Dentro del driver de broadcasting Mercure

// Supported: "reverb", "pusher", "ably", "mercure", "redis", "log", "null"

'mercure' => [
    'driver' => 'mercure',
    'url' => env('MERCURE_URL'),
    'public_url' => env('MERCURE_PUBLIC_URL'),
    'secret' => env('MERCURE_JWT_SECRET'),
    'encryption_key' => env('MERCURE_ENCRYPTION_KEY'),
    'claims' => [
        'iss' => env('MERCURE_JWT_ISSUER'),
        'client_id' => env('APP_NAME'),
    ],
    'cookie_name' => env('MERCURE_COOKIE_NAME'),
    'subscribe_expiration' => (int) env('MERCURE_SUBSCRIBE_EXPIRATION', 5),
],

public function mercure(array $config)
{
    if (empty($config['url'])) {
        return $this->frankenPhpMercure($config);
    }

    // ...
}

protected function mercureSecret(array $config, string $side)
{
    $secret = ($config[$side.'_secret'] ?? null) ?: ($config['secret'] ?? null);

    // ...

    $minimumLength = ['HS256' => 32, 'HS384' => 48, 'HS512' => 64][$algorithm] ?? 0;

    if (strlen($secret) < $minimumLength) {
        throw new InvalidArgumentException(/* ... */);
    }

    return $secret;
}

public function auth($request)
{
    $channelNames = (array) $request->input('channel_names', []);

    if ($channelNames === [] ||
        count($channelNames) > 100 ||
        $channelNames !== array_filter($channelNames, 'is_string')) {
        throw new AccessDeniedHttpException;
    }

    // Evalúa la autorización por canal y acumula los grants...

    return (new JsonResponse([/* ... */]))
        ->cookie($this->makeAuthorizationCookie($request, $grants, $user));
}

'encryption_key' => env('MERCURE_ENCRYPTION_KEY'),

php -r "echo base64_encode(random_bytes(32));"

if ($this->clientEvents && $whisperTopics !== []) {
    $grants[] = new Grant([Grant::ACTION_SUBSCRIBE, Grant::ACTION_PUBLISH], $whisperTopics);
}

protected string $topicPrefix = 'https://laravel.alt/echo/',

protected function channelTopic($channelName)
{
    return $this->topicPrefix.'channel/'.rawurlencode($channelName);
}

throw new BroadcastException(sprintf(
    'Mercure error: %s. Adjust the Mercure "public_url" configuration value so the hub [%s] shares a registrable domain with the application host [%s].',
    rtrim($e->getMessage(), '.'), $this->hub->getPublicUrl(), $request->getHost()
), 0, $e);

if (str_starts_with($hub->getCookieName(), '__') &&
    parse_url($hub->getPublicUrl(), PHP_URL_SCHEME) === 'http') {
    throw new InvalidArgumentException(/* ... */);
}