use Crypt::OpenSSL::DSA;
  my $dsa_priv = Crypt::OpenSSL::DSA->read_priv_key( $filename );
  my $sig_obj = $dsa_priv->do_sign($message);
  my $dsa_pub = Crypt::OpenSSL::DSA->read_pub_key( $filename );
  my $valid = $dsa_pub->do_verify($message, $sig_obj);