NSM flaw?

Shawn Steele Shawn.Steele at microsoft.com
Thu Sep 17 18:11:40 CEST 2009

I agree, this isn't a protocol issue.

From: idna-update-bounces at alvestrand.no [idna-update-bounces at alvestrand.no] on behalf of Mark Davis ☕ [mark at macchiato.com]
Sent: Thursday, September 17, 2009 7:57 AM
To: Harald Alvestrand
Cc: Vint Cerf; Abdulrahman I. ALGhadir; idna-update at alvestrand.no
Subject: Re: NSM flaw?

As you say, multiple occurrences of NSMs are permitted and necessary. They are described on p51, Section 2.11 "Combining Characters", subsection "Multiple Combining Characters" (although people want to read starting at p48). Briefly, the NSMs should be (in the absence of language/script-specific layout) be stacked outwards from the base.

I don't know of a language that needs two of the same NSM in a row; but that doesn't mean that one doesn't exist. And it is an issue if the rendering engine incorrectly places the NSMs so as to coincide. But forbidding two of the same in a row doesn't solve the problem: if a rendering engine overlaps NSMs, then it is easy to "hide" an NSM by just using it with different NSMs *or* even just base characters that turn on the same pixels. Should one disallow Arabic FATHA because it can be hidden if placed on an A-acute in a particular font on a particular system? Probably not.

These are issues that client software on a particular system can warn about, but IDNA cannot reliably detect or address in the protocol.

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