Delayed self-encrypting messages

Can I do this with crypto technology? Can I send someone a message, and 
possible a program, such that the message can only be decrypted after a 
predetermined date?



    ++PLS


From owner-cypherpunks@toad.com Fri Jun 10 15:13 CDT 1994
Date: Fri, 10 Jun 1994 13:04:07 -0700
From: Paul Schauble <pls@crl.com>
To: Cypherpunks@toad.com
Subject: Delayed self-encrypting messages
Precedence: bulk

I have a need to distribute some information fairly widely, but it's
critical that it not be openly revealed before a certain date. Consider the
model of an embargoed press release.
Can I do this with crypto technology? Can I send someone a message, and
possible a program, such that the message can only be decrypted after a
predetermined date?
++PLS
----- End Included Message -----
You could do the simple way, distribute the message, then a key at the
later date. To make sure the encrypted message is genuine, sign the message
encrypt it, then sign it again, to ensure that people know that the
encrypted text is okay.

> Paul Schauble <pls@crl.com> writes:
> Can I do this with crypto technology? Can I send someone a message, and 
> possible a program, such that the message can only be decrypted after a 
> predetermined date?

I think you would need a trusted time server that sends out signed
timestamps... you can't trust other time sources, including WWV (just
broadcast a signal locally to swamp the signal).

> 
> 
> > Paul Schauble <pls@crl.com> writes:
> > Can I do this with crypto technology? Can I send someone a message, and 
> > possible a program, such that the message can only be decrypted after a 
> > predetermined date?
> 
> I think you would need a trusted time server that sends out signed
> timestamps... you can't trust other time sources, including WWV (just
> broadcast a signal locally to swamp the signal).
> 
Use the GPS systems. They are synced to the NITS (or whatever they call it 
now) and have time stamps coded in them. As to swamping out a sat or two,
yeah could be done but they would have to be mighty high above you in
a airplane or blimp or something...

I have a need to distribute some information fairly widely, but it's 
   critical that it not be openly revealed before a certain date.  

The problem is underspecified.  What is the threat model?  That is,
what are to trying to prevent from happening, and what are you trying
to ensure will happen?

If you're just worried that the information will get suppressed if it
sits in one place, encrypting with symmetric cipher and a random key
and publishing the ciphertext does quite well.  You can then give
trusted parties the key.  This has been suggested.

If you want to make sure the message can be decrypted without further
intervention on your part, you need to farm that job out to someone
else.  Use another person, or a public key beacon, but some
other party will be involved.  If you can make that party a public
service (like a beacon), then you've depersonalized the problem.

The simplest public key beacon works as follows.  The operators of the
beacon publish a list of public keys, one per time period--let's say
days here.  The beacon is programmed to give out any particulare
private key at the beginning of its day.  To use this, simply encrypt
with the public key of the date you want the message to be revealed.
The message will be decryptable on that date, when the beacon's key is
revealed.

An interesting research project would be to construct one of these to
sit in orbit.

Eric