MCE within Digital Signature XML Signature parts (OPC)

[MURATA Makoto]

Dear colleagues,

Although most of my changes in the latest WD are editorial,
 but there is one (arguably major) technical change.

The below text is new.  It is an attempt to clarify the use of MCE
in Digital Signature XML Signature parts.

> 12.5 Digital Signature Markup
> 12.5.1 General
> The content of a Digital Signature XML Signature part shall be an
> XML document.  The requirements (including MCE processing before
> validation and subsequent processing) specified in §8.2.5 apply.
>
>  After the removal of any extensions by an MCE processor as
>  specified in ISO/IEC 29500-3, the content of each Digital
>  Signature XML Signature part shall be a schema-valid XML document
>  against xmldsig-core-schema.xsd, as specified in the W3C
>  Recommendation “XML-Signature Syntax and Processing”, and
>  opc-digSig.xsd (see §C.4).

Is the use of MCE allowed in Digital Signature XML Signature parts?
In my understanding, it is allowed.  I thus wrote the above
paragraphs.

My understanding is based on 13.6.2 in the currently published OPC.

> 13.6.2 Markup Compatibility Namespace for Package Digital Signatures
>
> The package implementer shall not use the Markup Compatibility
> namespace, as specified in Annex F within the package-specific
> Object element. The package implementer shall consider the use of
> the Markup Compatibility namespace within the package-specific
> Object element to be an error. [M6.32]
>
> Format designers might specify an application-defined package part
> format that allows for the embedding of versioned or extended
> content that might not be fully understood by all present and future
> implementations. Producers might create such embedded versioned or
> extended content and consumers might encounter such content. [O6.12]
> [Example: An XML package part format might rely on Markup
> Compatibility elements and attributes to embed such versioned or
> extended content. end example]
>
> If an application allows for a single part to contain information
> that might not be fully understood by all implementations, then the
> format designer shall carefully design the signing and verification
> policies to account for the possibility of different implementations
> being used for each action in the sequence of content creation,
> content signing, and signature verification. Producers and consumers
> shall account for this possibility in their signing and verification
> processing. [M6.33]

However, I feel a bit nervous.  Do we really allow MCE anywhere in
digital XML signature parts?  For example, can the root element be
an MCE Choice element?  My paragraphs allow such an MCE Choice
element.

One could argue that MCE is allowed only within application-defined
Object elements.

But why do we have to mention MCE-processing-before-valiation then?
Absolutely anything is allowed within application-defined Object
elements.

Finally, I find a cut-and-paste error in 12.5.10.3
(Application-Defined Object Element).

> The Markup Compatibility namespace, as specified in Annex E, shall
>  not be used within the OPC-specific Object element.

This paragraph should be revised or removed.

Regards,
Makoto


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