Update to encrypting attributes and arbitrary data based on conve rsations with SYMM group

For the record, I'd like to make some slight modifications to the 
EncryptedDataManifest example.  These modifications are the result
of discussions with the W3C SYMM group.

In the example, we start with

   <video src="secret.mpg" alt="Alien spaceship in hangar at Area 51."/>

and we want to encrypt the MPEG and the value of the alt attribute.
In the earlier resultant
(see
"http://lists.w3.org/Archives/Public/xml-encryption/2001Feb/0022.html"), 
the src attribute remains the same and the alt attribute disappears.
SYMM would prefer to have the option of eliminating the src attribute and
keeping the alt attribute while modifying its value; the idea being that
SMIL specifically states that while non-SMIL namespace stuff may appear in
SMIL instances, there must be no alteration to the semantics of SMIL.

So, keeping the SYMM requirements in mind, we get this result upon
encryption:

<video src="NotAuthorized.mpg"
       alt="You need the proper authorizations to view the referenced
video."
       enc:EncryptedDataManifest="./EncryptedDataManifest"
       xmlns:enc="http://www.w3.org/xml/encryption/...">

  <EncryptedDataManifest xmlns="http://www.w3.org/xml/encryption/...">

    <EncryptedData Type="AttributeValue" Name="src">
      <CipherText URI="secret.enc"/>
    </EncryptedData>

    <smil:EncryptedSrc xmlns:smil="http://www.w3.org/2001/SMIL20/WD/"> 
      <EncryptedData Type="video/mpeg" Name="secret.mpg">
        <CipherText URI="secret.mpg.enc"/>
      </EncryptedData>
    </smil:EncryptedSrc>

    <EncryptedData Type="AttributeValue" Name="alt">
      <CipherText>AbCd...WxYz</CipherText>
    </EncryptedData>

  </EncryptedDataManifest>

</video>

Note that the new model allows the <video> element's src and alt attributes
to be used by non-encryption-aware SMIL-conformant process.  However, we
still specify enough information so that an encryption-aware SMIL processor
can display the "real" data.

The <smil:EncryptedSrc> is a SMIL-defined element that the SMIL WG could
define, if they wish, to indicate the encrypted video would normally be
pointed to by the <video> element's src attribute.  The goal is for XML
Encryption to define a reasonable degree of common syntax for encrypted 
data while NOT trying to predict how applications will want to process
that data.  

As iterated in the recent XML Encryption meeting, I'm happy to put
this topic on the backburner until we get more experience with XML
Encryption implementations.  However, I just wanted to capture the results
of the SYMM discussion for posterity.

Ed