确认比特币交易

Question

I'm new at Blockchain development and I need to verify that some amount of BTC has been sent from address a to address b. I'm calling Blockchain api , and browsing for transactions of the receiver address. The thing that confuses me is that in some cases there are multiple addresses in inputs. So how can I be sure that the one address I'm looking for sent the required amount? Also about transactions, can on BTC transaction hold multiple 'real' transactions? Thanks

Answer 1

Probably better off asked on bitcoin.se but every bitcoin transaction may have multiple inputs and outputs. If your wallet has 3 addresses to which 0.5 btc has been sent and not spent, and you then send 1.25 btc somewhere then all 3 of those 0.5 btc unspent transaction outputs will be used to build the 1.25 plus a new address for the 0.25 change. The sending wallet will also own the address to which 0.25 in change was sent. In this way a single btc wallet can end up owning millions of addresses

In practice it's quite likely that the total you send will be composed of multiple inputs so if you're trying to prove you send your grandma 1.25 BTC you don't look in transaction X inputs for your single address that originated 1.25, you look in tran X outputs for your grandma's address that received a total of 1.25. Bear in mind that in this day and age of multiple output transactions your btc transaction might cite 100 outputs, 12 of which are grandma's address and which add up to 1.25 btc

Yes, one btc transaction can hold tens or more payments to tens or more different individuals

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Edit in response to comment #1:

There might be any number of outputs, it's totally down to your wallet how it decides to send them out. Remember that if the inputs don't add up to the outputs then just like buying a packet of gum with a twenty dollar note, there will be some change. When you look at the transaction on a block explorer that your wallet built, there might be 100 inputs, 100 outputs (if you had a lot of people to pay) and even multiple changes if multiple inputs were split and sent back to yourself. If you think of your wallet bitcoin balance like blocks of cheese in the delicatessen, they all weigh different things, 3 people come and all want different amounts, the deli server person might get 6 blocks of cheese, have to cut them and combine them in different ways multiple times, then hand them over. They don't necessarily become recombined either; people might walk away with multiple little bits of cheese making the total amount they wanted, and then if they want to give that cheese as gifts, they subdivide it.. Ultimately subdivision of cheese continues until all you end up with are crumbs, that cannot be transacted because putting them all in one transaction makes the transaction too big (kilobytes) for the network to allow, so you might engage in an exercise of consolidating them; sending them to yourself in exact multiples so no subdivision occurs. Thus they become big lumps of cheese again. If you hear people talking about wallets being dusty, it means they're tracking large numbers of tiny amounts - dust.

I think I understand the confusion now though; If two people agree to buy something from you for 5 BTC, one sends you 5 but the other is dishonest and only sends you 1. These may occur in the same block, but they wouldn't occur in the same transaction because they originated from different wallets. If your puzzle is over how to tell them apart, ie which person sent you the 5 and which person sent you the 1, this is why you get your wallet to generate 2 different addresses and give one to each person. You could always tell that you were sent a 5 and a 1 because of separate transactions, but when you look at the different addresses the 5 and the 1 were sent to you can know which person sent the 1, because of the address they used. Again, you're looking at the output side of the transaction, not the input side, because the dishonest person who sent the 1 can look on the block explorer and find out the input details the honest person used, and then claim to be the honest person.

Answer 2

If you look at a transaction block which have multiple inputs and outputs, you cannot learn the exact directions of inputs. You are only able to know inputs, outputs, and their amounts.

For example, there are 3 inputs which are A, B, C, and 3 outputs which are D, E, F. They share same transaction block, so there is no way to say that A sent BTCs to D, E, or F. Possibility is 1/3, so this is what CoinJoin "Bitcoin transaction anonymizing technique" is based on.

More on CoinJoin .