[xmlsec] XmlDSig: The Reference Processing Model (Node set vs Octet stream)

[Alexwell Sandro]

Sorry for the confusion.

The digest transform require an "octet stream". So all retrieved data can
be used as "octet stream".

But, ArchiveTimeStamp from *XML Advanced Electronic
Signatures<http://www.etsi.org/deliver/etsi_ts%5C101900_101999%5C101903%5C01.04.02_60%5Cts_101903v010402p.pdf>
* (page 58) seems refers to "The Reference Process
Model<http://www.w3.org/TR/xmldsig-core/#sec-ReferenceProcessingModel>"
without the digest transform, because the digest algorithm may be weak.

*I will try to explain:*

If I signed a detached file, using Xpath to get the node "*child*" and
apply exc-c14n:

- FakeFile.xml

<file xmlns:ns1="ns1">
*<child>Text<child>*
<file/>

I know that the *<child>Text<child> *is a "node set" with in FakeFile.xml.
So I can use exc-c14n or inc-c14n.

But after retrieved process without apply digest transform. Is this "nod
set" or "octet strem"?

*The Reference process Model says:*

*Unless the URI-Reference is such a 'same-document' reference , the result
of dereferencing the URI-Reference MUST be an octet stream.
*
*
*
*URI="http://example.com/bar.xml#chapter1" *
*Identifies the element with ID attribute value 'chapter1' of the external
XML resource 'http://example.com/bar.xml', provided as an octet stream.*

"provided as an octet stream" is the resource or the element?

Consider a signature over the digest "sha1" of "*<child>Text<child>*"

*Now the algorithm is weak, and I want to add ArchiveTimeStamp, the
Specification says:*

*Process the retrieved ds:Reference element according to the reference
processing model of XMLDSIG.*
*
*
*If the result is a XML node set, canonicalize it. If ds:Canonicalization
is present, the algorithm indicated by this element is used. If not, the
standard canonicalization method specified by XMLDSIG is used.*

*I can understand this like: Get data using URI and ds:Transforms without
apply digest transform.*

So I will get *<child>Text<child> (*without digest transform)*, *if this is
XML node set, canonicalize it.
*
*
But this is a problem.
*
*
If I consider the result of extern URI as a "node set" to apply
ArchiveTimeStamp and the canonicalize transform used by Archive is
inc-c14n, I will archive another file.
*
*
*<child *xmlns:ns1="ns1"*>Text<child>*
*
*
*If the documento change:*
*
*
<file xmlns:ns1="ns1" xmlns:ns2="ns2">
*<child>Text<child>*
<file/>
*
*
*I will try to verify the **ArchiveTimeStamp with:*
*
*
*<child *xmlns:ns1="ns1" xmlns:ns2="ns2"*>Text<child>. *
*
*
*The process to verify will fail. Because the Archive is over:*
*
*
*<child *xmlns:ns1="ns1"*>Text<child>**
*
*
*
*The question.*
*
*
*For these References:*

<ds:Reference Id="myId" URI="http://fakefile.xml">
...
(empty transform list)
...
</ds:Reference>
*Result 1#: ( <file> ... childs ... <file> ).*

<ds:Reference Id="myId" URI="http://fakefile.xml">
...
<ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
...
</ds:Reference>
*Result 2#: (xml with exc-c14n).*

<ds:Reference Id="myId" URI="http://fakefile.xml">
...
<ds:Transform "fake_Xpath_transform_to_get_all_childs"/>
<ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
...
</ds:Reference>
*Result 3#: ( only childs with exc-c14n <child_1/>...<child_x/> ) This is
not valid XML file to parse, does not has single root. But can be "node
set" with in fakefile.xml context.*

*Are results (1#, 2# and 3#) "octet stream" ?*

Because: *Unless the URI-Reference is such a 'same-document' reference, the
result of dereferencing the URI-Reference MUST be an octet stream.*

*Or are they "XML node set" required by XML Advanced Electronic Signatures
to canonicalize?*

Because: *(...) In particular, an XML document identified by URI is not
parsed by the signature application unless the URI is a same-document
reference or unless a transform that requires XML parsing is applied.*

*Or the (unless a transform that requires XML parsing is applied) is valid
only with in Reference context and the results are octet stream?*

Thanks



On Fri, Aug 30, 2013 at 12:05 PM, Aleksey Sanin <aleksey at aleksey.com> wrote:

