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Now, in this session, we could be looking at the simple substitution cipher now in the last session

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we have learned that the Afine Saiful has about a thousand possibilities, but that computers can still

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brute force through all of them easily.

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So we need a cipher that has so many possibilities that no computer can brute force through them all.

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So the simple substitution cipher is one such cipher that is effectively invulnerable to a brute force

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attack because it has enormous number of possible keys.

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Even if your computer could try a trillion keys every second, it would still take 12 million years

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for it to try every one.

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So in this particular session, we would write a program to implement a simple substitution cipher and

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learn some useful python functions and string methods as well.

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So topics covered in this particular session will be the smallest matter.

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Getting rid of the duplicate characters from a string of functions and using his opponent is lower string

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functions or methods.

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So moving on to how a simple substitution cipher.

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Now, to understand this, let us go to a blank Excel now to implement the simple substitution cipher.

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We basically choose a random letter to encrypt each letter of the alphabet and using each letter only

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once.

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Now the key for a simple substitution cipher is always a string of twenty six letters of the alphabet

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in any random order.

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And if you look at it, there are basically four zero three two nine one four six one one twenty six

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six zero five six thirty five five eight for so many number of different keys available.

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Now there are so many different possible key ordering for a simple substitution, and this is a lot

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more keys.

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So more importantly, this number is so large that it is impossible to brute forces.

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Now, let us try using a simple substitution cipher with paper and a pencil like we would be doing it

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on Excel, for example.

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They will encrypt the message said by hacking, for example.

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Now here we would be using certain keys.

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Now we would write down our alphabet and the corresponding keys under each alphabet.

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So say, for example, we would write down here all the alphabet, the capital alphabet.

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So A.B., let's take it from this.

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A, b, c, d, e, f, g.

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H i j k l m n o p q oreste.

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W, X, Y, Z and.

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Let's make it small.

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For example.

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For that, it is just viewed.

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So all together, we don't need to scroll.

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And below this, we would be writing down the keys and I'll say we would be using your keys as we G,

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that B, G and F to be L i d m x said w w then Q. You see why he h s o.

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So every character again used just once.

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So if you see hear this, every single character is having another substituted character for it, and

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these characters are also not repeating in it.

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So in that case, only if you see this same is substituted by those same characters.

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Viterbi same.

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But others are totally different now.

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This combination of random character generations could be a very large number.

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So in part, in this particular case, not to encrypt the message, we will find the plaintext from

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the top root and we substitute the letter in the bottom.

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So if we just write down it over here that we have that this post is B, so B, we'll take it.

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The Intrepid will be saved by then w then we have like so very having s the B is having C, H is having

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a substitution of E, always having substitution of B and end is having a substitution of X, then we

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are having hacking.

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So H is having a substitution of E A's having off.

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We then C is having a Z, he is having a substitution of pi.

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Similarly N is X and G is F, so this is what would be OK and cryptic text would look like, like.

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So not to decrypt the encrypted message you will find the letter in the cipher, text in the bottom

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row and you would replace it with the corresponding letter in the top.

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So like for example your W which will be replaced by B, so on and so forth.

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So unlike a Caesar cipher in which the bottom row shifts but remains in the alphabetical order in a

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simple substitution cipher, bottom row is completely scrambled.

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So this results in far more possible, which is a huge advantage of using a simple substitution cipher.

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And now the disadvantage is that the key is a twenty six characters long and more difficult to memorize.

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You may need to write down the key somewhere, but if you do so, make sure that no one is ever reset.

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OK, now let's come to a simple substitution cipher code.

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OK, we'll go back to let's close this.

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OK, now another potential.

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We would create a new file and we would enter the code in this file at the top and then save it and

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execute it again.

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We will place this in the same folder where we have the paperclip module.