IDN Display Algorithm

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

Implementing this plan is covered by bug 722299.


The Problem

If we just display any possible IDN label, we open ourselves up to 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).

Current Algorithm

Our current algorithm is to display as Unicode all IDN labels within TLDs on our 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 "/" and ".", and invisible characters.

Need For Change

This strategy provides pretty good user protection, and it provides consistency - every Firefox everywhere works the same. However, it does mean that IDNs do not work at all in many TLDs, because the registry (for whatever reason) has not applied for inclusion, or because we do not think they have sufficiently strong protections in place. In addition, ICANN is about to open a large number of new TLDs. So either maintaining a whitelist is going to become burdensome, or the list will become wildly out of date and we will not be serving our users.

Other Browsers

The Chromium IDN page has a good summary of the policies of Chrome/Chromium and the other browsers. Unfortunately, no consensus has emerged on how to do this. Those other mechanisms were considered, but many of them depend on the configuration of the user's computer (e.g. installed languages), and this does not give site owners any confidence that their IDN will be correctly displayed for all their visitors (and no way of telling if it's not).


The plan is to augment our whitelist with something based on ascertaining whether all the characters in a label all come from the same script, or are from one of a limited and defined number of allowable combinations. The hope is that any intra-script near-homographs will be recognisable to people who understand that script.

We will retain the whitelist as well, because a) removing it might break some domains which worked previously, and b) if a registry submits a good policy, we have the ability to give them more freedom than the default restrictions do. So an IDN would be shown as Unicode if the TLD was on the whitelist or, if not, if it met the criteria above.


If a TLD is in the whitelist, we will unconditionally display Unicode. If it is not, the following algorithm will apply.

Unicode Technical Report 39 defines a "Moderately Restrictive" profile. It says the following (with edits for clarity, after discussion with the author):

No characters in the label can be outside of the Identifier Profile (defined for us by the IDNA2008 standard, RFC 5892).

All characters in each label must be from Common + Inherited + a single script, or from one of the following combinations:

  • Common + Inherited + Latin + Han + Hiragana + Katakana; or
  • Common + Inherited + Latin + Han + Bopomofo; or
  • Common + Inherited + Latin + Han + Hangul; or
  • Common + Inherited + Latin + any single other "Recommended" or "Aspirational" script except Cyrillic or Greek

Unicode Technical Report 39 gives a definition for how we detect whether a string is "single script".

Some Common or Inherited characters are only used in a small number (but more than one) script. Mark Davis writes: "The Unicode Consortium in U6.1 (due out soon) is adding the property Script_Extensions, to provide data about characters which are only used in a few (but more than one) script. The sample code in #39 should be updated to include that, so handling such cases." This data is now available, but not yet in the Firefox platform (bug 844106). In the mean time, Common and Inherited characters are permitted without restriction.

We also implement additional checks, as suggested by TR #39 sections 5.3 and 5.4:

  • Display as Punycode labels which use more than one numbering system
  • Display as Punycode labels which contain both simplified-only and traditional-only Chinese characters -- bug 857481
  • Display as Punycode labels which have sequences of the same nonspacing mark

Possible Issues and Open Questions

The following issues are open, but should not block initial implementation.

Further suggestions from TR #39:

  • 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]


  • Should we document our character hard-blacklist as part of this exercise? It's already visible in the prefs. Are any characters in it legal in IDNA2008 anyway? We should review it to make sure we aren't disallowing characters now allowed by IDNA 2008.
  • 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? I suggest not...
  • Should we add Armenian to the list of scripts which cannot mix with Latin?


This system would permit whole-script confusables (All-Latin "scope.tld" vs all-Cyrillic "ѕсоре.tld"). However, so do the solutions of the other browsers, and it has not proved to be a significant problem so far. If there is a problem, every browser is equally affected.

If problems arose in the future (e.g. whole-script, or homographs between a particular single script and Latin), our response would be that in the end, it is up to registries to make sure that their customers cannot rip each other off. Browsers can put some technical restrictions in place, but we are not in a position to do this job for them while still maintaining a level playing field for non-Latin scripts on the web. The registries are the only people in a position to implement the proper checking here. For our part, we want to make sure we don't treat non-Latin scripts as second-class citizens.


In between adopting this plan and shipping a Firefox with the restrictions implemented, we will admit into the whitelist any TLD whose anti-spoofing policies at registration time were at least as strong as those outlined above.