RE: Requesting guidance on EXI fragment encryption and signature

Don,

 

I do not have guidance, but the questions are nonetheless of interest to me.
So, if you do not mind, let me make a few statements and then ask a few
questions for clarification.

 

I also looked at compressing EXI for an application I am working on, but
decided that compression (e.g., such as with zlib) was not desirable, as I
would lose the ability to manipulate the stream on the wire: I really
appreciate that attribute of both XML and EXI.

 

We looked at a few modes of EXI as compared to using zlib (without EXI at
all) and came up with this:

http://www.packetizer.com/xml/xml_compression.html

 

It would be interesting to see other comparisons.  The net is that "strict
compressed" yielded the best results, but we do not necessarily have the
schema in hand in our case, so that was out.  Zlib did extremely well for
XML documents between 300 bytes and 8K.  For smaller messages, EXI alone
(without compression) did better.  What was interesting was that above 8K,
it seemed that all modes (including zlib compression) largely converged in
size.

 

I am curious: will compression be necessary in your case, or would EXI alone
be sufficient?

 

Is the purpose for using EXI on only a part of the document to allow part of
the document to be visible by entities that need to know a bit about the
contents without decoding it?  If you have large documents and use any form
of EXI, wouldn't using EXI across the whole XML document be workable?
Inspecting the EXI binary seems to be relatively simple: at any rate, one is
going to employ a parser one way or the other.

 

Is the purpose to allow one to apply a signature over the EXI parts only,
with the unsigned parts outside?  In other words, you're defining a
"container", of sorts, to hold the real information, while placing the
signature that applies to that data outside in normal XML.  If so, s there
any reason to not simply define such en explicit wrapper?  The signature
issue is really a big deal and I've not been to happy with the existing
options.  So, the line of questioning is interesting - and I suspect to a
number of people.

 

Paul

 

From: public-exi-request@w3.org [mailto:public-exi-request@w3.org] On Behalf
Of Snyder, Sheldon (LT)
Sent: Tuesday, October 20, 2009 8:30 PM
To: public-exi@w3.org; Brutzman, Donald (Don) (CIV); McGregor, Donald (Don)
(CIV)
Subject: Requesting guidance on EXI fragment encryption and signature

 

EXI Working Group:

 

Requesting guidance on EXI fragment encryption and signature:

1.      Sometimes want to compress only part of an XML document and retain
the results in context within the new document.

2.      XML signature and XML encryption require XML inputs.

3.      Therefore an EXI document or EXI fragment cannot be signed or
encrypted because they are not XML.

4.      Need formal definition on how to embed an EXI fragment in an XML
document so that it can be signed and or encrypted.

 

Example:

Want to EXI the SequenceOfEvents and retain in the XML document for digital
signature and encryption:

 

<?xml version="1.0" encoding="UTF-8"?>

<PlanOfDay>

    <dayOfWeek date="Monday 18 October 2009"/>

    <Title>Some Tile for the day's Events</Title>

    <SequenceOfEvents>

        <Event>The first event of the day</Event>

        <Event>The second event of the day</Event>

        <Event>The third event of the day</Event>

    </SequenceOfEvents>

</PlanOfDay>

 

.....

 

<?xml version="1.0" encoding="UTF-8"?>

<PlanOfDay>

    <dayOfWeek date="Monday 18 October 2009"/>

    <Title>Some Tile for the day's Events</Title>

            EXI of SequenceOfEvents

</PlanOfDay>

 

Could do but:

<?xml version="1.0" encoding="UTF-8"?>

<PlanOfDay>

    <dayOfWeek date="Monday 18 October 2009"/>

    <Title>Some Tile for the day's Events</Title>

    <SequenceOfEvents>

            EXI Fragment of SequenceOfEvents

    </SequenceOfEvents>

</PlanOfDay>

 

However, once the XML is compressed to EXI it is no longer effectively
compressible because EXI will have few if any redundant bytes; no
compression algorithm will deliver good results.  

 

Some potential solutions of consideration:

1.      is a base64 formatting such as Message Transmission Optimization
Mechanism (MTOM)

2.      XML-binary Optimized Packaging (XOP)

3.      Williams, Jeffrey S., Document-Centric XML Encryption and
Authentication for Coalition Messaging, Masters Thesis, Naval Postgraduate
School, Monterey California, September 2009.

 

For additional guidance on the intent of this question please email Don
Brutzman: Brutzman@nps.edu