Computer Science Assignment: Cyber Security and Encryption Techniques

This assignment delves into the realm of cyber security and encryption, offering a comprehensive analysis of block ciphers, CBC mode, and CTR mode. The solution begins with an introduction to the concept of encryption within networked control systems, referencing relevant academic sources. The assignment then proceeds to explore basic encryption and decryption using a provided substitution cipher, highlighting its inherent security weaknesses. Following this, the solution demonstrates encryption and decryption using the same block cipher in CBC (Cipher Block Chaining) mode, incorporating the use of an initialization vector (IV). Finally, the assignment examines encryption and decryption using the block cipher in CTR (Counter) mode, comparing the ease of use and security implications of both CBC and CTR modes. The solution concludes with a discussion on the relative security strengths of CBC and CTR modes and provides references to supporting literature.

Computer Science
Cyber Security
As per Kogiso & Fujita (2015) the idea if controller encryption can be used to enhance the cyber
security of networked control systems. Kim et al (2016) has shown that the security can be
enhanced by protecting the signals from the sensor to the controller and controller to actuator.
This can be done using encryption.
Plaintext Ciphertext Plaintext Ciphertext
0 a 8 e
1 c 9 d
2 f a 0
3 6 b 7
4 3 c 5
5 8 d b
6 4 e 9
7 2 f 1
3. (3 pts)
a. Encrypt the 4-block message cafe using this cipher for each digit individually.
Solution:
Encryption – café = 5019
b. Decrypt the message 199b using this cipher.
Solution:
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Decryption – 199b = feed
c. Why is it insecure to use a block cipher in this way? Do not critique the block cipher
itself, but only how it is used.
Solution:
This is insecure because in case the substitution cipher table gets leaked, then all the
encrypted messages can be easily decrypted.
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4. (6 pts) For this question, you will perform encryption and decryption using the same
block cipher given above, but in CBC mode. Recall that you can convert hexadecimal
digits to binary to do the XOR operation.
i. Encrypt the 4-block message cafe using the above blockcipher in CBC mode with an
IV of 6 (0110in binary).
Solution:
In this case we will first Exor the Inititalization vector ( IV ) with the plain text. Then,
we apply the key and go to next step.
ii. Decrypt the message db70 using the above cipher in CBC mode with an IV of 6.
Solution:
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5. (6 pts) For this question, you will perform encryption and decryption using the same block
cipher given above, but in CTR mode.
a. Encrypt the message cafe using the above block cipher in CTR mode, with a counter
block that startsfrom 0.
Solution:
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b. Decrypt the message 5628using the above block cipher in CTR mode, with a counter
blockthat starts from 0.
Solution:
c. Was this easier or more difficult than encrypting/decrypting in CBC mode? Do you
suspect any difference in security? Why or why not?
Solution: This was easier than encrypting / decrypting in CBC mode. This does not involve
carrying forward the results of one stage to the next. In CTR, there is more parallel operation
involved. CBC mode is more secure than CTR mode. This is because in CBC mode, every
ciphertext block depends on all the previous plaintext blocks. This adds multiple levels which
makes it more secure.
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References
Kogiso, K., & Fujita, T. (2015, December). Cyber-security enhancement of networked control
systems using homomorphic encryption. In 2015 54th IEEE Conference on Decision and
Control (CDC) (pp. 6836-6843). IEEE.
Kim, J., Lee, C., Shim, H., Cheon, J. H., Kim, A., Kim, M., & Song, Y. (2016). Encrypting
controller using fully homomorphic encryption for security of cyber-physical systems. IFAC-
PapersOnLine, 49(22), 175-180.
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