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Project 3: Cryptography
Project Files
You can download a zip file with all necessary project files here.
Goals of the Project For Students
Will get an introduction to both symmetric and asymmetric cryptographic systems
Will gain an understanding of how these systems are implemented through examples
Will exploit systems that have certain vulnerabilities
Information
Refer to this video for an overview of the project and tips on how to approach the tasks – primer video
There is no required VM for this project. All that is required is a Python development environment. Make certain that you are using Python 3. To check your version of Python, open a command prompt and run the command:
python –version (You may need to use the python3 command instead.)
For the established algorithms that you may find it necessary to use, you are allowed to reference and implement pseudocode with citation (a comment in your code will suffice). What is Pseudocode? https:/ /en.wikipedia.org/ wiki/Pseudocode
UNDER NO CIRCUMSTANCES should you copy/paste code into the project. Doing so is an honor code violation (not to mention a real world security concern) and will result in a zero (refer to the syllabus for more information).
The Final Deliverables
Each task will have a separate autograder in Gradescope. The submission to each autograder may be a Python file, or a JSON file containing your answers.
Task Vigenere Ciphers (35 points)
The Vigenere cipher is an example of a symmetric key cryptographic algorithm. In such a system, a single key is used to both encrypt and decrypt messages (this is what makes it symmetric). The first step for both encryption and decryption is to build a Vignere square like the one pictured below. In each row of the Vigenere square, the letters of the alphabet are shifted to the left by one. The second step is to extend the key to match the length of the ciphertext/message by repeating it, taking care to remove any spaces or punctuation from the original message. For example, if our message is GEORGIA and our key is TECH we would end up with a key TECHTEC.
To encrypt a message, we lookup each letter of the message as a row, and find its intersection with the column whose label contains the corresponding letter from the key. Using our GEORGIA/TECH example, the intersection of the first letter of the message G with the first letter of the key T is Z. Going letter-by-letter in this manner we build our ciphertext until we get ZIQYZMC.
To decrypt a ciphertext, we start with the first letter of the key and traverse that row until we reach the corresponding letter from the ciphertext. The label of this column is our decrypted letter. Using our same GEORGIA/TECH example, to decrypt the ciphertext we would start with the first letter of our key T and traverse that row until we reached the first letter of the ciphertext Z. The label of this column is G – the first letter in our message.
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Vigenere Square [source: Wikipedia]
Armed with this information, the first two parts of this task will ask you to write the necessary code to handle the encryption and decryption functionality for a Vigenere cipher system.
For the third and final part, you will attempt to crack a Vigenere cipher using a dictionary attack. Ordinary words can make convenient keys because they are easy to remember, but this practice is far from secure. For this task, you are given a ciphertext and a list of some of the most common words in the English language. One of those words was used as the key to encrypt the ciphertext, and your job is to write the code to figure out which one it is. For simplicity, you can assume that all words in the original message are also chosen from the provided list of dictionary words.
def vigenere_decrypt_cipher(c: str, keyword: str) -> str:
# TODO: Write the necessary code to get the message (m) from the cipher (c) # using the keyword
m = ”
return m
def vigenere_encrypt_message(m: str, keyword: str) -> str:
# TODO: Write the necessary code to create a Vigenere cipher (c) of the # message (m) using the provided keyword
c = ”
return c
def vigenere_dictionary_attack(c: str) -> str:
# TODO: Write the necessary code to get the message (m) from the cipher (c)
m = ”
return m
Submission Details
You will write your code in the task.py file found in the task_vigenere_ciphers folder of the project files archive that you downloaded. Submit this file to the Project Cryptography – Vigenere Ciphers autograder in Gradescope.
Task RSA Warmup (10 points)
Now that we’ve reviewed a symmetric key cryptographic algorithm, we can move on to the world of asymmetric key cryptography. RSA is perhaps the best known example of asymmetric cryptography. In RSA, the public key is a pair of integers (N, e), and the private key is an integer d.
To encrypt integer m with public key (N, e), we use the formula c ≡ me mod N. To decrypt ciphertext c with private key d, we use the formula m ≡ cd mod N.
In this task you will write the code to perform the encryption and decryption steps for the RSA cryptographic algorithm. Finally, you will write the code necessary to calculate the private key d when given the factors of the public key N (i.e. p and q).
def rsa_decrypt_cipher(n: int, d: int, c: int) -> int:
# TODO: Write the necessary code to get the message (m) from the ciphertext (c) m = 0
return m
def rsa_encrypt_message(m: int, e: int, n: int) -> int:
# TODO: Write the necessary code to get the ciphertext (c) from the message (m) c = 0
return c
def rsa_calculate_private_key(e: int, p: int, q: int) -> int: # TODO: Write the necessary code to get the private key d from # the public exponent e and the factors p and q
d = 0 return d
Submission Details
You will write your code in the task.py file found in the task_rsa_warmup folder of the project files archive that you downloaded. Submit this file to the Project Cryptography – RSA Warmup autograder in Gradescope.
Task RSA Factor A S4-Bit Key (10 points)
Modern day RSA keys are sufficiently large that it is impossible for attackers to traverse the entire key space with limited resources. But in this task, you’re given a unique set of RSA public keys with a relatively small key size (C4 bits).
