Subject: timestamp messageImprint

From: Bernd Matthes <[email-masked]>
Date: Tue, 13 Nov 2001 12:31:43 +0100

Hi All!

I wish to timestamp a digital signature (not the document which is
signed),
that mean a digital signature (PKCS#7) should exist before a particular
time.
As RFC3161 puts it:
"The TSA's role is to time-stamp a datum to establish evidence
   indicating that a datum existed before a particular time.  This can
   then be used, for example, to verify that a digital signature was
   applied to a message before the corresponding certificate was
revoked..."

My question is: 
Which part of the signerInfo should be used to create the messageImprint
for the time-stamp query?

As first I could hash the DER encoded authenticatedAttributes.
That's would be the same hash value as in the encryptedDigest as
well, but readable without any decryption process. I think this is 
a security hole.

IMHO it's better to use the encryptedDigest itself to create the
messageImprint to ensure/proof that a signature was created 
with a particular signer private key before a particular time.

What's Your view of the matter?

with kind regards

-- 
Bernd Matthes               Celo Communications GmbH
Senior Software Engineer          - a Gemplus Company
Dipl.-Ing.(FH)              mailto:[email-masked]
------------------------------------------------------------
"Complexity breeds bugs. Bugs prevent adoption,
lack of adoption results in death. Death not good."

---

From: Jean-Marc Desperrier <[email-masked]>
Date: Tue, 13 Nov 2001 14:48:25 +0100

> for the time-stamp query?

Read APPENDIX A :
"The value of messageImprint field within TimeStampToken shall be a hash
of the value of signature field within SignerInfo for the signedData
being timestamped."

In this case, you time-stamp the fact that _someone_ had signed a data at a
given time, not just the fact that the data existed at that time, so you
need to base yourself on the signature content.