Are Footnotes in Scope?

I had a thought and I don't know whether it is in scope for this group -- or
even whether it is useful, possible or trivial.  Let me try it on and see
what you folks think:

Suppose one wants has a web service and one wants to include something that
is provided by another web service -- AND one wants to provide some proof
that the information really is valid and comes from that source.

For example, suppose one wants to express an amount in both dollars and yen,
based on an exchange rate at some time.  Further, suppose one wants to guard
against trickery based on using a bogus exchange rate.  That is, one wants
some way to say, "This exchange rate came from the following mutually
respected source" and have some way to ensure that this statement is true.

I can think of two approaches to this:

1 -- Incorporate some sort of validating information in a block within the
message returned by the web service.  As in, "This certifies that the
information on this line is guaranteed by Chase Bank, and that this is the
same Chase Bank as is registered in Verisign".  Or something along those
lines.

2 - Incorporate some sort of information about how the information was
obtained in such a way that the receiving party can repeat the query and
check the information if it likes.  As in, "This information was received
from Chase Bank from the following query -- if you like you can submit that
same query and you will get the same answer".

It seems to me that the second possibility is more interesting.  I think
that I am talking here about something that is a bit like footnotes, except
that they can be verified in an automated way.

Is this interesting?  In scope?  Possible?