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.