Changes

This page outlines a plan for changing the mechanism by which Firefox decides whether to display a given IDN domain label (a domain name is made up of one or more labels, separated by dots) in its Unicode or Punycode form.

If we just display any possible IDN domain namelabel, we open ourselves up to [http://en.wikipedia.org/wiki/IDN_homograph_attack IDN homograph attacks], where one identical-looking domain can spoof another. So we have to have some mechanism to decide which ones to display and which ones to not display, which does not involve comparing the domain in question against every other single domain which exists (which is impossible).

Our current algorithm is to display as Unicode all IDNs IDN labels within TLDs on our [http://www.mozilla.org/projects/security/tld-idn-policy-list.html whitelist], and display as Punycode otherwise. We check the anti-spoofing policies of a registry before adding their TLD to the whitelist. The TLD operator must apply directly (they cannot be nominated by another person), and on several occasions we have required policy updates or implementation as a condition of getting in.

We also have a character blacklist - characters we will never display under any circumstances. This includes those which could be used to spoof the separators "/" or and ".", and invisible characters. (XXX Do we need to update this to remove some of those, like ZWJ/ZWNJ?)

No characters in the identifier label can be outside of the [http://www.unicode.org/reports/tr39/#Identifier_Characters Identifier Profile].

All characters in each identifier label must be from a single script, or from

The Identifier Profile is defined for us by the IDNA2008 standard, [http://tools.ietf.org/html/rfc5892 RFC 5892]. So when we upgrade to IDNA 2008 (a separate discussion),that should hopefully eliminate a large number of non-alphabet characters for us. [http://www.unicode.org/reports/tr39/#Mixed_Script_Detection Unicode Technical Report 39] has some more ideas gives a definition for refinements.We should look at that carefully and decide which ones to adopthow we detect whether a string is "single script".

The key thing is defining "single script" in a way which works well enough for the languages people actually use.Unicode has properties There is no requirement that every word must be registrable as a domain name (the Irish have had to do withouto'malley.ie since the beginnings of the DNS) but we can use for this; don't want to disallow whole swathes of a language'svocabulary. We would need to see how well the TR 39 algorithm works in practice. The document itself lists a number of"possible enhancements", which we would need to check theyconsider: * Count Common or Inherited characters that are only used with a limited number of scripts as being in either or each script, instead of ignoring them completely. For example, U+0640 ARABIC TATWEEL is used with the scripts Arabic and Syriac, but not Latin or Hangul. This work would be potentially time-consuming and complicated; we may have to call in domain experts.* Check for mixing numbers from different systems, such as U+0660 ( ٠ ) ARABIC-INDIC DIGIT ZERO with U+06F0 ( ۰ ) EXTENDED ARABIC-INDIC DIGIT ZERO, or U+09EA ( ৪ ) BENGALI DIGIT FOUR with U+0038 ( 8 ) DIGIT EIGHT. Perhaps we could restrict non-Arabic numerals to particular languages, e.g. Bengali numerals to Bengali?* Check for strings which contain both simplified-only and traditional-only Chinese characters, using the Unihan data in the Unicode Character Database. Does our platform have access to this data? If not, how large is it?were adequate* Detect sequences of the same nonspacing mark.* Check to see that all the characters are in the sets of exemplar characters for at least one language in the Unicode Common Locale Data Repository.[XXX What does this mean? -- Gerv] Also: * Should we document our character hard-blacklist as part of this exercise? Are any characters in it legal in IDNA2008?* Do we want to allow the user to choose between multiple "restriction levels", or have a hidden pref? There are significant downsides to allowing this...* Do we ever want to display errors other than just by using Punycode?