etc.

Peer communications

Copay uses peer-to-peer (p2p) networking to communicate between wallet participants. Participants exchange transaction proposals, public keys, nicknames and information about the wallet configuration. Private keys are not shared with anyone.

Copay network communications use the webRTC protocol. A p2p facilitator server is needed to enable the peers to find each other. Copay uses the open-sourced peerjs server implementation for p2p discovery. Wallet participants can use a public peerjs server or install their own. Once the peers find each other, a true p2p connection is established between the peers and there is no further flow of information to the p2p discovery server.

webRTC uses DTLS to secure communications between the peers, and each peer uses a self-signed certificate.

Security model

On top of webRTC, Copay peers authenticate as part of the "wallet ring"(WR) using an identity key and a network key.

The identity key is a ECDSA public key derived from the participant's extended public key using a specific BIP32 branch. This special public key is never used for Bitcoin address creation, and should only be known by members of the WR. In Copay this special public key is named copayerId. The copayerId is hashed and the hash is used to register with the peerjs server. Registering with a hash avoids disclosing the copayerId to parties outside of the WR. Peer discovery is accomplished using only the hashes of the WR members' copayerIds. All members of the WR know the full copayerIds of all the other members of the WR.

The network key is a random key generated and distributed among the wallet members during wallet creation. The network key is stored by each peer in the wallet configuration. The network key is used in establishing a CCM/AES authenticated encrypted channel between all members of the WR, on top of webRTC. Use of this network key prevents man-in-the-middle attacks from a compromised peerjs server.

Secret String

When a wallet is created, a secret string is provided to invite new peers to the new wallet. This string has the following format:

• CopayerId of the peer generating the string. This is a 33 byte ECDSA public key, as explained above. This allows the receiving peer to locate the generating peer.
• Network Key. A 8 byte string to encrypt and sign the peers communication.

The string is encoded using Bitcoin's Base58Check encoding, to prevent transmission errors.

Peer Authentication

It is important to note that - except for private keys - all data in the wallet is shared with all members of the wallet. Private keys are never shared with anyone and are never sent over the network. There are no private messages between individual members of the wallet. All members of a wallet see everything that happens in that wallet.