Cryptography Reference
In-Depth Information
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Value to authenticate
Values to reveal
Figure 5.7.
Authentication using a Merkle tree.
5.5.2
Timestamps and Notary
Another problem is how to timestamp a digital document in a notary sense: when
producing the document together with a timestamp, we aim at proving that the document
was deposited as is to a notary at a given date. This problem was addressed by Stuart
Haber who actually made a business out of this problem.
7
Provided that the notary is trusted, the problem is trivial: the notary only has to sign
the document and the date. His signature is the timestamp, and verifying the signature
consists of checking the signature.
The problem is more complicated when the notary is not trusted. For this, we
first need to realize that the notion of time is society-related. A date is a frontier
in the timescale which separates two classes of events: events which occur before
and events which occur after. We can prove that a document was deposited after a
given date by signing the document together with a proof that past societal events
occurred. Since depositing a document becomes an event itself, we can prove that
it was deposited before another given event by having a linear and linked history of
events. Haber and Stornetta proposed the notion of
timestamp linkage
(see Ref. [84]).
In this protocol, a notary-like service records sequences of digital documents
y
n
to-
gether with identities (or pseudonyms) of depositors ID
n
. It computes sequences of
links
z
n
and issues timestamps. Upon reception of
y
n
and ID
n
, the notary proceeds as
follows.
7
See the activities of the Surety Technologies company
http://www.surety.com
, which is a kind of
digital notary.
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