问JWT令牌生成失败，MUX响应错误

EN

define('MUX_TOKEN_ID', 'xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx');
define('MUX_TOKEN_SECRET', '<base64-encoded-string-here>');

// load libs
require_once 'vendor/autoload.php';

// load MUX credentials
require_once 'mux-credentials.php';

use \Firebase\JWT\JWT;


$myId = "<MY-ASSET-ID-HERE>";       // Enter the id for which you would like to get counts here
$myIdType = "asset_id";   // Enter the type of ID provided in my_id; one of video_id | asset_id | playback_id | live_stream_id
$keyId = MUX_TOKEN_ID;      // Enter your signing key id here
$keySecret = MUX_TOKEN_SECRET;  // Enter your base64 encoded private key here

$payload = array(
        "sub" => $myId,
        "aud" => $myIdType,
        "exp" => time() + 600, // Expiry time in epoch - in this case now + 10 mins
        "kid" => $keyId
);

$jwt = JWT::encode($payload, base64_decode($keySecret), 'RS256');


print "$jwt\n";

PHP Warning:  openssl_sign(): supplied key param cannot be coerced into a private key in /home/example/vendor/firebase/php-jwt/src/JWT.php on line 225
PHP Fatal error:  Uncaught DomainException: OpenSSL unable to sign data in /home/example/vendor/firebase/php-jwt/src/JWT.php:227
Stack trace:
#0 /home/example/vendor/firebase/php-jwt/src/JWT.php(195): Firebase\JWT\JWT::sign()
#1 /home/example/people-watching.php(30): Firebase\JWT\JWT::encode()
#2 {main}
  thrown in /home/example/vendor/firebase/php-jwt/src/JWT.php on line 227

$jwt = JWT::encode($payload, base64_decode($keySecret));

curl 'https://stats.mux.com/counts?token=<JWT-HERE>'

{"error":{"type":"internal error","messages":["Could not get signing key."]}}

curl -X POST \
-H "Content-Type: application/json" \
-u <MUX_TOKEN_KEY>:<MUX_TOKEN_SECRET> \
'https://api.mux.com/system/v1/signing-keys?product=data'

{"error":{"type":"not_found","messages":["The requested resource either doesn't exist or you don't have access to it."]}}