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[Moderator Action] Re: First draft of Section 7.6 and proposed sections 4.3.1 and 4.3.3

From: Lightning <lightning@pacificcoast.net>
Date: Tue, 12 Oct 1999 16:07:20 -0400
Message-Id: <3.0.5.32.19991012160720.00938430@localhost>
To: "IETF/W3C XML-DSig WG" <w3c-ietf-xmldsig@w3.org>
Hi Donald,

Great feedback.  Here are some points of agreement and some points of
consideration (though I find your points amenable).

=====================================
Regarding the addition of fragments to Location:

I was also quite troubled by not being able to do a simple ID reference in
the Location.  Either way this ends up going, it shouldn't impact a lot of
the material in 7.6.  However, I did do a fair amount of background reading
through many of the specs (which is why it took me so long to write, even
though for the sake of terseness it doesn't necessarily come across).
According to RFC2396, the fragment part after URI# is not assumed to be an
ID reference into XML.  "It is a property of the data resulting from a
retrieval action, regardless of the type of URI used in the reference.
Therefore, the format and interpretation of fragment identifiers is
dependent on the media type [RFC2046] of the retrieval result".  So, if we
follow the consistency angle to the end, it seems that we have to be
prepared for someone putting a full-blown XPointer after the URI#, or for
that matter any document-specific, arbitrarily complicated reference
expression after the URI#.

Actually, this isn't overly troublesome since applications will still have
the Type information (Section 4.3.2) to help decide which parser to run on
the material after the # (or if the application can process the fragment at
all).  The parts that are a little more troublesome are as follows:

1) It is not possible to distinguish between XPath and XPointer in Location
whereas it is possible under the current formulation of section 7.6.  Aside
from inconsistency, this actually could be useful for those who are in those
constrained situations and feel that XPath support is sufficient whereas
XPointer is too burdensome.

2) Applications would need two quite different algorithms for determining
whether they could support partial document signatures.

To be honest, I like using a URI-reference rather than a URI in Location,
and would be fine with seeing it in both places.  I just wanted you to know
that it wasn't a spur-of-the-moment recommendation.  These are the issues I
came up with, and they seemed important enough to put forward for
consideration.  Hopefully we can discuss this at the next teleconference and
decide how it should be.  I'll be happy to reword the sections in accordance
with the decisions made.

===============================================
Regarding default canonicalization versus no Transformations

>I believe consensus was no transformation.  If some default
>canonicalization was applied, it would have to be data type dependent
>since even minimal canonicalization doens't make much sense for, say,
>a JPEG file.

I agree wholeheartedly and will be quite happy (relieved in fact) to change
this.  The comment was actually copied from the existing spec.  Given the
other changes I recommended, perhaps it would've been wiser to recommend
changing that too.  However, I assumed the default would end up being null
c14n anyway.  It seems best to leave the data alone unless an 'explicit'
statement of Transformation is made.

==================================
Regarding Handling of Encoding Information

>><p>The <code>Transformations</code> element contains an ordered list
>>of <code>Transformation</code> elements.  The output of each
>><code>Transformation</code> serves as input to the next
>><code>Transformation</code>.  The input to the first
>><code>Transformation</code> is the raw data result of obtaining the
>>resource given by <code>Location</code>.
>>The output from the last <code>Transformation</code> is the input for the
>>digest algorithm.</p>
>
>I believe that encoding information should be input to the first
>transformation and passed along, possibly changed by some
>transformations.

I don't understand. The two places where encoding comes into play are 1)
encoding the actual Transformation element's content, and 2) Encoding of the
object indicated by Location, which will be decoded by some Transformation.

