private function parseAuthnStatement(DOMElement $xml) : void
    {
        /** @var \DOMElement[] $authnStatements */
        $authnStatements = Utils::xpQuery($xml, './saml_assertion:AuthnStatement');
        if (empty($authnStatements)) {
            $this->authnInstant = null;

            return;
        } elseif (count($authnStatements) > 1) {
            throw new \Exception('More than one <saml:AuthnStatement> in <saml:Assertion> not supported.');
        }
        $authnStatement = $authnStatements[0];

        if (!$authnStatement->hasAttribute('AuthnInstant')) {
            throw new \Exception('Missing required AuthnInstant attribute on <saml:AuthnStatement>.');
        }
        $this->authnInstant = Utils::xsDateTimeToTimestamp($authnStatement->getAttribute('AuthnInstant'));

        if ($authnStatement->hasAttribute('SessionNotOnOrAfter')) {
            $this->sessionNotOnOrAfter = Utils::xsDateTimeToTimestamp(
                $authnStatement->getAttribute('SessionNotOnOrAfter')
            );
        }

        if ($authnStatement->hasAttribute('SessionIndex')) {
            $this->sessionIndex = $authnStatement->getAttribute('SessionIndex');
        }

        $this->parseAuthnContext($authnStatement);
    }