As an assignment in a digital security course, a group of 100 students was asked to transfer a small amount in bitcoins to a certain wallet. They promised they'd buy something for us from this money. As a proof that we made the transaction, we've been asked to hand in the transaction id.

I'm wondering what exactly restricts me of using blockchain.info to search for this wallet address, wait until someone else has made such a transaction, and then just use that transaction id.

Am I right that merely requesting the transaction id isn't enough here?

Of course, they could notice when two people hand in the same transaction id, but we're in different groups, so they may not notice.

  • 1
    And this is just aside from the question whether such an assignment should be allowed. Basically it's requiring students to spend money on something without telling them what it is, if they want to have a higher grade (even though this assignment doesn't weight too heavily, and we don't have to transfer so much - this is a principality).
    – user22832
    Jan 9 '15 at 11:19

See Is there a way to prove ownership of a transaction?, which suggests using the private key to sign a message. You could send a signed message to your professor with the transaction id. However, since you say you're in different groups, it can still be that someone from another group just uses your transaction id, and doesn't get caught.

I would suggest asking your professor to give everyone a different wallet address to transfer the money to, or to request that everyone signs his transaction id with the private key used for the transaction.


You are answering your own question: Both signing the transaction id with the private key corresponding to the sender's address, or individual destination addresses would prevent others from giving another's transaction id as proof.

Of course, the teachers can easily find out duplicates if they collect all submitted transaction ids in a single list.

Perhaps you should suggest that they do so, in case they haven't thought of it.

  • I'm new to bitcoin, so that's why I felt the need to verify :) thanks!
    – user22832
    Jan 9 '15 at 11:34

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