/**
*
* @param <type> $text
* @param <type> $mimetype
* @param Magicsig $magicsig Magicsig with private key available.
*
* @return MagicEnvelope object with all properties set
*
* @throws Exception of various kinds on signing failure
*/
public function signMessage($text, $mimetype, Magicsig $magicsig)
{
assert($magicsig->privateKey instanceof Crypt_RSA);
// Prepare text and metadata for signing
$this->data = Magicsig::base64_url_encode($text);
$this->data_type = $mimetype;
$this->encoding = self::ENCODING;
$this->alg = $magicsig->getName();
// Get the actual signature
$this->sig = $magicsig->sign($this->signingText());
}