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CIS 5371 – Cryptography

Fall 2024

Homework 3: Deadline Thursday 11/14

(Implementing padding-oracle attack) (180 points) Your task here is to implement the padding-oracle attack. The target server is linprog2.cs.fsu.edu (IP address is 128.186.120.158), and the port number is 31537. Due to some administrative restriction, to have a socket connection to this server, you need to run your code at the same machine. (You can log into this server using your linprog account.)

The server maintains a secret string M which your program needs to recover. Currently, this string is (without the quote and case sensitive) “C0ngr4tul4ti0n5!!! Y0u 5ur3 kn0w ur crypt0!!! :)”, and thus the byte length is 48. However, when I test your programs, I may change it to a different string, of a different length. You should not assume that the byte length of the secret is a multiple of 16.

Interacting with the server. The server provides two types of requests. If you want an encryption of P∥M, for some chosen prefix P, then you need to send a request of the form “-e P”. The server will send you a ciphertext C of P∥M under the TLS 1.0 encryption on a random IV L. If you want to send a ciphertext core C with IV L for validation, you need to send a request of the form “-v C L”, and the answer is either “Valid” or “Invalid”. Your goal is to recover the message M, and output it to stdout.

For an example of how to interact with the server, see the Python script client.py in the course website. When I run python3 client.py in my terminal, the script waits for me to enter requests to the server. If I type -e abcdef0123456789 (meaning my prefix is the hexadecimal string abcdef0123456789), I get back

b‘Encryption: 80\n49 8d 8e 57 ...6a 3d de \n’

IV: b‘a5244e79b9f94b4f5634a8b00e06e46c’

-E abcdef0123456789

Here the cipherext core C (in hex encoding) is 49 8d 8e 57 ...6a 3d de that is 80-byte long, and the IV L (in hex encoding) is a5244e79b9f94b4f5634a8b00e06e46c If you want to encrypt with the prefix as the empty string, just use “-E”. Likewise, if I type -v aaaaaa 40beed9c1b5bfcbc997bc025a42b4c0a, I get back ‘Invalid’. This means the ciphertext core aaaaaa with IV 40beed9c1b5bfcbc997bc025a42b4c0a is not a valid ciphertext.

For another example of how to write code to connect with the server, see the file connect.py in the course website. (The latter is written in Python2, so you’ll need to run python connect.py.)

Another testing server. To help you to test your program on fragmentary secrets, I also set up another server at the same IP address, but the port is now 31536, and the secret is “Hello World”.

Deliverables. Upload to Canvas a zip file containing your source code, which includes a README.txt that informs me how I should run the program.