4 ms·

Since it is a micropayment, it is possible (or acceptable) to accept payment with zero confirmation (in less than 10 seconds), as pointed out by Satoshi in the

Since it is a micropayment, it is possible (or acceptable) to accept payment with zero confirmation (in less than 10 seconds), as pointed out by Satoshi in the "Bitcoin snack machine" post: https://bitcointalk.org/index.php?topic=423.msg3819#msg3819 https://bitcointalk.org/index.php?topic=423.msg3819#msg3819 Yes, there is a risk of double-spending, but since this is a small amount, the cost of the attack may be higher than the benefit. The payment processor can also deploy more full nodes in different geographic locations to monitor and detect potential double-spending transactions in the transaction memory pool.

fxtentacle 6y ago
Since all you need is the software to generate your fake transaction with the right technical format, the cost of the attack can be zero. And then a million fraudulent microtransactions will add up. Of course, that would never happen in practice, because the Blockchain cannot handle a million transactions in any reasonable time frame.
- CalmStorm 6y ago
  In case there are multiple conflicting transactions (for double-spending purpose), the network will only accept the first one and reject the rest. So the micropayment processor needs to wait for a few seconds when it receives the first transaction in the mempool. If no other conflicting transactions arrive, the first transaction will be acccepted. These few seconds give the first transaction big advantage over the double-spending ones, as it will most likely be included in the block. If the attacker sends the legit transaction and double-spending one at the same time, it will be detected due to the few seconds wait by the payment processor.
  - paulmd 6y ago
    there's no protocol-level guarantee of that, both transactions are perfectly valid, signed transactions and it's purely dependent on the good behavior of node operators to reject these and in fact it is quite common to accept "double-spending" since that is really the only way to increase fees to push through a "stuck" transaction. Otherwise you have to wait an indeterminate (potentially forever) period of time until it exits the mempool of all network participants
    - CalmStorm 6y ago
      If there are two valid, signed transactions that spend the same unspent input transaction (UTXO), only one of them will be included in the block. This is indeed guaranteed by the protocol [1]. [1] https://bitcoin.stackexchange.com/questions/81624/can-two-or-more-transactions-sent-from-the-same-address-be-mined-in-the-same-blo https://bitcoin.stackexchange.com/questions/81624/can-two-or...
      - fxtentacle 6y ago
        Yes, but "included in the block" means you either wait some hours or pay higher fees.
    - sp332 6y ago
      Yes but it is in the best interest of the node operators to behave well in that way, so it's reasonably trustworthy even without protocol-level guarantees.
lostmsu 6y ago
Huh, I might misunderstand it, but that property appears to be broken in account-based cryptocurrencies like Ethereum. Since there's no concept of unspent outputs, what could protect ETH etc users in a similar way? Wallet transaction index?
- somebodythere 6y ago
  It is broken in Ethereum. In Ethereum, since consistency is enforced by transaction nonce rather than UTXOs, there is no way for a "watcher" to keep the payer honest by rebroadcasting a previously signed transaction output. Indeed, in Ethereum you can "cancel" a transaction in the mempool by signing a blank transaction with a higher fee and same nonce, which is by design.
  - lostmsu 6y ago
    How does Bitcoin decide which of the two transactions spending the same UTXO to pick?
  - decentralised 6y ago
    Mind you that a tx in the mempool has not yet produced any changes to state. In BTC you can also replace-by-fee for instance (https://en.bitcoin.it/wiki/Transaction_replacement https://en.bitcoin.it/wiki/Transaction_replacement). This page (https://github.com/ethereum/wiki/wiki/Design-Rationale https://github.com/ethereum/wiki/wiki/Design-Rationale) contains a good overview of the UTXO vs Account model with each's strengths and weaknesses.
- murbard2 6y ago
  There's a nonce on each account. If you see a transaction from the same account with the same nonce it's a double spend . Nonce can't be skipped either, so a double spend requires two transactions with the same nonce.

fxtentacle 6y ago

Since all you need is the software to generate your fake transaction with the right technical format, the cost of the attack can be zero. And then a million fraudulent microtransactions will add up. Of course, that would never happen in practice, because the Blockchain cannot handle a million transactions in any reasonable time frame.

CalmStorm 6y ago
In case there are multiple conflicting transactions (for double-spending purpose), the network will only accept the first one and reject the rest. So the micropayment processor needs to wait for a few seconds when it receives the first transaction in the mempool. If no other conflicting transactions arrive, the first transaction will be acccepted. These few seconds give the first transaction big advantage over the double-spending ones, as it will most likely be included in the block. If the attacker sends the legit transaction and double-spending one at the same time, it will be detected due to the few seconds wait by the payment processor.
- paulmd 6y ago
  there's no protocol-level guarantee of that, both transactions are perfectly valid, signed transactions and it's purely dependent on the good behavior of node operators to reject these and in fact it is quite common to accept "double-spending" since that is really the only way to increase fees to push through a "stuck" transaction. Otherwise you have to wait an indeterminate (potentially forever) period of time until it exits the mempool of all network participants
  - CalmStorm 6y ago
    If there are two valid, signed transactions that spend the same unspent input transaction (UTXO), only one of them will be included in the block. This is indeed guaranteed by the protocol [1]. [1] https://bitcoin.stackexchange.com/questions/81624/can-two-or-more-transactions-sent-from-the-same-address-be-mined-in-the-same-blo https://bitcoin.stackexchange.com/questions/81624/can-two-or...
    - fxtentacle 6y ago
      Yes, but "included in the block" means you either wait some hours or pay higher fees.
  - sp332 6y ago
    Yes but it is in the best interest of the node operators to behave well in that way, so it's reasonably trustworthy even without protocol-level guarantees.

lostmsu 6y ago

Huh, I might misunderstand it, but that property appears to be broken in account-based cryptocurrencies like Ethereum. Since there's no concept of unspent outputs, what could protect ETH etc users in a similar way? Wallet transaction index?

somebodythere 6y ago
It is broken in Ethereum. In Ethereum, since consistency is enforced by transaction nonce rather than UTXOs, there is no way for a "watcher" to keep the payer honest by rebroadcasting a previously signed transaction output. Indeed, in Ethereum you can "cancel" a transaction in the mempool by signing a blank transaction with a higher fee and same nonce, which is by design.
- lostmsu 6y ago
  How does Bitcoin decide which of the two transactions spending the same UTXO to pick?
- decentralised 6y ago
  Mind you that a tx in the mempool has not yet produced any changes to state. In BTC you can also replace-by-fee for instance (https://en.bitcoin.it/wiki/Transaction_replacement https://en.bitcoin.it/wiki/Transaction_replacement). This page (https://github.com/ethereum/wiki/wiki/Design-Rationale https://github.com/ethereum/wiki/wiki/Design-Rationale) contains a good overview of the UTXO vs Account model with each's strengths and weaknesses.
murbard2 6y ago
There's a nonce on each account. If you see a transaction from the same account with the same nonce it's a double spend . Nonce can't be skipped either, so a double spend requires two transactions with the same nonce.