Slack encrypts messages between clients and servers.

Which type of attack does this prevent?

Define "Confidentiality".

In this exercise we consider the Simple Substitution Cipher as a 5-bit block cipher. The letters A = 1, B = 2, C = 3…..Z = 26 are first represented as binary strings (5 bits per letter) as follows: A = 00001, B = 00010, C = 00011, …Z= 11010. Let the key of a Simple Substitution Cipher be given by (plaintext in top row above is replaced by bold ciphertext in bottom row):

(a)  Write the plaintext TODAY as a binary string. 

How many bits of the resulting plaintext are incorrect?

Hint: Convert the plaintext into bit form like you did in Part A and compare with your answer for part A

Encrypt TODAY using the above cryptosystem and write the ciphertext as a binary string.

Change the second bit in each block of the above ciphertext and then decrypt this altered ciphertext.

In this exercise we consider the Simple Substitution Cipher as a 5-bit block cipher. The letters A = 1, B = 2, C = 3…..Z = 26 are first represented as binary strings (5 bits per letter) as follows: A = 00001, B = 00010, C = 00011, …Z= 11010. Let the key of a Simple Substitution Cipher be given by (plaintext in top row above is replaced by bold ciphertext in bottom row):

(a)  Write the plaintext TODAY as a binary string. 

In Python or another language of your choice, implement a brute force attack.

• Allow a user to enter a 5-character, uppercase only, password of their choosing
• Write a script which generates all possible 5-character, uppercase only passwords
• Show that you've been able to find the user-inputted password.

Save this script as you'll need it in future weeks. Upload the script here.

Extension: Can you do the same for a password containing letters and numbers? What about if you were to include special characters? What do you notice about run time of your script with these differing password structures?

Now open Cryptool Online and navigate to Frequency Analysis. Copy the Ciphertext from the previous question. Change the 'Fragment Length' from 1 to 2 or 3. What do you notice about the frequency of 2 letter and 3 letter words?

Explain how you might adjust the normal English letter frequencies shown in the lecture slides in order to conduct single letter frequency analysis, given that the context of a plaintext is a letter in French.