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RE: Support of IEEE float; Canonical XML"

From: Taki Kamiya <tkamiya@us.fujitsu.com>
Date: Wed, 20 May 2009 12:26:03 -0700
To: "'Paul Pierce'" <prp@teleport.com>, <public-exi-comments@w3.org>
Message-ID: <C808C7CBCBEF496AB87FB61E54E21604@homunculus>
Hi Paul,

You brought up a good point with regards to a better way for EXI to interact
with the function of signature/encryption. Indeed, EXI WG has been in conversation
with XML Security WG for jointly investigating the need of EXI C14N for
enabling both fast canonicalization/signature computation and compact signed

The discussion is refected in EXI Impacts document. It describes EXI C14N [1]
as an item that needs to be defined in the future.

At the same time, it is incumbent and no less important for EXI to be compatible
with the XML stack as it exists today which is entirely built on top of XML
Infoset. For this reason, EXI enables the reproduction canonicalized XML from
EXI given an adequate fidelity option, as it is mentioned in EXI Best Practices
document [2].

We thank you for your attention to EXI in general, as well as voicing your
concern on this particular issue. It helps raise awareness and underscores
the importance of this issue.


[1] http://www.w3.org/TR/exi-impacts/#xml-security
[2] http://www.w3.org/TR/exi-best-practices/#security

Taki Kamiya (for the EXI Working Group)

-----Original Message-----
From: public-exi-comments-request@w3.org [mailto:public-exi-comments-request@w3.org] On Behalf Of Paul Pierce
Sent: Tuesday, May 12, 2009 10:12 PM
To: EXI Comments
Subject: "RE: Support of IEEE float; Canonical XML"

Thank you all very much for posting the summary of the difficult deliberations
surrounding IEEE floating point. It makes it much easier for those of us who
feel it deserves more public consideration to directly address all the proper
issues instead of just whining in the dark. (Also thank you for the response to
my comment on number represenation, given the upcoming XML Schema feature you
pointed out that fully covers this case I see no reason to pursue it further,
but that response also highlights the problem I wish to address in this

In the tension between EXI being XML enough versus binary enough, a most
important decision was to keep EXI compatible with Canonical XML so that the
existing XML security mechanisms would work. This is an extreme constraint that
I feel will ultimately cripple the intended extensibility of EXI to the extent
that EXI will fail to reach anything like its real potential. It also creates
an environment in which non-conforming implementations are likely. But more
important is that it is poor practice.

In security, good practice is to "compress first, then encrypt". Documents
should be hashed, signed or encrypted in their most compact form, and the form
in which they are published (except for encryption.)

With XML, different forms are allowed mostly for readability. An infoset can be
represented in many forms, some more readable than others. Canonical XML
defines a single representation for an infoset that is quite compact (and not
very readable) so that its possible to generate a repeatable cryptographic hash
for signing. Good security practice would be to transform an infoset into
Canonical XML, sign it, and publish or transmit it in that form. If its
desirable to make it more readable down the line, it can be done without losing
the ability to convert it back to canonical form to check the signature. But
publishing the document in canonical form is the best way to help recipients
check the signature.

This good practice is not possible with EXI as its now defined. A signed
document cannot be published in EXI in the same form as was hashed. A signature
cannot be checked without (temporarily) expanding the document to many times
its original size, using conversions to characters that may be meaningless to
the data carried within.

There is a good alternative path for EXI. Its perhaps more difficult to define
but should be much easier to implement reliably. If you step back from
Canonical XML and look at how it is used in the XML Security framework, I think
you will find that EXI has all the necessary features, such as fragment
support. The challenge then is to find the EXI replacement for Canonical XML.

Since EXI does not have any need for readability, it is not necessary to allow
multiple forms except to accommodate generation options and make use of
available schema. It is tempting, given an XML infoset, a set of EXI options
and user-defined data types, and the set of schema referenced by the infoset,
to declare that exactly one valid EXI representation is possible. That bit
stream can be signed, and as long as the options etc. are retained the infoset
can be used to regenerate the same bit stream for signature verification.

There is a confounding issue in that the formal XML Infoset defines all values
in terms of characters, so that it rather than Canonical XML is the real
problem for those of us who would wish EXI to be more binary. This problem must
also be addressed for any future binary API for XML. But if EXI steps back from
Canonical XML to the XML Infoset, then the resolution of this issue for any
binary API or whatever could apply immediately to EXI. Otherwise EXI will be
left in the dust.

In any case, its vital to align EXI better with good practice in security. The
current situation amounts to paying lip service to compatibility with XML
Security and will certainly hurt EXI in the long run.

Paul Pierce
Received on Wednesday, 20 May 2009 19:26:54 UTC

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