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In the last lecture, we talked about the different types of encryption, let us get started with the



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different types of ciphers.



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Well, the first is the classical cipher.



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And in this and there are modern ciphers.



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So classical ciphers were used in the war time period in order to send messages.



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And at that time, cryptography was not developed.



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So what is the first classical cipher?



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Well, the first classical cipher is substitution, cipher substitution ciphers and cryptic plain text



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by swapping each letter or symbol in the plain text by a different symbol as directed by the key.



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Now, when you are going to encrypt, we have already seen that we need a key.



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So we have encryption algorithm, we have the key and then we get the data.



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So perhaps the most simplest substitution cipher is the SESAR cipher, which is named after the man



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who used it for the first time.



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A cipher is perhaps better known to the captain.



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Midnight Colgrove sees a cipher maybe differentiated from other, more complex substitution cipher,



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but that is one of the most easiest SESAR cipher.



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So let us see.



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You have to encrypt this as B and why you want to tell someone that I am a spy.



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And the key here is, let's say E so the key here is e what you are going to do is you are going to



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apply this key and then you are going to get the encrypted text, which is.



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Now, if you use a numbering alphabet here, if you use this series, just let me show you the series



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right here.



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Yeah, so I was talking about this series, so now here every single letter has a number associated.



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For example, zero is E, 18 is S, 23 is X.



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So when you write spy, OK, when you when you write spy over here, what exactly it's doing is.



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S is 18, so s is it here?



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B, B is 15, so B is 15.



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Y is twenty four wise twenty four.



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And the key here is for the key here is for so remember one thing.



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Plain text.



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OK, plain text.



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Plus, the key gives you the ciphertext city.



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All right, so our plaintext is this key is for so substitution cipher does is it adds all the digits



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or 18 plus four gives us twenty two.



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So 18 plus four gives us 20 to 15 plus four gives us nineteen and twenty four plus four gets twenty



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eight.



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OK, so 22 here is W 22, here is W.



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19, here is D..



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Now, do we have 28?



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Well, this is twenty five, twenty six, twenty seven, and then you have twenty eight, so.



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If you are a spy and if you want to tell your friend that you are a spy, you just tell him that, OK,



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the key is E and you give this ciphertext.



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So when he can, you know, have this WTC.



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Now, let's say your friend has this WPEC and now he's confused about how you're going to decrypt it.



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Well, you have these letters and you have the key.



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So friend knows the keys and friend your friend knows these letters.



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So what he has to do is while encrypting, you added so much decrypting, he has to subtract.



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So 20 to minus four gives him 18, 19, minus four gives him 15.



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Twenty eight minus four gives him.



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What does it give?



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No, so in that case, what he has to do is if it's greater than 25, he has to jump back four places



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here and he has to add twenty eight minus four anyways, is 24.



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So 18 again, he checks is as 15.



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He checks is B and twenty four he checks is via hurry.



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So he got the word spy.



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This is how substitution ciphers actually work.



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So this is just a basic example of how you encrypt and how you decrypt.



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So if I, if I take a different Molcho all the way right here, this process, this process here is



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encryption process and this process here is the decryption process.



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So you have done encryption and you have the decryption.



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And that is how SESAR cipher or the substitution ciphers used to work in the early periods.



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Not the next type of cipher we are going to see is transposition ciphers.



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But before that I would just like to clear my screen.



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Now, let's continue with the next type of Saiful, which is transpositions, Saiful transpositions



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Saiful is another form of an ciphering technique and it involves rearranging of the letters of a plain



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text message.



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So if you want to, let's say write again, spy, then you rearrange it using a key.



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But this time you do not add, you just rearrange or you just shift by some particular numbers.



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So what is about classical ciphers?



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All substitution ciphers is the key is very weak.



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And if someone finds out a key, then there is no point in having that encryption at all.



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No modern type of ciphers are based on two factors, the first factor being and the first factor being



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the private key.



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So modern ciphers are basically based on two types, on the type of key and on the type of data.



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So when it comes to the type of key, there are two keys which we had discussed, public and private



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key.



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So this private key stays with you.



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You do not tell anyone about this private key.



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And the public key is to public knowledge.



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Anyone in the world can know your public.



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So I hope now you have got more hints to the question which I had told you to look upon in the last



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lecture.



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And then based on type of data, whether it is a stream cipher or whether it is a block cipher seems



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a trifle.



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See, I'm sorry, stream cipher is basically a bit by bit.



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You know, it's it's a stream of data that gets encrypted simultaneously.



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So ACMS encryption, ICMS encryption becomes encryption, whereas in case of Block Saiful a block of



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data, a chunk of data comes into the algorithm, it gets encrypted and then only the next data, next



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block of data gets encrypted.



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So it's a it's not a continuous thing, but it's a block by block thing, a block of 64 bits or a block



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of let's say eight bits or a block of 128 bits.



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So what happens in case of block cipher is sentences may get break and it becomes very difficult to



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rearrange to get a meaningful data.



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But when it comes to Stream Saiful, the data gets encrypted in one go.



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Now this was about the different types of ciphers.



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In the next lecture we'll talk about the most important and the most fundamental topic of symmetric



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encryption.



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All right.



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I will see you in the next lecture.