From: Otto Stolz (Otto.Stolz@uni-konstanz.de)
Date: Tue Mar 28 2006 - 11:14:37 CST
> Reading this [RFC 2152], I got the feeling it only encodes UTF-16 encoded Texts,
> but I think that's not true.
The description in RFC 2152, chapter 4, is probably misleading to
the uninitiated. The key to understanding is that all UTFs are
equivalent: they encode the same character set, viz. the whole Uni-
code, and any string encoded in one UTF can be easily transformed
into any other.
So, all references in chapter 4 of RFC 2152 to UTF-16, and to
16-bit code elements, are only meant to facilitate the description
of the algorithm. You can describe the UTF-7 encoding algorithm
(with a grain of salt) thusly:
1. encode the source string in UTF-16 (regardless of its previous
2. convert every three UTF-16 code units into 8 bytes using a modified
base-64 algorithm (hence, every byte encodes 6 bit);
3. enclose the result between a plus and a minus sign.
Alternatively, runs of "harmless" characters may be encoded in ASCII,
instead of applying steps 1..3, above.
The latter alternative renders UTF-7 indeterminate: a character
string may be encoded in several ways, cf. my example in
-- in contrast to UTF-8, UTF-16, and UTF-32. I guess, this is the
main reason for not having UTF-7 in the Uncode standard.
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