dereference filter

Hi all,

i have a question regarding the use of (pre-defined) transforms one can use 
in a signature. Let me introduce a use case to explain my question. I have 
an XML resource which contains an <item> element with a ref attribute 
containing a URI value (http://foo/bar.html) pointing to a second resource.

<root>
....
<dsig:Signature>...</dsig:Signature>
....
<item ref="http://foo/bar.jpeg"/>
....
</root>

Now, the second resource is the object I would like to sign. Note, that the 
signature itself might also be included in this first XML Resource. So, I 
could use a signature XPath transform to grab the value of the ref attribute 
of the item element. However It is unclear to me which transform I should 
use to derefence the obtained URI. Are there any existing filters I can use 
with this respect?

I am aware that I can put the ref value directly in the URI attribute of the 
dsig:Reference element, but let's assume for now that due to some 
application restrictions, I cannot do so.

Someone?
Thanks
Thomas

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