Welcome to the discussion thread about this lecture section. Here you can feel free to discuss the topic at hand and ask questions.
I don’t know if I understand it correctly.
Alice sends the transaction with right signatures, nodes validated the transaction and they put it into the mempool. Bob finds the transaction by the transaction ID and he changes the signature to random number. Once the transaction has got into mempool nobody is checking the signatures again. Miners just come and take the transaction, since it is in the mempool it suposed to be alright and these transaction is appended to the blockchain but with random signature.
Since the signature is changed by Bob, Alice can’t find it and she decides to send it again.
Is that right?
I understood that when the segwit was introduced there is no need for node to keep signature data, so in this case he is blindly copying the blockchain from other nodes?
Great question. The signature is altered by Bob, yes. But that doesn’t invalidate the signature. It’s still a valid signature and a valid tx. So nodes doesn’t care. But the TX id is changed and therefore Alice’s old tx looks like it’s lost to her. And then she might resend a new tx to Bob, and he gets both.
With segwit, Bob can still alter the signature, but that doesn’t effect the tx id (since the signatures are outside of the tx). So it doesn’t have any effect.
Thank you for answer, but definitelyI need to learn more about it, I understand the general concept, but I still don’t get it 100%.
Sounds good. If you want more information here is a good start: https://en.bitcoin.it/wiki/Transaction_malleability
But it’s beginning to get quite complex when you dig deeper, because now you are getting down to cryptography and discrete math.