Block Cipher¶

In this chapter you will:¶

area	topic
🚀	implement a transposition cipher
⚙	slice lists and strings
💡	use the `join` method of the `string` data type
🔀	use a loop variable for indexing
🔀	use indexing to re-arrange a list

Indexing and Slicing¶

Strings and lists are both ordered sequences of elements. In both, you can address elements by their position. However, Python is counting differently than humans:

Exercise 1: Indexing lists¶

What do the following expressions result in?

numbers = [1, 4, 9, 16, 25, 36]

numbers[4]
movies[0]
movies[-1]
numbers[-3]

Exercise 2: Slicing strings¶

What do the following commands result in?

name = "hello world"

name[5:] name[5:10] name[:10:2] numbers[2:-2] numbers[::2]

Exercise 3: Ranges¶

Use list(range()) to create the following lists:

[4, 7, 9, 12, 15]

[10, 20, 30, 40, 50, 60]

[33, 32, 31, 30]

Exercise 4: Decypher¶

The following text contains an encrypted word:

name = "CSAIPRALKAINACZEYLVOST"

Print every second character, starting with the 2nds.

Exercise 5: Slicing puzzle¶

Use the expressions to modify the list as indicated. Use each expression once.

Exercise 6: Blocks¶

The following code is creating the first two blocks for a transposition cipher . Complete the code by creating the other two blocks as well.

message = "MEETINGATDAWNATTHEBRIDGE"

block1 = message[0::4]
block2 = message[1::4]
___
___
encrypted = block1 + block2 + block3 + block4

Exercise 7: Transposition Cipher¶

Complete the program that encrypts a text using a transposition cipher:

message = input("enter the text to encrypt: ")
encrypted = ""
for start in range(4):
    ___

Exercise 8: Decrypt¶

Write a program to decrypt an encrypted message again.

Exercise 9: Encryption Key¶

Use an encryption key like 2031 that specifies a new order for the blocks. Implement the following:

create an empty list
create the blocks as above and add them to the list
go through each position of the encryption key
select the block with the index given by the digit from the key (convert it to int)
add the block to the result string

Reflection questions¶

what is indexing?
what do the three numbers in slicing do?
what do you think about the transposition cipher. Is it secure?
how could you decrypt a transposition cipher without the key?