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Soap Message Canonicalization (SM-C14N)

From: Rich Salz <rsalz@zolera.com>
Date: Fri, 15 Feb 2002 10:59:25 -0500
Message-Id: <200202151559.g1FFxPi27177@os390.zolera.com>
To: xml-dist-app@w3.org
Here's my action item to write up how to canonicalize SOAP messages.
I'm basing it on Henrik's proposal for what message rewrites are allowed [1].

In doing so, I came across a problem.  The proposal allows an intermediary
to remove the actor attribute if it's targeted to the ultimate recipient.
If this remains, it means that only entities that know the recipient can
verify a signature.  Speaking as someone who sells generic DSIG servers,
I think that's a mistake. :)  I see three choices (in my decreasing order
of preference):
1   Remove that from the proposal
2   Require a "parameter" to the SM-C14N so the recipient can be
    identified.  E.g., in an XML DSIG you'd have a transform like this:
	<disg:Transform disg:Algorithm="[[value; see below]]">
3   Limit verification to those who know the recipient


Second, since intermediaries can add and remove headers, it's necessary
to define an ordering.  I chose alpha-order, as that will often not
require the full rendering of all elements to be buffered.

Anyhow, my proposed text appears below.  The prose is a little turgid,

[1] http://lists.w3.org/Archives/Public/xml-dist-app/2002Feb/0183.html

Soap Message Canonicalization (SM-C14N)

Because intermediaries have some flexibility in serializing messages that
pass through them, it is necessary to define a canonicalization method so
that all semantically equivalent serializations will render identically.
This is necessary, e.g., to generate a message digest for a digital
signature, maintain a replay cache, and so on.

This mechanism is called SOAP Message Canonicalization (SM-C14N).
It is identified by the following URI: [[value needed]].  SM-C14N can be
targetted at an individual header or body element, a set of such elements,
or an entire SOAP message.

When targetted to an individual element information item, the following
steps are performed:
1.  Any namespace declarations that are inherited from the outer SOAP
    element, and used within the element, are treated as if they were
    declared by the element.
2.  if the SOAP mustUnderstand attribute information item is present
    with a non-false value, the value is taken to be "1"
3.  if the SOAP mustUnderstand attribute information item is present
    with a false value, the attribute information item is ignored
4.  if the SOAP actor 
The element is then processed according to XML-C14N.

When targetted to multiple element information items, each is processed
as described above.  The elements are then sorted lexigraphically,
and a single newline (&#A) is inserted between each one.

When targetted to a SOAP Header or Body element information item, the
result is computed as if the canonicalization was applied only to all
the immediate child elements of the SOAP element.  Note that the SOAP
Header or Body element information item itself is not directly used.

When targetted to a SOAP message information item, the result is
calculated as the concatenation of targetting the SOAP Header element
information item, a single newline, and targetting the SOAP Body element
information item.  Note that the SOAP Message element information item
itself is not directly used.
Received on Friday, 15 February 2002 10:58:11 UTC

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