Homework 2 COM-301

Homework 2 COM-301
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”.
Since Alice is not responding to clear text messages, you need to use her encrypted channel
to talk her out of her idea. To alleviate your load, the TA developed ‘AliceGram’ a script
that creates a connection between you and Alice so that you can easily send the messages,
but you are tasked with handling the encryption part. Since convincing people to use secure
messaging is a hard task, ‘AliceGram’ includes an artificial TA which helps you creating
messages addressing Alice’s concerns.
Warning: You are not allowed to share your code with other students.
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.
Start your conversation with Alice, receiving her first encrypted message. Use the artificial
TA to get convincing arguments to rely to Alice. You may need to exchange more than
one message with her. You will know she is convinced when she sends you a unique “I’m
convinced” message that has a token. Submit this token and your code to our grading system
to pass this task.
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.
Start a new conversation with Alice as you did before. Again, use the artificial TA to get
convincing arguments to rely to Alice until she is convinced and gives you a new token.
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.
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Deadline
29/10/2019 Homework 2 COM-301
Hint 2 : pay close attention to IVs.
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