IDNA decode?

Simon Josefsson simon at
Fri May 27 11:32:29 CEST 2011

John C Klensin <klensin at> writes:

> Yes.   In retrospect, the reverse conversation might have been
> spelled out a little better.  But the general design of the
> IDNA2008 relationships (and RFC 5891 in particular) is to
> establish the equivalence relationship between U-labels and
> A-labels.  Conversions are valid if they preserve that
> equivalence and invalid otherwise.  Unlike IDNA2008 or your
> discussion below, there deliberately is no algorithm in
> pseudo-code.

Thanks -- this helps my understanding.  Having an algorithm defined only
implicitly in terms of equivalences is nice from a theoretical view.
>From a practical view, I think there are risks in having each
implementer (attempt to) translate the properties into an algorithm, and
pondering about each corner case.  When I revisit this aspect for my
implementation, I'll write down the algorithm I will use with more
details and publish that as a draft -- hopefully that can serve as an
informational help to other implementers.


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