Time-limited spending and physical bitcoins, brainstorming

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I'm interested in finding ways to enable paper bitcoins in an easy, personally mass-producible way. The reasoning would be for a fair with many vendors, communal yard sale, etc, where people could purchase tickets with something like a private key that has been promised money for the following day on them, without needing holographic tamper resistant sticker or equivalent.

It seems relatively easy to have the clients accept transactions whose output can't be spent for 'x' amount of time, and transactions initiated before that time would be ignored. The problem I see with this is that an attacker could scan all the private keys they see, and try to race the legitimate holders at the expiration of the spend restriction.

Can anyone think of a scheme that would allow the issuing of distributedly redeemable coins using just normal paper and open information?

-s

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[cbeast]

We will probaby need to wait until Bitcoin can be escrowed by an m-of-n transaction for security. Otherwise there are invisible inks that may work.

[cbeast]

We will probaby need to wait until Bitcoin can be escrowed by an m-of-n transaction for security. Otherwise there are invisible inks that may work.

Thanks, cbeast! I would imagine that multi-signature transactions will be available soon. Combine that with Eto's sweeping function in Armory, and the mind reels!