Re: Can watermarking Unicode text using invisible differences sneak through Emacs, or can Emacs detect it?

[Eli Zaretskii <eliz@gnu.org>]

> From: Richard Stallman <rms@gnu.org>
> Cc: psainty@orcon.net.nz, luangruo@yahoo.com,
> 	kevin.legouguec@gmail.com, emacs-devel@gnu.org
> Date: Mon, 07 Feb 2022 22:55:54 -0500
> 
>   > > I think there are only around 20 diacritics.
> 
>   > You are thinking of some subset, I think.  The real number is more
>   > like 80,
> 
> I am amazed.  Where can I see a list that shows more of them?

Type "C-x 8 RET COMBINING", press TAB, then filter out of the
candidates those which pertain to Cyrillic, Greek, and other specific
scripts, leaving just Latin and those which don't belong to specific
scripts.

>   >   That's a great simplification from a table
>   > > of hundreds of elements, set up by hand.
> 
>   > Setting by hand was already done, and we have it in latin1-disp.el so
> 
> Do you mean, the table that presents a-with-breve-and-tilde as `a)?'?
> I don't think that works well.

I think it works as well as it could, but in any case, seeing all the
combinations explicitly is needed to provide reasonable results.

>   > > I don't follow you here.  In particular, what does "complete
>   > > equivalent" mean?
> 
>   > For example, "o?'" instead of "o" + "?" + "'" (to emulate ?\ṍ).
> 
> I don't understand the difference between "o?'" and "o" + "?" + "'".

Your proposal is to have separate rules to produce the equivalent of
each diacritic, so you will never see "o?'", only its components
separately; I denoted the latter by "o?'" and "o" + "?" + "'".

>   >   What would you do with the likes of ?\ǿ (which we currently
>   > represent as "o/'")?  Its base character, ø, doesn't have a
>   > decomposition in Unicode.
> 
> For my terminal, I'd like it to send ø literally since my terminal
> can display that.  `ø'' would be a good way to display it.
> But on a terminal that can't display ø, `o/'' would be a good choice.

My point is that there isn't a mechanical way of producing "o/" from
ø, because Unicode decompositions don't support that.

>   > > Not on a Linux console, I think.  When I have f and i in the buffer,
>   > > Emacs does not convert them into a ligature.  The only time it has to
>   > > try to deal with a ligature is when there is a Unicode ligature
>   > > code point in the buffer.
> 
>   > Once again, on a TTY frame Emacs does NOT produce the ligatures nor
>   > combine base characters with the diacritics.
> 
> You have told me this several times, and I believe you.  But how does
> it relate to the case I am talking about?  I don't see a relationship.

As I said, that remark was for other people, those who will read my
email on GUI displays.