07-03. Caesar cipher

This programme provides a simple encryption routine using a caesar cypher, it turns plain text into ciphertext.

As always, we have some boilerplate code.

This time we'll start with the main programme.

First, we'll ask the user to enter a message and store that in a variable identified as actual message.

The stored message will become the result of encrypting the message using the actual message.

Then we'll output the stored message to the screen.

Okay, now we can write the encryption function that will take the message in as plain text as a parameter, and return the ciphertext static string encryptmessage.

With the parameter message, the encryption key will be five.

That's the number of characters we will shift.

So, so A will become F.

The encrypted message starts as an empty string and then we'll build it up by shifting each character in the message one at a time.

We can do that with a for loop for each character in message.

To shift each character, we need to know the ASCII code of the character and then add the key to it before turning it back into a character.

an integer ASCII code becomes the integer of the character.

This is an alternative way of casting a character into a number.

In effect, what it does is convert and store the UNICODE for the character.

Okay, step back here.

Every character in a computer, that's letters, numbers and symbols are all stored in binary zeros and ones, so each character has a binary number.

Standards such as Unicode and ASCII map characters to binary numbers to be stored in a computer.

It's helpful to us to convert our character into a number so that we can add the key to it.

We called our variable ASCII code because the first 128 characters in each standard are the same.

It was just a decision I took when identifying the variable.

I could have called the variable Unicode instead.

ASCII code then becomes whatever it is now plus the key.

Now we have a problem, because what do we do if we shift beyond the number 126 in the ASCII table?

Because 127 is delete, we need to cycle back to the first character again.

So if ASCII code is greater than 126, ASCII code becomes 32.

That's the space character plus ASCII code minus 127.

That does the looping back around for us.

We could also have used modulo here if we wanted to.

The encrypted message is then whatever characters it already includes plus the character of the ascii code we just calculated.

Note how we turn a number back into a character.

Pretty neat.

Once all the characters have been converted, we can return the encrypted message.

This is a variation of the Caesar Cypher.

because it uses some of the symbols in the ASCII table too, we could restrict it to just letters as an extension.

We'll give the programme a run with the message.

I love computer science.

Not a great encryption algorithm, but a great example of using the ASCII table in code.