[whatwg] URL spec and IDN

Joshua Cranmer Pidgeot18 at verizon.net
Wed Feb 19 15:53:29 PST 2014

I've noted that the URL specification is currently rather vague when it 
comes to IDN, and has some uncertain comments about issues related to 
IDNA2003, IDNA2008, and UTS #46.

Roughly speaking, in my experience, there are three kinds of labels: 
A-labels, U-labels, and "displayable" U-labels. A-labels are the 
Punycode-encoded version of the labels used for DNS (normalized to ASCII 
lower-case, naturally). U-labels are the results of converting an 
A-label to Unicode. "Displayable" U-labels are A-labels converted to 
U-labels only if they do not contain a Unicode homograph attack. My 
drawing a distinction between the "displayable" U-label and the regular 
kind is out of concern that the definition of "displayable" may change 
over time (e.g., certain script combinations are newly 
permitted/prohibited), whereas the U-label derived from an A-label 
should be constant.

Given these three kinds of labels, it ought to be possible (IMO) to 
convert a generic domain in any (i.e., unnormalized) format. The 
specification currently provides for a domainToASCII and a 
domainToUnicode function which naturally map to producing A-labels and 
U-labels, but contains a note suggesting that they shouldn't be 
implemented due to the "IDNA clusterfuck." The way to a "displayable" 
U-label would seem to me to come most naturally via |new URL("http://" + 

Looking at the spec, it's not clear if the host, href, and other methods 
are supposed to return U-labels or A-labels (or some potential mix of 
the two). Running some tests appears to indicate that Firefox, Chrome, 
and IE actually all do different things:
* Firefox appears to use "displayable" U-labels (i.e., the result 
matches what is seen in the address bar)
* Chrome appears to always use A-labels
* IE appears to always use U-labels (on http://☃.net, the address bar 
displays http://xn--n3h.net but location.href returns http://☃.net).

I'm guessing the reason why the domainTo* methods are unspecified are 
due to inconsistent handling of IDNA2008 by current web browsers, 
although it appears to me that a consensus has emerged on 
implementation. Chrome and IE appear to roughly implement UTR #46 
transitional mode:
* Uses Unicode $RELATIVELY_NEW for case folding/mapping
* Processes eszett, final sigma, ZWJ, and ZWNJ according to IDNA2003 
(UTR #46's transitional) instead of IDNA2008
* Symbols and punctuation are allowed, in violation of IDNA2008.
* The BiDi check rules appear to follow IDNA2008 instead of IDNA2003. 
Neither Chrome's documentation nor IE's documentation are explicit about 
the changes (sufficiently so, at least, for someone like me who doesn't 
know all that much about how IDNA* is implemented but rather wants to 
make sure things work "correctly" and safely).
* They do not appear to enforce IDNA2008's contextual rules (again, the 
documentation is slightly unclear).
* Chrome's documentation calls out ignoring STD3 rules (i.e., permitting 
more ASCII characters) and disallowing unassigned code points. IE's 
documentation does not suggest what they do here.
Firefox still implements IDNA2003, but this is only because the person 
who would be implementing IDNA2008 lacks the time.

Given these facts, I'd like to propose changes to the URL spec to better 
specify and more reliably reflect IDN processing as is currently done:
1. Expressly identify how to normalize and process an IDN address under 
IDNA2008 + UTR #46 + other modifications that reflects reality. I'm not 
qualified to know what happens at precise edge cases here.
2. Resolve that URL should reflect U-labels as much as possible while 
placing the burden of avoiding Unicode homograph attacks on the browser 
implementors rather than JS consumers of the API.
3. The comments about avoiding implementation on domainTo* methods 
should be dropped.
4. Tests should be added to ensure that domain labels are processed 
correctly. There is already a testsuite for UTR #46 processing available 
at http://www.unicode.org/Public/idna, which I suspect could be adapted 
into a testsuite for processing domain labels.
5. Browser vendors should implement domainToASCII and domainToUnicode to 
at least as well as they internally implement these methods, even in 
lieu of precise definitions on how to handle IDN. In any case, I'd 
rather have IDN handling that matches how my browser implements it 
internally than one that matches an official, blessed spec, if the two 
diverge. Note that this includes handling deciding when U-labels are 
"safe" to display (assuming that browsers are competent to enough to 
already think about this).

Beware of bugs in the above code; I have only proved it correct, not tried it. -- Donald E. Knuth

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