# File lib/omniauth/strategies/saml.rb, line 86
      def response_fingerprint
        response = request.params["SAMLResponse"]
        response = (response =~ /^</) ? response : Base64.decode64(response)
        document = XMLSecurity::SignedDocument::new(response)
        cert_element = REXML::XPath.first(document, "//ds:X509Certificate", { "ds"=> 'http://www.w3.org/2000/09/xmldsig#' })
        base64_cert = cert_element.text
        cert_text = Base64.decode64(base64_cert)
        cert = OpenSSL::X509::Certificate.new(cert_text)
        Digest::SHA1.hexdigest(cert.to_der).upcase.scan(/../).join(':')
      end