Overview

This is a library that handles encryption of messages passed back and forth between two parties, using a "double ratchet" algorithm. It uses Curve25519 for generating keys and signing.

Requirements

• PHP 7.1.3 or greater

Installation

composer require andrewdo/php-double-ratchet

Usage

First thing you'll want to do is either generate or load up your identity private key:

// generate random private key
$ourIdentity = KeyPair::getNewKeyPair();

// load from a base64 string
$ourIdentity = new KeyPair(new PrivateKey(base64_decode($base64String)));

In order to start a session with another party, you'll need a signed Pre Key from them. Signature verification is optional but suggested. You should probably also hard code the other party's public key if you know it ahead of time.

$theirPublicKey = new Key(...their 32 byte public key...);
$theirPreKeyPublicKey = new Key(...their 32 byte pre key public key...);
$theirPreKeySignature = ...signature of the pre key...;
if (!$theirPublicKey->isValidSignature($theirPreKeySignature, $theirPreKey->getValue())) {
    throw new Exception('Sketchy...');
} 

Then you can create a SessionManager instance.

NOTE: If you are the one that generated the Pre Key and you have its private key, you will need to pass it along.

$sessionManager = new SessionManager(
    $ourIdentity->getPrivateKey(),
    $theirPublicKey,
    $theirPreKeyPublicKey,
    $preKeyPrivateKey ?: null,  // if we are the receiver of the first message
    $logger ?: null,
    $options ?: []
);

The session manager will handle encryption and decryption of JSON payloads, along with the double ratcheting of keys. It will include the Ratchet Key in each JSON request/response. By default, the key for this field is ratchet_key but can be changed using the options parameter of the SessionManager.

$encryptedMessageStr = $sessionManager->encryptData(['grandmas_cookbook' => '...']);
$decryptedData = $sessionManager->decryptMessage($encryptedMessageStr);
// var_dump($decryptedData)
// array(2) { ["ratchet_key"]=> string(4) "asdf" ["grandmas_cookbook"]=> ... }

If you need to persist the SessionManager instance, you can use:

$sessionManagerString = $sessionManager->getAsEncryptedString();
// var_dump($sessionManagerString)
// string(2801) "/6Iq=...A==:eoNnkPa2sCE0F8ezC9TJzA=="

$sessionManager = SessionManager::getFromEncryptedString(
    $ourIdentity->getPrivateKey(),
    $sessionManagerString
);

Generating Pre Keys

In order to receive an ecrypted message from someone, you will need to somehow give them your public key and a signed Pre Key. To generate one:

$preKey = KeyPair::getNewKeyPair();
$preKeyPublicKey = $preKey->getPublicKey();
$signature = $ourIdentity->getSignature($preKeyPublicKey->getValue());