My understanding is that a signing party is vouching for the message. This
means that it is providing an assurance that the message contents, including
originating address fields, are authorised. If the signing party is
unwilling or unable to provide this assurance, then they should not apply a
signature. The receiving party can place a value on this assurance
depending on a variety of factors (relationship to originating address,
reputation, etc).
I doubt that it's a good idea to insist that those semantics be
associated with every signature, as it would drastically impede the
ability of intermediaries to sign messages. For instance, a list
should be able to sign a message in such a way as to mean "this
message was sent to you from this list" but not to make any assurances
about the content of the message.
Basically all that signing a message inherently means is "I saw the
message when it looked like this". This is a useful service by itself,
but there are situations when we'd like a signature to say more than
that. If we want to add additional semantics to a particular signature
they should be (a) explicit, and (b) decoupled from the message itself.
What (b) probably implies is that any explicit semantics associated
with a signature need to be contained in the message header(s) that
represent the signature, rather than in any of the headers that are
signed by the signature.
Keith
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