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Subject: Re: [xacml-comment] What is "URI equality" ?

Hi Steven and Erik,

I think the 3986 spec is not quite as bleak as Steven's last email.
Basically, what I believe it says is that you can determine "equality"
unambiguously. However, the same cannot be said for "inequality".
i.e. if your Policy depends on 2 URI's being equal, then you should
be OK. However, there may be cases where they are found to be
"unequal", but, in fact, could later be proved to be equal.

Also, I recommend fixing the 3.0 core spec to refer to 3986 in the
3 places 2396 is ref'd, which is in the list of refs, plus 2 other
places that you can find searching for "2396".

Also, we should make explicit the meaning of "equal" and "unequal"
in the spec by basically repeating the salient points from 3986
(as usual, all above is "imo").


On 10/27/2011 5:13 AM, Erik Rissanen wrote:
Hi Steven, Rich,

I remember that we discussed uri equality for the anyURI-equals function. If I recall correctly, in XACML 2.0 it referenced a draft specification of XPath 2.0, which defined a uri equals function. For XACML 3.0 we updated all XPath 2.0 references to the final W3C recommendation. However, the final recommendation had dropped the uri equality function. So what we did was to look back at the XPath draft and we copied in the definition from there into the XACML 3.0 spec. Hence the anyURI-equals function says "if the values of the two arguments are equal on a codepoint-by-codepoint basis".

We did not notice though that there are other cases where URIs are compared, not just the equals function. We should have updated the other similarly. The very least, good implementation advice would be to use the same definition as for the anyURI-equals function for the other URI equality tests.

Best regards,

On 2011-10-27 08:32, Steven Legg wrote:

Hi Rich,

Thanks for the prompt reply.

RFC 3986 doesn't so much define URI equality as leave it to
applications to decide how thoroughly they want to compare URIs.
Since XACML is mute on the issue that effectively makes URI
equality in XACML implementation defined. Perhaps that should
be acknowledged in the spec ? Or at least a mention that it's
not the same thing as anyURI-equal.


On 27/10/2011 3:06 AM, rich levinson wrote:
Hi Steven,

"URI equality" is defined in the URI specification RFC 3986:
"Uniform Resource Identifier (URI): Generic Syntax"
which in section 6, goes into detail as to what is involved:

The bottom line is that there is no accepted unambiguous way
to determine absolutely that 2 URIs are "equal", and the
objective has been reduced to:

    "Therefore, comparison methods are designed to
    minimize false negatives
    while strictly avoiding false positives. "

In other words (I interpret this to mean),

  * a determination of "equivalent" should, in general,
    be considered to be "accepted as absolutely true",
  * while a determination of "non-equivalent" should,
    in general, be considered to be accepted as "true",
    but, with the qualification that further investigation
    may under some circumstances find that the result
    may be "equivalent" when those additional
    circumstances are included in the evaluation.

Note that the core spec refers to the August 1998 version:
"Uniform Resource Identifiers (URI): Generic Syntax"
which elaborates less on this issue but does say in section 6:

    "In general, the rules for equivalence
    and definition of a normal form, if any,
    are scheme dependent."

-> XACML 3.0 core spec should be updated to refer to RFC3986,
which has "obsoleted" RFC2396.


On 10/25/2011 7:36 PM, Steven Legg wrote:

Sections 5.29 and 7.3.4 of the Committee Specification 1 XACMLv3 core
specification define the matching of Category, AttributeId and DataType
XML attributes according to "URI equality". I've always assumed that
URI equality was the same as the matching performed by the
urn:oasis:names:tc:xacml:1.0:function:anyURI-equal function, but I can't
find anything in the specification to justify that assumption since
"URI equality" is not defined anywhere. It would help if the specification
clarified what "URI equality" actually means.

Similarly, there is also a reference to undefined "string equality"
matching in section 5.24.


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