Description

After attending Com 301 lectures, Alice decides that she will only communicate with others using encrypted messages. To achieve this, she has given each of her friends a secret key in a USB and has asked them to create their own scripts to encrypt and decrypt messages using the AES symmetric block cipher.
One of TAs heard about this story and said: “Remember, do not implement your own crypto protocols to communicate! It is hard to get all the properties right! You must convince Alice to use one of the well-known secure messaging apps”.
Part 1
When she gave you your secret key, Alice told you that she will use use AES-128-bit in cipher block chaining mode (CBC). Alice encodes strings as ‘utf-8’ and uses PKCS padding to allow messages whose size is not dividable by the block size. note: usually libraries handle the padding implicitly.
Part 2
Even though she said you she is convinced, Alice does not install a secure messaging app. Instead, she decides that the solution is to consider the messages in a conversation as a stream of data, and use a block cipher mode of operation which allows her to encrypt streams.
Hints and suggestions
Suggestion 1 : We have installed “petlib” library on the com301’s VM. You can use this library for encryption.
Hint 1 : “petlib” handles the padding on its own.
1

Hint 2 : pay close attention to IVs.
2