Re: Unicode confusables and reordering characters considered harmful, a simple solution

[Gregory Heytings]

> The right balance is where the percent of false positives is very low.

IMO, that's not the right balance: the right balance is where the 
percentage of false negatives is zero.  When security is at stake, I very 
much prefer too many false positives to missing one danger.  In particular 
because such warnings give you the feeling that there is no danger when 
there is no warning.

> I encourage you to read the comments in the implementation I wrote, to 
> see which cases I consider "suspicious".

This "I consider" is the problem of your approach.  Malevolent actors are 
always more inventive, and will find a way to escape the safety net you 
created.  The cases you consider suspicious are cases where the 
directionality of one or more characters is overridden by reordering 
control characters, but this is not what the "Trojan Source" paper is 
about.  The problem it points to is much broader, it's about using these 
invisible control characters to make the source code appear different to a 
human reader and to a compiler.

In fact, it did not take me much time to create a case that your algorithm 
doesn't detect (and AFAIU cannot detect without also displaying warnings 
about many legitimate uses).  I attach the example code, how that code is 
displayed by Emacs, and how that code would be displayed with the patch I 
proposed.

bidi-reordering.c
Description: Text Data

bidi-reordering.png
Description: PNG image

bidi-reordering-display.png
Description: PNG image