Subject: RE: Time stamp request

From: Russ Housley <[email-masked]>
Date: Tue, 01 Dec 1998 09:28:11 -0500

At 11:23 AM 11/25/98 -0500, Sarbari Gupta wrote:
>[snip]
>
>On the other hand, currently (in draft-ietf-pkix-time-stamp-00.txt), the
>MessageImprint data structure seems to imply that only a cryptographic
>hash of the message is permitted. However, it would be relatively easy
>to accommodate other types of message imprints, including the actual
>message (there is no better imprint of a message than the message
>itself!!) and arithmetic checksums, etc. 

I do not think that arithmetic checksums are useful.  It is trivial to
generate two documents that have the same arithmetic checksum.  So, a
timestamp on such a checksum is essentially meaningless.

I suggest that a one-way hash value of the document or the document itself
be used.

Russ

---

From: Sarbari Gupta <[email-masked]>
Date: Wed, 2 Dec 1998 09:37:22 -0500

Russ:

Arithmetic checksums are weak - there is no argument about that. I was
just pointing out that the MessageImprint type could be broadened easily
to accommodate the various needs of clients of a timestamping service.

In fact, if the actual message itself was being sent to the Time Stamp
Authority (TSA), I can envision two modes of usage:
	1) the TSA calculates a hash on the message and uses the hash
value to generate the time stamp token
	2) the TSA uses the message in its entirety to generate the time
stamp token.

The first mode is obvious. The second mode allows time stamp tokens to
serve as standalone data vehicles that can be used to verify the content
as well as carry the content - this would typically be useful for small
messages. 

- Sarbari 

-----Original Message-----
From: Russ Housley [mailto:[email-masked]]
Sent: Tuesday, December 01, 1998 9:28 AM
To: Sarbari Gupta
Cc: [email-masked]
Subject: RE: Time stamp request

I suggest that a one-way hash value of the document or the document
itself
be used.

Russ