William Ottaway wrote:
If a signer verifies the signature it is countersigning then everything is
fine and dandy. However, when I receive a countersignature I have no way of
knowing if the signature being signed has been checked first. Infact I
would expect that the entity applying the countersignature is more likely
not to check the signature over the original content because it doesn't
need to and may not be able to.
This is correct but there are cases where this is unncecessary: an
example is a time stamping authority. All a time stamping authority is
saying is that "at this time I produced this signature".
There are several circumstances where this is important for example
archived messages signed with revoked certificates or copyright
disputes. I'll expand on this if it isn't clear.
A receiving agent would presumably regard the countersignature as
valueless is the original signature is either invalid or could not be
Excerpt from last paragraph of section 11.4 in draft-ietf-smime-cms-05.txt
"The fact that a countersignature is computed on a signature value means
that the countersigning process need not know the original content input to
the signing process."
If the original content is not available then the signature being
countersigned can not be validated.
Not by the signer at least but as I said there are times when this
In this case the signature value being countersigned compactly
represents: some signed content, the signed attributes of that content
and the signers public key. Asking the relevant countersigning agent to
validate possibly huge quanties of data and signatures would not be
practicable in all circumstances.
There could, of course, be circumstances where the countersigning agent
would check the original content if it wanted to place some other value
on the counter signature. As in the case with signed messages in general
the value of the counter signature is dependent on the CA doing the
Dr Stephen N. Henson.
UK based freelance Cryptographic Consultant. For info see homepage.
PGP key: via homepage.