# We can construct authenticated ciphers:
  my $authcipha = new Crypt::CBC({header=>"randomiv", key=>$K_01_a->as_bytes, cipher=>"Blowfish"});
  my $authciphb = new Crypt::CBC({header=>"randomiv", key=>$K_01_b->as_bytes, cipher=>"Blowfish"});

  # or we can use the secrets to construct authentic clear-text messages:
  
  my @verifiable_message;
  SCOPE1: {
      my $msg = "This is verifiable.";

      my $sha = new Digest::SHA1;
         $sha->add( $msg );
         $sha->add( $K_01_a->as_bytes );

      my $MAC = $sha->digest;

      @verifiable_message = ( $msg, $MAC );
  }

  SCOPE2: {
      my $sha = new Digest::SHA1;
         $sha->add( $verifiable_message[0] );
         $sha->add( $K_01_b->as_bytes );

      my $MAC = $sha->digest;

      print "Verified message from ID_0:  $verifiable_message[0]\n\n"
          if $MAC eq $verifiable_message[1];
  }