Your goal is to get the factors (p and q) of each key. You can use whatever methodology you want. Your only deliverable is a formatted json file containing p and q. To get your unique set of keys, you must update the task.py file located in the task folder with your U-digit GT ID, and then run it. Find the section below in task.py:
##############################################
# Change this to your 9-digit Georgia Tech ID! STUDENT_ID = ‘123456789’ ##############################################
Running the command “python task.py” should output your assigned keys. Your JSON submission file should look like the image below. NOTE: Choose the lower of the two values as p and the higher one as q.
{
“test_1”: {“p”: 0, “q”: 1},
“test_2”: {“p”: 0, “q”: 1},
“test_3”: {“p”: 0, “q”: 1},
“test_4”: {“p”: 0, “q”: 1},
“test_5”: {“p”: 0, “q”: 1}
}
Submission Details
You will put your answers in the submission.json file found in the task_rsa_factor_C4_bit_key folder of the project files archive that you downloaded. Submit this file to the Project Cryptography – RSA Factor C4-Bit Key autograder in Gradescope.
Task RSA Weak Key Attack (15 Points)
Read the paper “Mining Your Ps and Qs: Detection of Widespread Weak Keys in Network Devices”, which can be found at: https:/ /factorable.net/weakkeys12.extended.pdf. The essay is essential to understanding this task, please read it.
You are given a unique RSA public key, but the RNG (random number generator) used in the key generation suffers from a vulnerability described in the paper above. In addition, you are given a list of public keys that were generated by the same RNG on the same system. Your goal is to write the code to get the unique private key (d) from your given public key (N, e) using only this provided information.
def rsa_weak_key_attack(given_public_key_N: int, given_public_key_e: int, public_key_list: typing.List[int])
-> int:
# TODO: Write the necessary code to retrieve the private key d from the given public # key (N, e) using only the list of public keys generated using the same flawed RNG d = 0
return d
Submission Details
You will write your code in the task.py file found in the task_rsa_weak_key_attack folder of the project files archive that you downloaded. Submit this file to the Project Cryptography – RSA Weak Key Attack autograder in Gradescope.
Task RSA Broadcast Attack (15 Points)
A message was encrypted with three different 1,024-bit RSA public keys (N_1, N_2, and N_3), resulting in three different ciphers (c_1, c_2, and c_3). All of them have the same public exponent e = 3.
You are given the three pairs of public keys and associated ciphertexts. Your job is to write the code to recover the original message.
def rsa_broadcast_attack(N_1: int, c_1: int, N_2: int, c_2: int, N_3:int, c_3: int) -> int: # TODO: Write the necessary code to retrieve the
# decrypted message (m) using three different
# ciphertexts (c_1, c_2, and c_3) created using
# three different public key N’s (N_1, N_2, and N_3) m = 0
return m
Submission Details
You will write your code in the task.py file found in the task_rsa_broadcast_attack folder of the project files archive that you downloaded. Submit this file to the Project Cryptography – RSA Broadcast Attack autograder in Gradescope.
Task RSA Parity Oracle Attack (15 Points)
By now you have seen that RSA treats messages and ciphertexts as ordinary integers. This means that you can perform arbitrary math with them. And in certain situations a resourceful hacker can use this to his or her advantage. This task demonstrates one of those situations.
Along with an encrypted message (c), you are given a special function that you can call – a parity oracle. This function will accept any integer value that you send to it and decrypt it with the private key corresponding to the public key that was used to encrypt the given ciphertext (c). The return value of the function will indicate whether this decrypted value is even (true) or odd (false). Armed with this function and a little modular arithmetic, it is possible to crack the encrypted message. Your goal is to write the code necessary to decrypt the original message (m) from the given ciphertext (c).
Hint: Look into Decimal library to avoid rounding errors when you divide and multiple large numbers
def rsa_parity_oracle_attack(c: int, N: int, e: int, oracle: Callable[[int], bool]) -> str:
# TODO: Write the necessary code to get the plaintext message from the ciphertext (c) using # the public key (N, e) and an oracle function – oracle(chosen_c) that will give you
# the parity of the decrypted value of a chosen cipertext (chosen_c) value using the hidden private key (d) m = 42
# Transform the integer value of the message into a human readable form message = bytes.fromhex(hex(int(m_int)).rstrip(‘L’)[2:]).decode(‘utf-8’) return message
Submission Details
You will write your code in the task.py file found in the task_rsa_parity_oracle_attack folder of the project files archive that you downloaded. Submit this file to the Project Cryptography – RSA Parity Oracle Attack autograder in Gradescope.
Important Notes:
All necessary starter code and unit tests for each task is located in the corresponding folder in the provided zip file.
You may import any python packages that are part of the standard library. Some useful ones are already imported for you, and additional ones are not strictly necessary.
For each task you are also given a unit test file (it starts with test_)to help you develop and test your code. We encourage you to read up on Python unit tests, but in general, the syntax should resemble either:
python -m unittest test_task_rsa_encrypt_message
or:
python test_task_rsa_encrypt_message.py
However, keep in mind that passing the unit test(s) does NOT guarantee that your code will pass the autograder!
Each autograder will timeout after 10 minutes. If your implementation is timing out, then there is very likely something wrong with your implementation. It is possible to solve each task in a few seconds. We encourage you to test locally to avoid unnecessary submissions.
Gradescope can get very busy and even potentially unavailable near submission deadlines. Please do not wait until the last minute to make your submissions to the autograder. Submit early and often. There will be no late submissions accepted, as per the syllabus.
Good luck!
This course material is derived from EECS 388 at the University of Michigan and CS 461 / ECE 422 at the University of Illinois, and is provided under a Creative Commons License.
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