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In the last lecture, we talked about the digital signatures.



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Now let us have a practical look at how digital signatures work.



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The reason I have used graphics here and animations is for you to understand because I'm a man of practicality



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and pragmatism and I believe in using practical things to demonstrate I'm not just going to write out



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BOYENS and that you read them.



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So again, this is our dear friend Bob.



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He has a document and now he wants to send this document over email to Alice, obviously his friend,



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and now he wants Alice to believe that Bob has signed the document.



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Let's see, this is a contract and Bob has signed the contract.



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So Bob has to attach his digital signature.



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So how is the process?



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So this is the original document.



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OK, you're on the same page, everyone.



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Now, what Bob does is Bob hashes this original document.



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And even though the document is of five pages, 11 pages, 200 pages on one page, output of S.A.G.



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256 is going to be of a fixed length.



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So this is the hash of this document.



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And now what Bob does is Bob and Grip's this hash with his own private key.



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All right.



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He's not going to encrypt it.



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Alice's public key, because this time Alice wants to check if the document I have received is from



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Bob's and if Bob has signed.



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So if Alice is able to unlock this document or unlock this hash value with the help of Bob's public



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key, then only she'll believe that this document has been sent over by Bob.



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So, Bob, Bob does is Bob and this hash of this document.



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So Bob and groups this hash of this original document here.



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And what he does is he gets encrypted hash value, so and this is the digitally signed document, so



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digitally signed a document is a combination of the original document and the encrypted hash.



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So when you attach the original document, along with the encrypted hash and then send it over to someone



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that is known as a digitally signed document.



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OK.



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Now, let's have a look at this scenario.



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So now Alice has received the digitally signed document from Bob, and now Alice wants to know if Bob



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is the one who has sent and signed the document.



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So along with the digital sign document, she knows that it's a combination of original document and



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it's a combination of encrypted hash.



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As I told you a couple of minutes ago, that if not, this encrypted hash is this is an encrypted hatch.



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So if she wants to get the original hash, she has to decrypt the hash.



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Since Bob has encrypted this hash with his private key and we know that asymmetric encryption works



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if something is encrypted with private key and decrypted it publicly or vice versa of the same person.



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So now, since this hash is encrypted with Bob's private key, Alice has to find out his public key,



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which is publicly available.



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So what she does first is she has got the original document.



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So she again uses the same as a 256 algorithm to generate a hash value.



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OK, so now, Steve, she has this hash value over here.



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And again, she uses Bob's public key to get the hash, which was sent by both of the original document.



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So if these two hashes match.



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OK, so if these two hashes match, then Alice can see that the document has been digitally signed by



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document, obviously, because, see, this is the hash value of the document, which was first sent



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by the Bob.



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Let's go down to the back previous slide over here so we can see Bob has hash, the original document



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over here, and this is the hash value, which he has encrypted.



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So Alice is just following the reverse process of this.



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She's just following the reverse process of what Bob had done earlier.



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So this is very this is the way in which digital signatures work.



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And if these two hash values are exactly the same, we can see that Bob is the one who has signed the



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document digitally.



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I hope my explanation with graphics was pretty clear, if you if you have any doubts, please feel free



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to ask.



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I will be happy to answer your questions.



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And if not, I don't think will get a pretty clear explanation out there about digital signatures.



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But this is the best possible explanation that I've given to you.



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All right.



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So in the next lecture, we'll talk about SSL.



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That is secure sockets layer.



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I will see you in the next lecture.