Bitcoin Forum

I recently watched a video about "How Bitcoin works under the Hood" to know more about how bitcoins work, but after watching the video i still had one question:

If you have a transaction, for example Eve sends Marc 5 Bitcoins, then there will be generated a digital signature that can only be used once in the blockchain. Because everyone else trying to use the same signature will be rejected. But what if Eve wants to send Marc another 5 Bitcoins later on, why is the digital signature different then? Wouldn't it otherwise also be rejected?

Thanks for helping.

Kuriv

A Bitcoin transaction consists of a list of inputs (ie. coins received) and a list of outputs (ie. the coins sent). Obviously the sum of the outputs may not be larger than the sum of the inputs, as otherwise money would be created out of nothing. The sum of inputs may be larger than the sum of outputs, as this is the transaction fee a miner receives when adding your transaction to a block.

So when Eve sends Marc 5 BTC, her transaction refers to inputs that have not been spent yet (ie. became outputs). When signing a transaction to send 5 BTC to Marc, Eve refers to these unspent inputs. When these inputs have been spent (ie. Marc received the 5 BTC and the transaction has been confirmed by a miner), Eve can not use them when signing subsequent transactions. Thus she has to use different inputs when sending another 5 BTC to Marc.

So the thing is, the signature of a transaction does not merely confirm that 5 BTC have been sent from Eve to Marc. The signature also confirms which inputs have been used. So the message is not just "Eve sends 5 BTC to Marc" (a message whose signature would always look the same, when being resent), but actually something like "Eve uses inputs A, B, C to send 5 BTC to Marc", after that "Eve uses inputs D, E, F to send 5 BTC to Marc", etc... thus the message is always different and thus the signature is always a different one (since the signature depends on the content being signed).

Does this make sense to you?

Yes it makes sense for me, I get it now. Thanks for taking the time to answering me :D.

The signature used in these transactions re way different to handwritten ones and refers to the coins that the user is trying to send. When coin has been sent to the receiver, the sender cannot use the signature anymore, and hence, needs to create a new one each time for a new transaction or sending.

Digital signatures are always different because they are based of user private keys and transaction details.

A user's wallet is not like a regular wallet, it does not store coins inside. A wallet is a public key, an address in the network.
A wallet is a collection of transactions, associated with your address.
So when you are sending BTCs to someone, the system checks your history of transactions to see if you really have enough BTCs transferred to you.
And since each transaction has a unique identifier, transaction details are always different, which gives a unique digital signature.

So if you are still not sure how bitcoin transaction work, read this article: https://finrazor.com/news/bitcoin-transactions-how-do-they-work

I am confused about the digital signature still. It’s never make sense to me. What should I do for a  clear concept about that?

"Every signature is different than another whether you send the same amount to the receiver account or not. This is not a QR code but a program code which each are different than another. This article may help you to know more in detail:
https://www.cryptocompare.com/wallets/guides/how-do-digital-signatures-in-bitcoin-work/"

First of all, this signature does not work like real life signature which is basically identity of the user. the signature used in these transactions refers to the coins that the user is trying to send. When coin has been sent to the receiver, the sender cannot use the signature again. So, for sending the same amount of coins again, the sender has to create another signature which will refer to new coins he is trying to send to the receiver.

You see, the signature does not act like real life signature that we use, rather the signature is like stamp on the current amount of coins that you are trying to send. Once you are done sending you coins, the signature is lost after the receiver gets it. So, for the next transaction you need to use new signature.

The signature is for one time use during a particular transaction. Once the transaction is over and coins have been received on the other side the signature gets abolished. That is why, there are different signatures for each transaction from the same person.