Confirmed transactions refer to Bitcoin transactions that appear in a valid block. These transactions are checked before being included in a block to prevent double-spending attacks; since they already appear in a block in the Bitcoin blockchain, they cannot be modified easily. In the paper, the authors refer to a transaction that has acquired X confirmations (i.e., X −1 blocks appear in the chain after the block that confirms the transaction) by an X-confirmation transaction.

Bitcoin is a decentralised payment system that is based on Proof-of-Work. Bitcoin is currently gaining popularity as a digital currency; several businesses are starting to accept Bitcoin transactions. An example case of the growing use of Bitcoin was recently reported in the media; here, Bitcoins were used as a form of fast payment in a local fast-food restaurant. In this paper, the authors analyse the security of using Bitcoin for fast payments, where the time between the exchange of currency and goods is short (i.e., in the order of a few seconds).

They focus on double-spending attacks on fast payments and demonstrate that these attacks can be mounted at low cost on currently deployed versions of Bitcoin. They further show that the measures recommended by Bitcoin developers for the use of Bitcoin in fast transactions are not always effective in resisting double-spending; they show that if those recommendations are integrated into future Bitcoin implementations, double-spending attacks on Bitcoin will still be possible. Finally, they leverage on our findings and propose a lightweight countermeasure that enables the detection of double-spending attacks in fast transactions.