presto-main/src/main/java/io/prestosql/server/security/CertificateAuthenticator.java/CertificateAuthenticator/authenticate
class CertificateAuthenticator:    @Override
    public Principal authenticate(HttpServletRequest request)
            throws AuthenticationException
    {
        X509Certificate[] certs = (X509Certificate[]) request.getAttribute(X509_ATTRIBUTE);
        if ((certs == null) || (certs.length == 0)) {
            throw new AuthenticationException(null);
        }
        return certs[0].getSubjectX500Principal();
    }