Re: A Problem with the "delegated:" Prefix

[Steven Legg <steven.legg@enitiatives.com.au>]

On 21/05/2012 2:11 PM, Steven Legg wrote:
> Step 1.d. in Section 4.5 of Committee Specification 1 of the XACML v3.0
> Administration and Delegation Profile Version 1.0 simply assumes that
> concatenating "urn:oasis:names:tc:xacml:3.0:attribute-category:delegated:" with
> the Category URI results in a valid URI. While this is true when the Category
> URI is a URN, it is not necessarily the case in general for any arbitrary URI
> because there are certain characters that are allowed in URIs, but are reserved
> or excluded in the Namespace Specific String (NSS) of a URN (in being the
> suffix, the Category URI becomes part of the NSS in the resulting URI).
>
> For example, consider a Category URI using the http scheme, e.g.,
> "http://example.com/MyCategory";. The "/" characters in the URI are reserved
> characters for a URN and RFC 2141 (which defines the URN syntax) says
> "developers SHOULD NOT use these characters in unencoded form". The draft for
> an update of RFC 2141, draft-ietf-urnbis-rfc2141bis-urn-02, goes further in
> disallowing an unencoded "/" in the NSS. Thus for the concatenation to be a
> valid URI now and in the future the "/" characters must be substituted with
> %2F. In general, any character in the Category that is reserved or excluded for
> URNs must be %-encoded in the concatenation.
>
> To address this problem XACML could either:
>
> (1) specify in 4.5 Step 1.d. that reserved and excluded characters in the
> Category must be %-encoded in the concatenation, or
>
> (2) restrict the URIs of categories to be URNs, or
>
> (3) use a separate, optional XML attribute to hold the prefix, i.e., in each of
> the six places where a Category attribute appears add a CategoryPrefix XML
> attribute alongside it. The section 4.5 mapping would populate the
> CategoryPrefix attribute instead of concatenating. AttributeDesignator and
> AttributeSelector (two of the six places) would only match XACML attributes
> with the same CategoryPrefix.

Two more possible solutions:

(4) Change the type of the Category and XPathCategory XML attributes to
normalizedString. This works provided the proposal to compare URIs code-point
by code-point is accepted.

(5) Change the type of the Category and XPathCategory XML attributes to a
list of anyURI, with comparisons treating the list as ordered. This has the
advantage of category identifiers still clearly being URIs. The procedure
for generating an administrative request would put the "delegated:" URI
at the front of the list. The trailing ":" wouldn't be needed.

I now prefer (5) over (3).

Regards,
Steven

> Personally, I would go with (3). It is more flexible and easier for
> implementors to get right compared to (1). It would also help to resolve a
> problem I raised previously with respect to an original access request that
> contains an <Attributes> element with a category that is the same as the
> category of another <Attributes> element in the same request except for the
> prefixing of "urn:oasis:names:tc:xacml:3.0:attribute-category:delegated:" (an
> email entitled "Multiple Decision Requests During Reduction"). Under (3), this
> would be two <Attributes> with the same category, only one of which also has a
> CategoryPrefix attribute. Either CategoryPrefix could be banned in access
> requests, or it could be ignored when deciding whether there were any
> duplicated categories in an access request.
>
> Regards,
> Steven