I am having real difficulty in seeing any value from including a signature
purpose attribute. Some of the examples given are trying to authenticate the
context or the transport of the signed document. Other examples such as time
stamping do not seem ambiguous.
If Bob receives a document signed by Alice, he does not and cannot know it
was sent by Alice, only that Alice signed the document. This seams to me
inherent feature of using a hostile infrastructure. I certainly do not trust
the document any the less because of its uncertain means of
transportation(email or http), since I trust the signature. If Alice
constructs a document solely intended for Bob, but addresses it "to whom it
may concern" then she has opened the door herself for Bob to forward the
document. Any document Carol receives signed by Alice and beginning "Dear
Bob" must be understood by Carol for what it is. I cannot see any
technological add-on will help Carol at this point. It is at the end of the
day people using these systems, not ASN.1 experts.
Signature Purpose - R.I.P.
Trevor