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Everyone and welcome back to this class natural language processing in Python.

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In this lecture, we're going to talk about ciphers specifically, we're interested in one of the simplest

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ciphers, the substitution cipher.

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Basically, this should be exactly what you think it is.

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We create our encoded message by substituting each letter with a different letter.

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Let's start with some terminology and a high level picture.

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First, we're going to assume that there is some senator and some receiver, and the sender is sending

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a message to the receiver.

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You are a spy who has intercepted this message.

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But of course, it's encrypted, so you can't just read it plainly.

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The message was encrypted by the sender before sending the message, and the receiver will decrypt the

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message upon receiving it.

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Both the sender and the receiver have a dictionary or a mapping such that they can look up at which

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letters in the unencrypted message get mapped to which letters in the encrypted message.

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This is a one to one mapping, so in order to decrypt the message, we just have to reverse the mapping.

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Of course, you as the spy don't have this mapping, so therefore you can't read the message without

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having to do some extra work, which is what this section is all about.

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Let's do a very simple example.

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So you get the basic idea.

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Suppose the message we want to send is I like cats.

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Now we need to encrypt this message.

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We're given this mapping, which tells us how to translate each of the letters here into its encrypted

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form.

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Since there are 26 letters in the alphabet, let's just focus on the letters we need.

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Specifically, I should be replaced with why L should be replaced with W AI again.

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It should be replaced with why K should be replaced with R e should be replaced with n c should be replaced

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with J.

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A should be replaced with l t should be replaced with O and s should be replaced with B.

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So our encrypted message becomes y w y are n jello B. Therefore, our sender will send the message y

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y are in jail, though b across the internet without transmitting the key, it's assumed that both the

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sender and receiver already know how to decrypt the message.

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Next, let's pretend that we have a receiver and we just receive this encrypted message.

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Our job is now to decrypt this message.

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In order to do this, we must apply the same mapping as before, but in reverse order specifically,

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why should be replaced with I w should be replaced with L y again.

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It should be replaced with AI.

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Ah, should be replaced with K and should be replaced with E j should be replaced with C l should be

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replaced with a o should be replaced with T and B should be replaced with s.

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Therefore YWCA in a jail or B becomes I like cats.

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So hopefully, this lecture helps you to understand the basic mechanism behind the substitution cipher

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and to understand the basic terminology, you know how to encrypt and decrypt a message and why we want

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to solve this problem in the first place.

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You now understand your role in the process, which is to crack the code and decrypt the messages without

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being given the key.

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In the next few lectures, we'll talk about how our understanding of probability will help us in that

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pursuit.