<?php
namespace Pusher;
class PusherCrypto
{
private $encryption_master_key;
// The prefix any e2e channel must have
public const ENCRYPTED_PREFIX = 'private-encrypted-';
/**
* Checks if a given channel is an encrypted channel.
*
* @param string $channel the name of the channel
*
* @return bool true if channel is an encrypted channel
*/
public static function is_encrypted_channel(string $channel): bool
{
return strpos($channel, self::ENCRYPTED_PREFIX) === 0;
}
/**
* Checks if channels are a mix of encrypted and non-encrypted types.
*
* @param array $channels
* @return bool true when mixed channel types are discovered
*/
public static function has_mixed_channels(array $channels): bool
{
$unencrypted_seen = false;
$encrypted_seen = false;
foreach ($channels as $channel) {
if(self::is_encrypted_channel($channel)) {
if ($unencrypted_seen) {
return true;
} else {
$encrypted_seen = true;
}
} else {
if ($encrypted_seen) {
return true;
} else {
$unencrypted_seen = true;
}
}
}
return false;
}
/**
* @param $encryption_master_key_base64
* @return string
* @throws PusherException
*/
public static function parse_master_key($encryption_master_key_base64): string
{
if (!function_exists('sodium_crypto_secretbox')) {
throw new PusherException('To use end to end encryption, you must either be using PHP 7.2 or greater or have installed the libsodium-php extension for php < 7.2.');
}
if ($encryption_master_key_base64 !== '') {
$decoded_key = base64_decode($encryption_master_key_base64, true);
if ($decoded_key === false) {
throw new PusherException('encryption_master_key_base64 must be a valid base64 string');
}
if (strlen($decoded_key) !== SODIUM_CRYPTO_SECRETBOX_KEYBYTES) {
throw new PusherException('encryption_master_key_base64 must encode a key which is 32 bytes long');
}
return $decoded_key;
}
return '';
}
/**
* Initialises a PusherCrypto instance.
*
* @param string $encryption_master_key the SECRET_KEY_LENGTH key that will be used for key derivation.
*/
public function __construct(string $encryption_master_key)
{
$this->encryption_master_key = $encryption_master_key;
}
/**
* Decrypts a given event.
*
* @param object $event an object that has an encrypted data property and a channel property.
*
* @return object the event with a decrypted payload, or false if decryption was unsuccessful.
* @throws PusherException
*/
public function decrypt_event(object $event): object
{
$parsed_payload = $this->parse_encrypted_message($event->data);
$shared_secret = $this->generate_shared_secret($event->channel);
$decrypted_payload = $this->decrypt_payload($parsed_payload->ciphertext, $parsed_payload->nonce, $shared_secret);
if (!$decrypted_payload) {
throw new PusherException('Decryption of the payload failed. Wrong key?');
}
$event->data = $decrypted_payload;
return $event;
}
/**
* Derives a shared secret from the secret key and the channel to broadcast to.
*
* @param string $channel the name of the channel
*
* @return string a SHA256 hash (encoded as base64) of the channel name appended to the encryption key
* @throws PusherException
*/
public function generate_shared_secret(string $channel): string
{
if (!self::is_encrypted_channel($channel)) {
throw new PusherException('You must specify a channel of the form private-encrypted-* for E2E encryption. Got ' . $channel);
}
return hash('sha256', $channel . $this->encryption_master_key, true);
}
/**
* Encrypts a given plaintext for broadcast on a particular channel.
*
* @param string $channel the name of the channel the payloads event will be broadcast on
* @param string $plaintext the data to encrypt
*
* @return string a string ready to be sent as the data of an event.
* @throws PusherException
* @throws \SodiumException
*/
public function encrypt_payload(string $channel, string $plaintext): string
{
if (!self::is_encrypted_channel($channel)) {
throw new PusherException('Cannot encrypt plaintext for a channel that is not of the form private-encrypted-*. Got ' . $channel);
}
$nonce = $this->generate_nonce();
$shared_secret = $this->generate_shared_secret($channel);
$cipher_text = sodium_crypto_secretbox($plaintext, $nonce, $shared_secret);
try {
return $this->format_encrypted_message($nonce, $cipher_text);
} catch (\JsonException $e) {
throw new PusherException('Data encoding error.');
}
}
/**
* Decrypts a given payload using the nonce and shared secret.
*
* @param string $payload the ciphertext
* @param string $nonce the nonce used in the encryption
* @param string $shared_secret the shared_secret used in the encryption
*
* @return string plaintext
* @throws \SodiumException
*/
public function decrypt_payload(string $payload, string $nonce, string $shared_secret)
{
$plaintext = sodium_crypto_secretbox_open($payload, $nonce, $shared_secret);
if (empty($plaintext)) {
return false;
}
return $plaintext;
}
/**
* Formats an encrypted message ready for broadcast.
*
* @param string $nonce the nonce used in the encryption process (bytes)
* @param string $ciphertext the ciphertext (bytes)
*
* @return string JSON with base64 encoded nonce and ciphertext`
* @throws \JsonException
*/
private function format_encrypted_message(string $nonce, string $ciphertext): string
{
$encrypted_message = new \stdClass();
$encrypted_message->nonce = base64_encode($nonce);
$encrypted_message->ciphertext = base64_encode($ciphertext);
return json_encode($encrypted_message, JSON_THROW_ON_ERROR);
}
/**
* Parses an encrypted message into its nonce and ciphertext components.
*
*
* @param string $payload the encrypted message payload
*
* @return object php object with decoded nonce and ciphertext
* @throws PusherException
*/
private function parse_encrypted_message(string $payload): object
{
try {
$decoded_payload = json_decode($payload, false, 512, JSON_THROW_ON_ERROR);
} catch (\JsonException $e) {
throw new PusherException('Data decoding error.');
}
$decoded_payload->nonce = base64_decode($decoded_payload->nonce);
$decoded_payload->ciphertext = base64_decode($decoded_payload->ciphertext);
if ($decoded_payload->ciphertext === '' || strlen($decoded_payload->nonce) !== SODIUM_CRYPTO_SECRETBOX_NONCEBYTES) {
throw new PusherException('Received a payload that cannot be parsed.');
}
return $decoded_payload;
}
/**
* Generates a nonce that is SODIUM_CRYPTO_SECRETBOX_NONCEBYTES long.
* @return string
* @throws \Exception
*/
private function generate_nonce(): string
{
return random_bytes(
SODIUM_CRYPTO_SECRETBOX_NONCEBYTES
);
}
}
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