> I don't understand your question. At the end, digest is always
> applied to the octet stream. The rules in the spec describe how
> to get to the octet stream. In human language, there are only
> two rules:
>
> 1) Do not parse to XML unless it is necessary: it is already
> parsed (same document) or there is an XML transform (e.g. XPath)
> 2) To convert back to octet stream use the specified c14n method.
>
> IMHO, it's pretty straightforward and logical.
>
> Best,
>
> Aleksey
>
> On 8/30/13 6:50 AM, Alexwell Sandro wrote:
> > I do not know if this is the list for this discussion.
> >
> > I added this question to stackoverflow
> > <
> http://stackoverflow.com/questions/18522211/xmldsig-the-reference-processing-model-node-set-vs-octet-stream
> >.
> >
> > I'm studying XML Advanced Electronic Signatures
> > <
> http://www.etsi.org/deliver/etsi_ts%5C101900_101999%5C101903%5C01.04.02_60%5Cts_101903v010402p.pdf
> >.
> >
> > *To create "ArchiveTimeStamp" (page 58) the Specification says:*
> >
> > /Process the retrieved ds:Reference element according to the reference
> > processing model of XMLDSIG./
> > /
> > /
> > /If the result is a XML node set, canonicalize it. (...)/
> >
> > *The Reference Processing Model is over this:*
> >
> > <*ds:Reference *Id="myId" URI="http://fakefile.xml">
> >     <*ds:Transforms*>
> >         <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#
> "/>
> >     </ds:Transforms>
> >     <ds:DigestMethod/>
> >     <ds:DigestValue/>
> > </ds:Reference>
> >
> > The process uses "URI=..." and "ds:Transforms" to retrieve the data.
> >
> > *Below are some parts extracted from (4.3.3.2 The Reference Processing
> > Model <http://www.w3.org/TR/xmldsig-core/#sec-ReferenceProcessingModel
> >):*
> >
> > /The data-type of the result of URI dereferencing or subsequent
> > Transforms is either an octet stream or an XPath node-set. (...)/
> > /
> > /
> > /In this specification, a 'same-document' reference is defined as a
> > URI-Reference that consists of a hash sign ('#') followed by a fragment
> > or alternatively consists of an empty URI (...)/
> > /
> > /
> > /Unless the URI-Reference is such a 'same-document' reference, the
> > result of dereferencing the URI-Reference MUST be an octet stream. In
> > particular, an XML document identified by URI is not parsed by the
> > signature application unless the URI is a same-document reference or
> > unless a transform that requires XML parsing is applied./
> > /
> > /
> > /The following examples demonstrate what the URI attribute identifies
> > and how it is dereferenced:/
> > /
> > /
> > /URI="http://example.com/bar.xml" /
> > /Identifies the octets (...)/
> > /
> > /
> > /URI="http://example.com/bar.xml#chapter1" /
> > /Identifies the element with ID attribute value 'chapter1' of the
> > external XML resource (...), provided as an octet stream. (...)/
> > /
> > /
> > /URI="" /
> > /Identifies the node-set (...)/
> > /
> > /
> > /URI="#chapter1" /
> > /Identifies a node-set containing the (...)/
> >
> > *The question.*
> >
> > *For these References:*
> >
> > <ds:Reference Id="myId" URI="http://fakefile.xml">
> > ...
> > (empty transform list)
> > ...
> > </ds:Reference>
> > *Result 1#: ( <file> ... childs ... <file> ).*
> >
> > <ds:Reference Id="myId" URI="http://fakefile.xml">
> > ...
> > <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
> > ...
> > </ds:Reference>
> > *Result 2#: (xml with exc-c14n).*
> >
> > <ds:Reference Id="myId" URI="http://fakefile.xml">
> > ...
> > <ds:Transform "fake_Xpath_transform_to_get_all_childs"/>
> > <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
> > ...
> > </ds:Reference>
> > *Result 3#: ( only childs with exc-c14n <child_1/>...<child_x/> ) This
> > is not valid XML file to parse, does not has single root. But can be
> > "node set" with in fakefile.xml context.*
> >
> > *Are results (1#, 2# and 3#) "octet stream" ?*
> >
> > Because: /Unless the URI-Reference is such a 'same-document' reference,
> > the result of dereferencing the URI-Reference MUST be an octet stream./
> >
> > *Or are they "XML node set" required by XML Advanced Electronic
> > Signatures to canonicalize?*
> >
> > Because: /(...) In particular, an XML document identified by URI is not
> > parsed by the signature application unless the URI is a same-document
> > reference *or unless a transform that requires XML parsing is applied*./
> >
> > *Or the (unless a transform that requires XML parsing is applied) is
> > valid only with in Reference context and the results are octet stream?*
> >
> > Articles are welcome.
> >
> >
> > _______________________________________________
> > xmlsec mailing list
> > xmlsec at aleksey.com
> > http://www.aleksey.com/mailman/listinfo/xmlsec
> >
>
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