What is a Caesar Cipher? If we wanted to print the letter $\text{'}$I$\text{'},$ in Python we would write that as:

print$($fruit$[3])$

Strings are Really Numbers

At the beginning of the term, we learned that letters are actually interpreted as numbers. These numbers are derived by the ASCII table. As you will recall the a lower case a is actually assigned to $97.$ The lower case letter $\backslash ($ z $\backslash )$ is assigned to $122.$ The computer uses these numeric values in order to do comparisons. We are going to use them to formulate our cipher.

The Algorithm to Encrypt

Below is the algorithm we will use to write a program that will encrypt text entered by the user:

$```$

For index $=0$ to length of string, step $1$

Assign one letter to the variable one$\_$letter

Determine the ASCII value

If ascii$\_$letter$\_$value $=122$ then $($ASCII equivalent of z$)$

$```$

$```$

Blse if ascii$\_$letter$\_$value $=90$ then

Set new$\_$asciii$\_$value $=65($this sets all Z$\text{'}$s to A$)$

Else

Set new$\_$ascii$\_$value $=$ ascii$\_$letter$\_$value $+1($shift to next letter$)$

End if

Assign letters to our new string based on the ASCII values

$```$ Translation to Python

The function below translates our algorithm above into Python so we can start to encrypt our messages. It is designed to take in to it a string message. It will then pass through each character of the string assigning the ASCII number. It utilizes the ord$()$ method in order to determine the ASCII numeric value. It also uses the chr$()$ method to take our ASCII numbers and convert them to text.

$```$

def encryptstr$($message$)$:

#declare variables

newstring $=$ str$0$

ascii$\_$letter$\_$value $=$ int$()$

one$\_$letter $=$ str$()$

new$\_$ascii$\_$value $=$ int$()$

#loop to assign new ASCII value

for index in range$(0,$ len$($message$))$:

one$\_$letter $=$ message$[$index$]$

ascii$\_$letter$\_$value $=$ ord$($one$\_$letter$)$

#checking if the letter is Z or Z

if ascii$\_$letter$\_$value $==122$:

new$\_$ascii$\_$value $=97$

elif ascii$\_$letter$\_$value $==90$:

new$\_$ascii$\_$value $=65$

#assigning a new ascii value if not Z or Z

else:

new$\_$ascii$\_$value $=$ ascii$\_$letter$\_$value $+1$

#creating the new string

newstring $=$ newstring $+$ chr$($new$\_$ascii$\_$value$)$

return newstring

$```$ Finishing Up

Using this code, complete the following:

$1.$ Write an additional function to decrypt a message

HINT: The body of this function is very similar to the encrypt function. You are simply doing the inverse of the encrypt function.

$2.$ Build a main$()$ function that:

a$.$ Contains a loop that will asks the user if they want to encrypt text, decrypt text or end the program. This loop will continue to execute until the user decides to quit the program.

b$.$ If the user wishes to encrypt or decrypt text, the program will ask them what word they want to either encrypt or decrypt.

c$.$ If the user selects encrypt, main$()$ will call the encrypt function and pass into it the word the user wishes to encrypt.

d$.$ If the user selects to decrypt, main$()$ will call the decrypt function and pass into it the word the user wishes to decrypt.