Cesar Cipher

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In this chapter you will:

area	topic
🚀	encrypt and decrypt text
⚙	index the positions of a string
⚙	loop over a string
💡	use the find method of the string data type
🔀	build a longer string by concatenation
🔀	use the same index for two strings
🐞	fix index errors

Exercise 1: Cesar Cipher

Execute the following code calculating a Cesar Cipher:

plain = "ABCDEFGHIJKLMNOPQRSTUVWXYZ "
encrypted = "DEFGHIJKLMNOPQRSTUVWXYZABC "

for i in range(26):
    print(plain[i], " -> ", encrypted[i])

Explain what the code does.

Exercise 2: String methods

Find out what the expressions do to the string in the middle.

Exercise 3: Loop over a string

The following program should print the position in the alphabet of every character. Complete program by inserting char, message, plain and position:

plain = "ABCDEFGHIJKLMNOPQRSTUVWXYZ "
message = "MY SECRET MESSAGE"

for char in ___:
    position = ___.find(___)
    print(char, "is in position", ___)

Why is the space at the end of plain important?

Exercise 4: String concatenation

Explain the following code:

encrypted = "DEFGHIJKLMNOPQRSTUVWXYZABC "

s = ""
s += encrypted[4]
s += encrypted[1]
s += encrypted[8]
s += encrypted[8]
s += encrypted[11]
print(s)

Exercise 5: Encryption

Write a program that:

1. defines a plain and encrypted alphabet as 26-character strings

2. reads a message from the keyboard

3. defines an empty result string

4. goes through each character of the message

5. finds the position in the plain text alphabet

6. looks up that position in the encrypted alphabet

7. adds the encrypted character to the result string

8. when all characters have been processed, output the result

Hint

After each step, you should be able to run the program and see what it already does.

Exercise 6

Explain why the following code does the same as in exercise 1:

plain = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
for i in range(26):
    print(plain[i], " -> ", plain[(i + 3) % 26])

Can you use it to make your encryption program shorter?

Hint

If you are not sure what happens, print the value of i inside the loop.

Exercise 7

Also write a program for decryption.

Hint

How could you make sure that the input string is in upper case?

Reflection questions

• what happens when a string is followed by square brackets?

• describe two ways to loop over the characters of a string?

• what does the str.find() method do?

• how can you add characters to a string?