2key-ratchet

2key-ratchet is an implementation of a Double Ratchet protocol and X3DH in TypeScript utilizing WebCrypto.

The Double Ratchet protocol and X3DH were designed with goals of providing both forward secrecy and cryptographic deniability. Importantly there have been several independent security reviews that concluded they deliver on those goals.

The term “Double Ratchet” comes from how the protocol makes sure each message gets a new key: their Diffie-Hellman keys are “ratcheted” by each new message exchange; and so are the send/receive chains (the “symmetric-key ratchet”).

There are a few differences between the original specifications and 2key-ratchet, the most significant being, as it’s name suggests, it uses two keys, one for authentication and another for key exchange. The other big one is that secp256r1 is used instead of curve25519 because browsers do not yet support this curve natively.

See the ARCHITECTURE document to better understand the library structure.

For ideas on where you might use 2key-ratchet see the SCENARIOS document.

For licensing information, see the LICENSE file.

Overview

IdentityKeys

Each peer in the protocol has an IdentityKey, these are secp256r1 keys. These keys are used to authenticate both PreKeys and ExchangeKeys. IdentityKeys are used similarly to the public key in an X.509 certificate.

ExchangeKeys

ExchangeKeys are introduced by 2key-ratchet, they are used to derive PreKeys. The ExchangeKey is signed by a peers IdentityKey.

PreKeys

In 2key-ratchet a PreKey is a secp256r1 public key with an associated unique id. These PreKeys are signed by the IdentityKey.

On first use, clients generate a single signed PreKey, as well as a large list of unsigned PreKeys, and transmit all of them to a server.

Server

The server in the protocol is an untrusted entity, it simply stores preKeys for retrieval when the peer may be offline and unreachable.

Sessions

The Double Ratchet protocol is session-oriented. Peers establish a session with each other, this is then used for all subsequent exchanges. These sessions can remain open and be re-used since each message is encrypted with a new and unique cryptographic key.

Size and Dependencies

NOTE: You will also have to import tslib and protobufjs for use in the browser.

Instructions

Installation

npm install 2key-ratchet

Usage

Include 2key-ratchet and its dependencies in your application.

NODEJS:

let DKeyRatchet = require("2key-ratchet");

BROWSER:

<script src="2key-ratchet.js"></script>

The DKeyRatchet namespace will always be available globally and also supports AMD loaders.

Generate an IdentityKey

The first step is to create an IdentityKey.

let AliceID;
DKeyRatchet.Identity.create(16453, 1, 1)
    .then((id) => {
        AliceID = id;
    });

Then create your PreKey message bundle:

let bundle = new DKeyRatchet.PreKeyBundleProtocol();

bundle.identity.fill(AliceID)
    .then(() => {
        bundle.registrationId = AliceID.id;
        const preKey = AliceID.signedPreKeys[0];
        bundle.preKeySigned.id = 1;
        bundle.preKeySigned.key = preKey.publicKey;
        return bundle.preKeySigned.sign(AliceID.signingKey.privateKey);
    })
    .then(() => {
        return bundle.exportProto();
    })
    .then((ab) => {
        console.log(ab); // ArrayBuffer { byteLength: 374 }
    });

And then import the generated PreKey message bundle:

DKeyRatchet.PreKeyBundleProtocol.importProto(ab)
    .then((bundle) => {
        // check signed prekey
        return bundle.preKeySigned.verify(AliceID.signingKey.publicKey);
    })
    .then((trusted) => {
        if (!trusted)
            throw new Error("Error: The PreKey is not trusted");
    })

Create a session

With the previous steps complete you can now create a session:

NOTE: For data conversion was used module pvtsutils.

DKeyRatchet.AsymmetricRatchet.create(BobID, bundle)
    .then((cipher) => {
        return cipher.encrypt(Convert.FromUtf8String("Hello world!"));
    })
    .then((preKeyMessage) => {
        return preKeyMessage.exportProto();
    })
    .then((BobMessage) => {
        console.log(BobMessage); // ArrayBuffer {byteLength: 408}
    });

On the other side you would do the same:

// Parse received bytes to proto
return DKeyRatchet.PreKeyMessageProtocol.importProto(BobMessage)
    .then((proto) => {
        return DKeyRatchet.AsymmetricRatchet.create(AliceID, proto)
            .then((cipher) => {
                return cipher.decrypt(proto.signedMessage);
            })
            .then((message) => {
                console.log(Convert.ToUtf8String(message)); // Hello world!
            });
    });

We have a complete example you can look at here.

Contributing

If you've found an problem with 2key-ratchet, please open a new issue. Feature requests are welcome, too.

Pull requests – patches, improvements, new features – are a fantastic help. Please ask first before embarking on any significant pull request (e.g., implementing new features).

Note

Bruce Schneier famously said "If you think cryptography can solve your problem, then you don't understand your problem and you don't understand cryptography". The point being, using 2key-ratchet, or any other "cryptography related" library, will not necessarily make your product secure.

In short, there is a lot more to making a secure product than adding cryptography, this is a great book to get you familiar with thinking defensively.

WARNING

Though this library is based on the Double Ratchet Algorithm and the X3DH Key Agreement Protocol several changes have been made that could change the security properties they offer. At this time you should consider this implementation appropriate for experimentation until further security reviews are completed.

Acknowledgements

Both Double Ratchet and X3DH were designed by Trevor Perrin and Moxie Marlinspike, we thank them for their work.

Related

• A Formal Security Analysis of the Signal Messaging Protocol
• WhatsApp Security Paper Analysis
• Web Cryptography API
• The X3DH Key Agreement Protocol
• The Double Ratchet Algorithm
• Google Key Transparency
• OMEMO Multi-End Message and Object Encryption
• Matrix OLM
• Double Ratchet & the Quantum Computers
• CryptoCat Encryption Overview