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RE: C14N Argument

From: Dournaee, Blake <bdournaee@rsasecurity.com>
Date: Fri, 27 Jul 2001 16:21:58 -0700
Message-ID: <E7B6CB80230AD31185AD0008C7EBC4D2DAEFDD@exrsa01.rsa.com>
To: "'Joseph M. Reagle Jr.'" <reagle@w3.org>
Cc: "'w3c-ietf-xmldsig@w3.org'" <w3c-ietf-xmldsig@w3.org>
Joseph, John, All,

The way I understand the spec is as follows.

Consider the given <Reference> element:

<Reference URI="#foo">
	<Transform Algorithm="&xpath;">some Xpath</Transform>

When this is processed, the URI reference produces a node-set (I'll call
this "N"). Further, define two 
functions XPATH() and Convert(). XPATH() accepts a node-set and produces a
node-set and Convert() is 
the "conversion" from node-set to octet stream, using the "specified
canonicalization algorithm." Convert() is really equivalent to one run of

N -> XPATH(N) -> Convert(N)-> SHA1()

Now, I would say that Canonicalization happens a single time here.

Next, let's expand on the above example as follows:

<Reference URI="#foo">
	<Transform Algorithm="&xpath;">some Xpath</Transform>
	<Transform Algorithm="$c14n;"/>

Now the processing looks like this:

N -> XPATH(B) -> C14N(N) -> Convert(N) -> SHA1()

Here, I would say that canonicalization happened *twice*, once explicitly
and once implicitly. 

This is where I think people will get confused, thinking that they must
canonicalize when in reality
they are just wasting processing power.

Is this clear?, or am I misunderstanding something here?

The crux of the problem is that it is stated that the "octet stream must be
converted to a node-set", this
means, really, "Canonical XML" because this is the way the algorithm is
defined (to produce a physical representation).

This is what I mean by implicit canonicalization. When you convert, you
canonicalize. You "convert" any time you
operate on a node-set somewhere in the transformation pipeline, therefore,
if you explicitly use C14N, the possibility
is there for redundant processing.

Blake Dournaee
Toolkit Applications Engineer
RSA Security
"The only thing I know is that I know nothing" - Socrates

-----Original Message-----
From: Joseph M. Reagle Jr. [mailto:reagle@w3.org]
Sent: Friday, July 27, 2001 2:29 PM
To: Dournaee, Blake
Cc: 'w3c-ietf-xmldsig@w3.org'
Subject: RE: C14N Argument

At 17:24 7/27/2001, Dournaee, Blake wrote:
>"Canonicalization is used implicitly when a node-set is converted to an
>octet stream in the transformation pipeline. Care should be made not to
>include it unnecessarily as an explicit transform. Doing so may affect core
>processing performance."

Hrmm... I don't want to encourage not using it explicitly. If I have a

   <Transform Algorithm="&xpath;">some Xpath</Transform>
   <Transform Algorithm="$c14n;"/>

Then exactly 1 canonicalization was done, the explicit one. (Could you given

an example of a mistaken double c14n (implicit/explicit)?)

Joseph Reagle Jr.                 http://www.w3.org/People/Reagle/
W3C Policy Analyst                mailto:reagle@w3.org
IETF/W3C XML-Signature Co-Chair   http://www.w3.org/Signature
W3C XML Encryption Chair          http://www.w3.org/Encryption/2001/
Received on Friday, 27 July 2001 19:18:28 UTC

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