In the former case it is obvious that encoding information should not be
passed along since it applies only to the immediate transform, which must be
decoded so we can find out what the transform is supposed to do (e.g. a Java
class for decompression).  In the latter case, it seems neither necessary
nor always feasible to pass encoding information along.  Suppose an
application puts some base64 encoded data into an element as follows:

<MyData id="Data1">
    asdfasdfasdfasdfasdfasdfadsf
</MyData>

If they wanted to mark Data1 as base64 encoded, they would have to use *our*
base64 encoding marker (currently urn:dsig:base64) rather than there own.
This is why I thought it would be best to denote the encoding in one of
*our* elements (namely, a Transformation that brings about base64 decoding).
Furthermore, this means that the decoding can be preceded by other
transformations.  For example, to meet requirement 3.1.7, the necessary
transformation sequence for recovering the original data out of Data1 is

<Transformations>
    <Transformation
Algorithm="urn:dsig:xpointer">id("Data1")/descendant::text()</Transformation
>
    <Transformation Algorithm="urn:dsig:base64"/>
</Transformations>

or, if one allows fragments in Location

<Location>#Data1</Location>
<Type>text/xml</Type>
<Transformations>
    <Transformation
Algorithm="urn:dsig:xpath">descendant::text()</Transformation>
    <Transformation Algorithm="urn:dsig:base64"/>
</Transformations>

Either way, it seems that the easiest way for an application to indicate
that the content was base64 encoded is to put a base64 decoding
transformation at the appropriate place in the list rather than having an
attribute on MyData that must be passed through the descendant::text()
transform (despite the intended semantic of throwing out the start and end
tags and the attributes).

=================================
Regarding Parameterization of Transforms

I am glad you also like the view that the transformation element content is
opaque to us if the Transform is not one of the defined algorithms.

========================================
Regarding Stating Recommendations in the Positive

Quite true, it reads a lot better that way.  Applications should use the
enumerated algorithms in Section 7.6 whenever possible.  I'll be happy to
change that immediately.

======================
Regarding These Comments


These were copied from the current spec.  I can reword the first, and Dave
can move and/or reword the second.

>><p class="comment">Implementation Comment: When transformations are
applied
>>the signer is
>>not signing the native (original) document but the resulting (transformed)
>>document that
>>is not captured explicitly in the signature syntax. Where transformation
>>processes are
>>well known and widely implemented an application might include native
>>content and specify
>>transformations by reference. Otherwise, an application may perform
>>transformations on the
>>content itself and use the resulting content within the signature. </p>

>I think I know what you are trying to say but I'm not sure it quite
>says it.  For example, the base64 of an "original" binary MPEG might
>be included and the Transform used to retore it to its original form.


>><p class="comment">Security Comment: Applications are recommended to
ensure
>>signers
>>understand the actual resulting content that is being signed after
>>transformations are
>>applied. Users should not be tricked into signing a native content that is
>>transformed
>>into something that the user would not have signed otherwise. This
>>recommendation applied
>>to transformations specified in the signature block, as well as
>>transformations found
>>within the document itself. </p>

>Comments along this line are definitely needed but should be in the
>Security Considerations section.  A reference to that section could be
>included here.




=====================================
Regarding  the Canonicalization Transformation

>
>I think there will be few enough standard canonicalization algorithms
>that they can have different algorithm values.
>
>Null, Minimal, DOMcanon, W3Ccannon.  DOMcanon might take a parameter
>or have versions to determin (1) if it discards comments and (2) if
>it discards Processing Instructions.
>

I agree that there are few c14n algorithms, but we already enumerate them in
the c14nalg element (or whatever it will end up being called).  It would be
fine with me if we wanted to enumerate them again as Algorithm values for
Transformation elements, but it seemed more appropriate at the time of the
first draft to reuse the existing markup definitions in Sections 4.1 and 7.5
so that changes to those sections would not imply changes to sections 4.3.3
and 7.6.


====================================
Regarding the XPath Transformation Algorithm


>
>The above it not enough to specify how the output is formed.  Are
>there any new lines?
>

I believe the statement I gave is precisely what is required, though I could
explain more about my readings of the XPath spec, which could be helpful to
readers of the dsig spec.  The linefeeds are in the XPath node-set if the
XPath specifies them as being in the node set.  They are represented by text
nodes just like all the other text in the document.  Actually, they do not
appear as separate text nodes if there is other text in the element.  So, if
your character sequence is

<Parent>\n\t<Child>multiline\ncontent</Child>\n</Parent>

Then you would have three text nodes, one for the "\n\t", one for
"multiline\ncontent" and one for "\n".

Thanks,
John Boyer
Software Development Manager
UWI.Com -- The Internet Commerce Company
Received on Tuesday, 12 October 1999 16:07:27 GMT

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