Re: trusted time stamping

[tcmay@netcom.com (Timothy C. May)]

Wei Dai wrote:

> 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).

Stu Haber (who reads this list, sometimes) and Scott Stornetta of
Bellcore developed a system which solves the more important problem of
the time stamper reliability, which I don't think W.D. has addressed.

I've written up a couple of summaries, the last of which got a
favorable reaction from Stu on. So I'll mail it later today, when I
fire up my off-line archives and retrieve it.

The hard part is time stamper reliability, i.e., how does the world
(and the courts) know that the time stamper(s) did not simply reset
his clock and thus fake the times?

Haber and Stornetta came up with two clever ideas:

1. Publish a one-way hash of the text to be stamped in a very public place,
e.g., one's latest bestselling novel or the "New York Times." This is
similar to the crypto methods used by scientists through the ages to
prove ownership. H & S call this a "widely witnessed event," the idea
being that millions of copies of archived issued of the NYT (or the
novel!) would have to be retrieved and reprinted in order to change at
a later date the text. Economically impractical.

2. But it may also be economically impractical for the NYT to print
page after page of such hashes...they may choose not to,
understandably. So H & S developed a "tree"-like way to merge
customer-provided hashes with many other hashes (and earlier hashes,
to, thus adding to the difficulty of faking) and so to only have to
publish a comparatively small number.

These two clevernesses are the crux of time-stamping.

They are trying to build a company to do this; perhaps Stu can update
us on the status.

--Tim May


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