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Hello! Welcome to the section 10 challenge.

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In this challenge, what I'm
asking you to do is to create

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a substitution cipher program.

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This is really straightforward,
but it'll give you some experience

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using the c++ string class.

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Now there's an easy way to do this
and a not so easy way to do this.

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I'm not going to tell you what
the easy way to do this is

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because I want you to learn and
try to figure it out on your own.

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Let me talk about the challenge.

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A simple very old method of sending
messages is the substitution cipher.

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I remember using this as a
kid with some of my friends.

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We used to have something
called a secret decoder ring.

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And the idea is you have a message
and then you look at every character

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in that message and you replace
it with the character on the ring.

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So for example, every time you
see a lowercase a it gets replaced

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with a capital x or every time you
see a lowercase b it gets replaced

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with a capital z and so forth.

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So it's called the
substitution cipher.

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It's really a straightforward cipher.

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It's not a very secure cipher at
all, but it's a fun cipher to do.

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So here's the idea.

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I want you to write a program that
asks the user to enter a secret

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message, that'll be a string.

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And then I want you to encrypt that
message using the substitution cipher

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and display the encrypted message.

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Then take that encrypted
message and decrypt it back

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to the original message,

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okay? You can use two strings
for your substitution.

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You can have one string
for the alphabet.

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And let me write this here
for you that way you maybe

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understand a little bit better.

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So let's say you have one
string which is the alphabet.

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And this string contains
all the characters that

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you want to substitute.

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So for example, let's say
we've got a b c and so forth.

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And capital a capital b capital c.

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Okay, then I've got another
string called the key.

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And you can name these
anything you like.

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And the idea here is that for every
character up in the alphabet string,

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you're going to have a character in
the key string that'll substitute it.

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So if i want to substitute that
lowercase a for an x, it'll be here.

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And let's say that the b gets a z
and the c gets a q and then this

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uppercase a gets a lowercase,
let's say, z, a lowercase q and

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a lowercase d, it doesn't really
matter what you replace them with.

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So now the idea is
really straightforward.

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Whenever you see the secret message,
let's say, the secret message is

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a b c, you look in the alphabet.

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You look for that
little a, you find it.

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You find it in position 0.

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Then you go to the key,
look in position 0, whatever

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is there you replace it.

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You do the same thing
with lowercase b.

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It's going to be replaced with the z.

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And you do the same thing
with the lowercase c, it'll

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replace it with a q.

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Okay, obviously you just keep going
down the string, whatever you find,

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you replace, so that would be my
original message, this is now my

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encrypted message. How would i
decrypt the message? Do the same

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thing just the other way around.

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You look in the key for an x.

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I find it at position 0.

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So I go to position zero in my
alphabet, and I grab the little a.

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Again, I look for the z.

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It's in position one.

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There's my b in position one.

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And then my q is going
to be in position three or sorry, position two.

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And it gives me my c.

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So there you go.

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Really, really straightforward to do.

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Make the cipher stronger if you wish.

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This is a really weak cipher.

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Right now it's only
substituting the letters.

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And let me scroll down here just
a little bit to show you what

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I'm giving you to start with.

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You can see what I'm
giving you to start with.

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I'm giving you an alphabet,
which is a string of all the

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lowercase and uppercase letters.

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I kept these in order just
to make it easier to debug.

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And then you've got the key.

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Those are the substitutions.

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So for every one of these letters,
there is an equivalent letter here

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that we're going to substitute.

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So for example whenever we
see a lowercase a in our

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message, we're going to
substitute it with a capital x.

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Whenever I see a capital z, I'm going
to substitute it with a lowercase r.

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That's for encryption.

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And then for decryption, you
just read the encrypted string

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and you go the other way around.

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So if you see a capital x in your
encrypted string, you'll replace it

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with a lowercase a to decrypt it.

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That's it, really straightforward.

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As I said, I'm only using
letters here, I'm not dealing

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with numbers or spaces.

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You can be as creative
as you want with that.

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There's a lot of ways to
make this a lot stronger

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and just have fun doing it.

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Remember, I'm writing this
right here on line 28.

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Remember, the less code you write,
the less code you have to test.

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You reuse existing
functionality in the libraries.

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Use the code that's
in the stream class.

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Don't try to create
all this on your own.

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Remember, the kiss principle, keep
this as simple as you can, reuse

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as much as you can and that makes
your program very likely a lot

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more correct than it will be if you
try to do everything on your own.

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Remember, those those library
functions and methods that we've

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used, they've been tested, and they've
been debugged for years and years.

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So they're pretty
solid, so reuse those.

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So let me give you a sample run so
that you can see what to expect.

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So I'm going to run this.

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And you can see here,
enter your secret message.

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And I'll say I'll meet you
at Billy's house at 10 a.m.

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Okay, now press enter.

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And it says encrypting message,
and here's the encrypted message.

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You can see it's replacing characters.

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It's pretty obvious what's
going on here, right.

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You can see the capital i in isle is
being replaced with a lowercase j.

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The apostrophe is not being
replaced at all because I'm

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only dealing with letters.

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The lowercase l is being replaced
with the capital d, you can see here.

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So you can see going across that
that's your encrypted message.

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Then I'm taking that encrypted message
and decrypting it just doing the

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opposite I just did, and I got back
the same message I started with.

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Okay, so you can see it's pretty easy.

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So my secret message would
be this is my secret message.

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There's the encrypted message.

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Okay, you can see the third
character here is an i, which is

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being replaced by a capital j.

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And you can see the i again
right here in the is, and

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there's the capital j again.

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Okay, so have fun, like I
said and make this stronger

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that's what I like to do.

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You know learn and play
around with ideas that you may

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have to make this stronger.

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It's a lot of fun to try.

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Okay, so when you're done, I'll
see you over in the solution,

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and I'll show you my solution.