W3C home > Mailing lists > Public > w3c-ietf-xmldsig@w3.org > April to June 2002

Re: exc c14n bugs

From: merlin <merlin@baltimore.ie>
Date: Thu, 06 Jun 2002 13:41:44 +0100
To: "John Boyer" <JBoyer@pureedge.com>
Cc: w3c-ietf-xmldsig@w3.org
Message-Id: <20020606124144.18E0A4432D@yog-sothoth.ie.baltimore.com>

>Well, we are already overriding the c14n handling of xmlns="", so it doesn't s
>eem a justifiable defense. Also, your conclusion that the xmlns="" is unnecess
>ary is based only on the specific implementation method of transferring E from
> one envelope to another by reparenting a node-set.  The developer cannot writ
>e a new envelope by writing "<envelope xmlns='http://mynamespace'>" + Exclusiv
>e canonicalization + "</envelope>", and it seems odd that such a simplistic th
>ing is broken.  Finally, why would we handle:
><Foo xmlns="http://example.org"/>
>differently from 
><Foo xmlns=""/>

We treat it differently because, in general, we omit redundant
namespace nodes; and, parsed in isolation, xmlns="" on a
top-level element is redundant.

I agree that omitting this definition does cause problems with
(non-pejoratively) na´ve textual wrapping. In particular; pure
c14n (as recommended by the spec) is not a good serializer
for XML encryption because the wrapping and parsing language
will cause problems.

However; I don't think that patching exc-c14n, in isolation,
is the solution. The fact is, the problem will remain for
c14n, for exc-c14n with #default in the prefix list, and
for most standard serializers.

I would rather characterize this as a common fault of
canonicalization algorithms derived from c14n and many standard
serializers, and describe a general solution for systems that
serialize XML for subsequent na´ve wrapping:

  When serializing XML for subsequent wrapping and parsing,
  serialization of the namespace axis should be changed as
  follows: The declaration xmlns="" SHOULD be emitted where it
  would normally be suppressed because no ancestor element in
  the node set has a namespace node in the node set declaring
  a value for the default namespace.

  The result may, therefore, not be in pure canonical form,
  even if a canonicalization algorithm was used. The resulting
  octet stream will not be used directly in a signature
  computation, so an exact specification is not necessary.

  This can be retroactively applied to an existing c14n
  implementation by canonicalizing each apex node and its
  descendants from the node set, inserting ' xmlns=""' at
  the appopriate points, and concatenating the resulting
  ctet streams.

As I say; I'm not disagreeing that this behaviour causes
problems. I just think that a partial fix is potentially
worse than no fix. I think that we will need language of
the above form in, at least, the encryption spec; and, if we
correct exc-c14n-without-#default then we will need further
language to disable this patch when that particular variant
of exc-c14n is used as a serializer.

If you think that this fix must go in exc-c14n then I will not
argue further, just point out that, unlike the current text
revisions which clarify exc-c14n, you will be breaking exc-c14n
so we will need a new namespace and consequent recycling, and
request that we also put xmlns="" in apex nodes if #default
is in the prefix list and the apex node is an element node
with no default namespace node in the node set. The encryption
spec can then provide a patch just for non-exc-c14n.

My preference would be not to patch exc-c14n, but instead
to provide text in XML encryption. The problem extends
beyond the use of canonicalization algorithms. If people
simply encrypt a serialized document from the filesystem,
which will probably omit xmlns="", na´vely expecting things
to work in decrypt-and-replace mode, they won't. This is a
general problem and needs a general fix.

Received on Thursday, 6 June 2002 08:42:17 UTC

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