Who needs time vaults anyway?

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I don't really understand the use for "can't be opened until
Christmas" tricks.  If you don't want anyone to see your info until
Christmas then just don't give them a copy until then!  If you want
to prove that you have it but not let them see it until later then
do timestamping of hashes, zero-knowledge proofs and so forth.


Can anyone explain what use this theoretical "time-sensitive" crypto
box would be good for?


Regards,

Bryce

signatures follow


            "To strive, to seek, to find and not to yield."   
    <a href="http://www-ugrad.cs.colorado.edu/~wilcoxb/Niche.html">

                          bryce@colorado.edu                   </a>



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> Can anyone explain what use this theoretical "time-sensitive" crypto
> box would be good for?

	Suppose you die.

-- 
sameer						Voice:   510-601-9777
Community ConneXion				FAX:     510-601-9734
The Internet Privacy Provider			Dialin:  510-658-6376
http://www.c2.org (or login as "guest")			sameer@c2.org

> > Can anyone explain what use this theoretical "time-sensitive" crypto
> > box would be good for?
>
> 	Suppose you die.

Suppose you get "silenced", or carted off by big brother for thought
crime.  A defense: if you try to silence me, this info gets plastered
all over the net.

Suppose you discovered a polynomial time factoring algorithm (dream
on:-), and wanted to sell it to the highest bidder - what odds that
the NSA would try to obtain it and silence you?  Time-release would be
useful for such things.

Adam

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Ack.  Corrections.

In article <483l88$1f0@yage.tembel.org>, I wrote:
> You could do this with a reflector $ct/2$ metres away, assuming your
> opponent and you are in the same location.

Also assuming your attacker cannot use the information until he can get
it back to Earth, or else he could just race towards the reflector and
catch it on the way back.

> the diameter of the solar system is about 5.4 light-hours

The *radius* of the solar system is about 5.4 light-hours.

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-- 
Shields.

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In article <199511111953.MAA26503@nagina.cs.colorado.edu>,
Bryce <wilcoxb@nagina.cs.colorado.edu> wrote:
> A single station could serve up multiple pieces.  It would only
> reveal the k piece if the querying agent can prove that he has the 
> k-1 piece.  Of course if the total number of stations is small then 
> the "physically move the pieces" trick might work.

But you're back to trusting an agent or device not to reveal a secret.
What have you gained?

The point about moving the elements of the message physically apart has
merit, though.  So the one-time pad of timerel, the ideally secure but
unworkable model, is to encrypt your message with an OTP, then securely
transport the pad and location to points that are $ct$ metres apart,
where $t$ is the length of time you want to keep your message secret.
You could do this with a reflector $ct/2$ metres away, assuming your
opponent and you are in the same location.

I suppose this could be useful for very short-term applications (for
reference, the diameter of the solar system is about 5.4 light-hours),
but like the OTP, its application is limited.

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-- 
Shields.

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 Bryce wrote:
>
> Can anyone explain what use this theoretical "time-sensitive" crypto
> box would be good for?


 Sameer wrote:
> 
> 	Suppose you die.


Hey!  Who do you think you are?


:-)


Just kidding.  When I woke up this morning I realized what I was
missing:  the decryption might be out of your hands, such as when
you die, or you might *want* it to be out of your hands for some
other reason.


With that in mind, I can think of only one unalterable lower-limit
on the time of as decryption--  the speed of light.  Suppose you
encrypt your data with successive layers of keys, K1-Kn.  Then you
encrypt each key with its predecessor, encrypting Kn with Kn-1,
encryping Kn-1 with Kn-2, etc.  Destroy all copies of unencrypted
keys except for K1, which has not been encrypted.  Now put all
odd-numbered keys in location A and all even-numbered keys in
location B, which is 1 light minute from location A.  Once an agent
has received Key 1, it will take at least n minutes to decrypt 
the data.  Of course, the agent could just take copies of all of the 
keys from location B on some physical media and transport the media 
to location A, which would make the lower bound on time to be "much 
longer than 1 minute".


Hm.  Suppose the n different keys are in n different physical
locations, and the agent does not know where the k+1 location is
until he decrypts the material at the k location.  The "scavenger
hunt" scheme for timed decryption.  Of course this doesn't mean that
you have to bury your crypto box and make a map with an "X" marking
the spot.  Each key could be held by a crypto box which is
publically accessible on the Net.  The important thing is that
the decrypting agent can't retrieve the k+1 piece until he has 
decrypted the k piece.  Then the lower bound on time of decryption 
is...  um...  Well it depends on the location of the decrypting agent 
with respect to the locations of the n pieces.  (Neglecting, still, 
transmission overhead and decryption time.)  I'm not sure what the 
lower bound actually is, but it can be increased simply by adding more 
pieces to the puzzle.


A single station could serve up multiple pieces.  It would only
reveal the k piece if the querying agent can prove that he has the 
k-1 piece.  Of course if the total number of stations is small then 
the "physically move the pieces" trick might work.


Bryce

signatures follow


            "To strive, to seek, to find and not to yield."   
    <a href="http://www-ugrad.cs.colorado.edu/~wilcoxb/Niche.html">

                          bryce@colorado.edu                   </a>



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According to rumor, Bryce said:
> 
> 
> -----BEGIN PGP SIGNED MESSAGE-----
> 
> I don't really understand the use for "can't be opened until
> Christmas" tricks.  If you don't want anyone to see your info until
> Christmas then just don't give them a copy until then!  If you want
> to prove that you have it but not let them see it until later then
> do timestamping of hashes, zero-knowledge proofs and so forth.
> 
> 
> Can anyone explain what use this theoretical "time-sensitive" crypto
> box would be good for?

An application that I've seen is financial data, more specifically
MBS payment info. There is 100s of megabytes of data, and it was
encrypted so that nobody could use the info before the release
date/time, but the data needed to be transmitted prior to release
because of bandwidth constraints. In this case, it wasn't real
time-release, because the key was manually transmitted to release
the information rather than implement some sort of "do not decrypt
until" scheme.


-- 
Kevin L. Prigge        |"A computer lets you make more mistakes faster 
UofM Central Computing | than any invention in human history--with the 
email: klp@cis.umn.edu | possible exceptions of handguns and tequila."
01001101100010110010111|- Mitch Ratcliffe