3

Let's say current BTCUSD rate is 1BTC = 3,000USD and average fee is 300satoshi per byte And I need to pay some bitcoin to P2PKH address owned by bitcoin exchange/cafe or bar, basically whoever allows Bitcoin payments.

Because Bitcoin allows that transactions which has multiple same address output, I can do something like this

  • Input
    • 0.0135 BTC($40)
  • Output
    • cafe address 0.00045BTC($1.332)
    • cafe address 0.00045BTC($1.332)
    • cafe address 0.00045BTC($1.332)
    • ... 27 more same outputs

I need to pay about $9 extra fee(because each output are 34 byte data therefore each output costs $0.306 (3000 * 0.00000034 * 300)).

Once the transaction is contained in the blockchain, I technically paid 0.0135BTC and I get a cup of coffee.

The problem is, the caffe owner almost never be able to use it. each P2PKH input needs around 148bytes and costs 0.000444BTC(0.00000148 * 300) as fee.

spending 0.00045BTC requires 0.000444BTC.

I don't know who would do this.But this might work.

Could this be happened?

2

Technically, sure, this is possible.

Practically, you might get away with it the first time. You'll have effectively paid an extra $9 for your $40 round of drinks, but you'll get the added satisfaction of having been a jerk. The bar owner will realize that he can't spend the money, shake his fist, and write off the cost of the drinks (which is probably more like $10).

The next time, you'll find a sign posted that says "Bitcoin payment rules: maximum one output". If you try your multi-output trick again, they'll point to the sign and say "nope, that doesn't count, you haven't paid". You'll have to pay again, and now you are out $89 for a $40 round of drinks.

(The third time, you'll find a photo of you behind the bar, marked "DO NOT SERVE". And a very large fellow in a T-shirt will ask you firmly to take your business somewhere else.)

  • haha You are right. The question was more likely for a exchange service I think. there is no need to do multiple times but one big trade.it always make 4 times bigger damage from a attacker pay.what If I were a owner of exchange service trying to shut down other exchange places? I attack to a exchange service that bitcoin deposit and withdraw system enabled.I pay $1mil($4million damage) to make the service out of service and I might get some new clients and compensate the cost. – suish Jun 9 '17 at 7:10
  • @suish: Indeed. So analogous to the above, it would be smart for the exchange to have a policy that they will not honor any incoming transaction with an excessive number of inputs. For that matter, even if they don't have such a policy, they could simply dishonor the transaction anyway and refuse to credit your account. Your only recourse would be to sue them in court. Maybe you win, maybe not; in the meantime you are out $5 million plus your attorney's fees, and they just get to keep operating. – Nate Eldredge Jun 9 '17 at 14:16
0

Yes, this could happen, and inadvertently this is what is happening to a lot of rebittance services and IIRC also what drove changetip to give up their business.

However, you must take into account that fees are not always at the same level. Fees have a daily and weekly cycle overlaid with incident motivated activity. While last week we saw fees up to 540 sat/byte, they are now down to 65 sat/byte and may even drop further during the weekend. People will then put in consolidation transactions to combine low-value unspent transaction outputs. I've recently read that someone has been doing 1 sat/byte transactions for over a year to consolidate all their inputs as even that goes through every once in a while.

So, yes, while this could be used as a way to be a jerk as Nate describes, with some patience a lot of these unspent transaction outputs may be salvaged.

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