[6743] in cryptography@c2.net mail archive
Re: time dependant
daemon@ATHENA.MIT.EDU (Steven M. Bellovin)
Tue Mar 7 23:36:09 2000
From: "Steven M. Bellovin" <smb@research.att.com>
To: "Arrianto Mukti Wibowo" <iscp9063@nus.edu.sg>
Cc: "Cryptography" <cryptography@c2.net>
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Date: Tue, 07 Mar 2000 17:10:28 -0500
Message-Id: <20000307221034.5281641F16@SIGABA.research.att.com>
In message <010601bf8879$2f4c1980$82d08489@muki>, "Arrianto Mukti Wibowo" write
s:
> Hi,
>
> I want to know whether there is a crypto building block which doesn't allow
> someone to open an encrypted message before a certain date.
>
> [Damn hard. Math functions don't grok "date". The only reasonable way
> to do this without a trusted third party is to pick an encryption
> algorithm that will take at least as long to decrypt (in likely
> available computer time) as are needed. -Perry]
>
> In another word, I need to know several "date/time-related" crypto papers
> around. Can somebody give me pointers?
>
In the future, it may be possible to base something like this on physical
principles. For example (and if I haven't dropped a decimal point), Jupiter
is never closer than about 2079 light-seconds from Earth. A message encrypted
with the public key of a satellite in that orbit could not be received in the
clear on Earth in less that 4158 seconds. One could postulate a network of
such satellites at various distances -- but it would sure strain our
technology to get any reasonable delays with a single hop.
Second-best would be to use repeated decryptions -- say, a message addressed
to Pluto Equilateral (about 5.5 light hours away), encrypted with its key, and
containing a message encrypted with an Earthstation key. That message in turn
is encrypted with Pluto Equilateral's key; iterate as needed.
--Steve Bellovin
