How does one make a secure gift certificate?

Hello! I would like to offer some gift certificates, but I'm not sure of the right way to go about making them so they cannot be forged. Right now, I think of making them out of a cube prim with special dimensions and transfer-only permissions. Since they will be transfer-only, no one would be able to know their exact dimensions but me (I'll choose something difficult like (0.013, 0.068, 0.167)). Additional protection would be provided from the Creator's name field. Am I right in assuming that this is safe, or am I asking for trouble? Are there any other ways that people have used to make such gift cards? Thanks in advance.

You can use one of the web shopping services like SL Exchange to sell the gift certificates. I believe you get a record of who purchases the item and also who receives it. After that point you can stipulate that it is not tranferable to anyone else without voiding the gift certificate.

Another way you can do it (if you don't want to limit the certificate to a single transfer) is to request that the gift giver record thier transaction ID in the title of a No Mod, No Copy note and transfer that with the gift certificate.This requires effort on the customers part, so it may not be a good solution. You can trace the purchase back to the original purchaser who will show as the creator of the note. If you get multiple certificates with that transaction ID, or one that doesn't match any purchases in your transaction history, then you know you have a forgery. This requires that you keep a transaction log since LL only stores 30 days of transaction data.

I would stay away from using any method that relys on vector data, texture data, or any other data associated with the actual object. It is easily forged.

Your gift certificate whispers something on channel -2389, the vendor (or any other script checking the certificate) answers on channel -9875 (just an example) and whispers a random number. Now both gift certificate and vendor script do a little calculation like (random number + 357) / 1.573 - 283. The resulting number is sent as an email subject to the vendor.
No one knows how the calculation is done, no one can listen in to the email. Should be pretty safe.

If the number of gift certificates isn't that high, you could also have your vendors read a notecard with asset UUIDs.

Thanks for the replies. I was hoping to have some gift cards lying around my store and that anyone would be able to purchase and use without too much hassle. I think I'll have to shelve this project for the time being. Your ideas do sound like they'd work, but I do not really want to send customers off-world to shop from a web service or check out their transaction IDs, and I do not have the scripting skills to write something like what Ishtara proposes. At any rate, thanks again for giving this a shot