From: weidai@eskimo.com (Wei Dai)
Date: Fri, 19 Aug 94 01:43:06 PDT
To: cypherpunks@toad.com
Subject: trusted time stamping
Message-ID: <199408190842.AA06184@eskimo.com>
MIME-Version: 1.0
Content-Type: text/plain


To: cypherpunks@toad.com

-----BEGIN PGP SIGNED MESSAGE-----

I thought my idea about having trusted entities digitally sign a document in
order to establish its existence at a particular time was a new idea, but I
just read about it in _Applied Cryptography_.  Anyway, I wrote some C code to
do automatic time stamping with PGP (source code is in the next e-mail).

If you just want to try it, simply send an e-mail to weidai@eskimo.com
with the subject "Time Stamp This Mail".  The body of the mail will
be signed with a PGP private key (public key is at the end of this message)
and returned to the sender.  E-mail with any other subject will be piped
to my regular mailbox.

What's the use of this?  Well, here is an interesting application of
time stamping that wasn't covered in _Applied Cryptography_.

Let's say Alice would like to publish an article anonymously but retain the
ability to claim authorship some time later.  She can follow this protocol:

1. Alice signs her article with RSA
2. She encrypts her signed article with IDEA
3. She sends the encrypted article to several trusted time stamping servers
4. She places the signatures she gets back along with the encrypted article
   in a safe place
5. She waits a random length of time
6. She posts the plain article (without encryption or signature) anonymously
7. When Alice wants to claim authorship, she publishes the encrypted article,
   the IDEA key, and the signatures she got back from the time servers

Now, people can be reasonably sure that Alice actually wrote the original
article because the time server signatures prove that she signed the
article before it was made public.  One problem here is that at
least one of the time servers she used must have remained secure until step 7.

Comments?

Wei Dai
PGP Public Key avaliable

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=w0Q+
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Version: 2.3a

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=vpgO
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PGP Public Key available