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[Music] welcome back aliens my name is 720 and let's continue this series on blockchain in this video we'll talk about digital signature why do we need it so let's go back to the previous video where we have talked about four problems on the network the first one was confidentiality second one was integrity third one was non-repudiation and fourth one is authentication so we have talked about confidentiality right how do we achieve that with the elf of cryptography of course what we do is we use a receivers public key to encrypt the message and then the moment this message received at the receiver end the receiver will decrypt the message with the help of his own private key and that's how it works and no one else can actually decrypt the message because no one has the receivers private key so that works right we have achieved confidentiality what about authentication how do we achieve that and that's where we have a concept of digital signature so let's imagine we have multiple nodes on this network and one of them is a and the other one is B and B has received a packet or the message by saying let's meet at 6 p.m. and packet claims that this is coming from node and a and this sort of starts you know how can you be sure that this packet has came from node a and how node a can say hey I have sent that message so to solve this problem you know in real world as well what we do when we write a letter we also sign the letters like that's your signature which specifies that this letter has been sent by you or written by you how can we do that in the virtual world of course we cannot use a pen well right and that's why we have to use a digital signature how can we do that so let's imagine we have two keys right so everyone will have two key a private key and a public key if I were to send a message so let's say a want to send a message to B and the message is let's meet at 6 p.m. now before sending that message what if Abel entered the message with the help of is private key and then this message goes to the B and is able if you decrypt it with a is public key but then hold on his public key will be available to everyone right that's the case we are not trying to achieve confidentiality here we are trying to achieve signature the moment a is able to sign the message by his own private key and it is decrypted by a public key on the B's and now be sure that this message is coming from a because this message was able to decrypt with the help of a sperm the key and the only way you can...