Substitution Cipher¶

In this chapter you learn:¶

area	topic
🚀	encrypt text with a substitution cipher
⚙	convert strings to lists and back
⚙	use a random seed
💡	use the `random` module
💡	use the `string` module
💡	use the `zip` function
🔀	use the `dict(zip())` pattern
🐞	fix IndexErrors

The following code creates a random chiffre that we will use for encryption. Insert alphabet, chars, join and random into the code:

import random

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

___ = list(___)
___.shuffle(chars)
chiffre = "".___(chars)
print(chiffre)

Run the program a couple of times.

Add the following line after the import:

random.seed(77)

Run the program a couple of times. What changes?

Also try running the program with different numbers.

Output each character from the chiffre and its index Simplify the following code using the function enumerate():

i = 0
for char in chiffre:
    print(i, char)
    i += 1

Create a dictionary that maps unencrypted to encrypted characters:

lookup = {}
i = 0
while i < len(alphabet):
    char = alphabet[i]
    lookup[char] = chiffre[i]
    i += 1
print(lookup)

Execute the program slowly on paper to make sure you understand the index operations.

Use the following code to make the program above simpler:

for char, chif in zip(alphabet, chiffre)
    print(char, chif)

Once you understand what happens, also try:

lookup = dict(zip(alphabet, chiffer))

Use the lookup dictionary to encrypt a message. Use the following lines as a starting point:

message = ...
result = ""

for char in message:
    result += ...

What happens when you enter lowercase letters or special characters in the message?

Try the expression:

print(lookup.get(";", "X"))

Fix the problem.

Decrypt a message when all you know is the encrypted message and the random seed.