[UA-discuss] Public domain/MIT licensed IDN library for C

Michael Casadevall michael at casadevall.pro
Tue Jul 2 03:50:55 UTC 2019

Short version: Is anyone aware of an implementation of IDNA2008 that is
in the public domain or MIT/BSD licensed?

Long version: During a discussion with Paul Hoffman, we were talking
about the state of IDN support in at a core programming language level,
and my efforts to write a yellow paper showing the full effort required
to implement support for IDNs/EAIs we talked about the licensing of
libidn2. Specifically, libidn2 is dual-licensed under the GPLv2/LGPLv3.
This is rather problematic for implementing IDN support in many
applications coded in C/C++.

GPLv2 requires that an application that uses libidn be free and open
source and compatible the GPLv2 licensing terms which is impossible for
any propitiatory software package, or those under a free software but
non-GPL compatible license such as the CDDL.

LGPLv3 in theory solves this problem, but adds even more complications.
LGPL allows libidn2 to linked to proprietary software but only in the
form of a shared library (dll or so) file. However, LGPLv3 requires
patent grants be given to any user of said library (GPLv3 section 11).
This clause adds an extremely high barrier to entry in corporate
environment which involves legal questions and may not be surmountable.

As we want IDN support to go as far as wide as possible, the best way to
accomplish that is an IDNA2008 implementation that is entirely
non-embured, either under the MIT license, another weak copyleft
license, or ideally in the public domain or CC-0 licenses. I searched
around and came up empty but I'm hoping the list can point me in the
right direction.
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