[dss] Use case: which signature?

A multi-party agreement may have multiple signatures associated with the 
document, detached or enveloped.  (Some may want to add enveloping, but 
I won't.)  A client should be able to send a verification request to the 
service and specify which signature(s) it wants verified.

> 2.3.2 Signed Data: Reference or direct provision
> 
> Data items to be signed/validated should be either provided
> to the service as a reference (URI), or directly as part of
> the request. The latter is important for situations where
> the data to be signed cannot be located by resolving a URI.

In some (many? :) cases, a DSS service will not be willing to retrieve 
data from arbitrary URL's on behalf of a client.  In this case, the 
client must be able to "push" the data, along with an indicator of the 
URL of the data.

Extending on this, a DSS service might be asked to verify a signature 
where it does not have the privileges to read the source document. A 
"just trust the References" mode would allow such a service to operate.

	/r$