anchor.diff:  
   prop_secretbox_correct() ->
       ?FORALL({Msg, Nonce, Key},
  -            {g_iodata(),
  +            {fault_rate(1, 40, g_iodata()),
                fault_rate(1, 40, nonce()),
                fault_rate(1, 40, secret_key())},
         begin
  
positive.code:                     Err = enacl:box_open([<<"x">>, CipherText], Nonce, PK1, SK2),
                      equals(Err, {error, failed_verification});
                  false ->
                      case box(iolist_to_binary(Msg), Nonce, PK2, SK1) of
                        badarg -> true;
                        Res ->
                          failure(box_open(Res, Nonce, PK1, SK2))
  
negative.code: 
  prop_stream_xor_correct() ->
      ?FORALL({Msg, Nonce, Key},
              {fault_rate(1, 40, g_iodata()),
               fault_rate(1, 40, nonce()),
               fault_rate(1, 40, secret_key())},
          case v_iodata(Msg) andalso nonce_valid(Nonce) andalso secret_key_valid(Key) of
  