ps4

Problem Set 4

    ┌────────────────┐  ┌────────────────┐       ┌────────────────┐
    │  plaintext[0]  │  │  plaintext[1]  │  ...  │  plaintext[n]  │
    └────────────────┘  └────────────────┘       └────────────────┘
             │                   │                        │
             ▼                   ▼                        ▼
       IV   .─.                 .─.                      .─.
     ─────▶( X )    ┌─────────▶( X )       ┌───────────▶( X )
            `─'     │           `─'        │             `─'
             │      │            │         │              │
             ▼      │            ▼         │              ▼
         ┌───────┐  │        ┌───────┐     │          ┌───────┐
         │       │  │        │       │     │          │       │
         │  AES  │  │        │  AES  │                │  AES  │
    K ──▶│Encrypt│  │   K ──▶│Encrypt│    ...    K ──▶│Encrypt│
         │ Block │  │        │ Block │                │ Block │
         │       │  │        │       │     │          │       │
         └───────┘  │        └───────┘     │          └───────┘
             │      │            │         │              │
             ├──────┘            ├─────────┘              │
             │                   │                        │
             ▼                   ▼                        ▼
    ┌────────────────┐  ┌────────────────┐       ┌────────────────┐
    │ ciphertext[0]  │  │ ciphertext[1]  │  ...  │ ciphertext[n]  │
    └────────────────┘  └────────────────┘       └────────────────┘

Let's take a tour of a few unauthenticated block cipher modes!

Cheating

All the code you submit must be written by you. Submitting code written by anyone else is cheating in the official sense. Please don't do that.

It's ok (and encouraged!) for students to work together by discussing problems without sharing code. It's also ok to have someone look at your code to help you debug a specific issue. But it's not ok to look at someone else's code to copy it or to have anyone write code for you. Please don't explore the gray area between these things.

For common operations like "open a file", "parse a JSON string", or "call a library function", it's ok to copy two or three lines of code from documentation, Stack Overflow, etc. But copying more than two or three lines of code is probably not ok. If you do, you must clearly cite your source in an inline comment that begins with COPIED. Citing your source doesn't automatically make copying ok, but failing to cite your source turns one violation into two violations. Please don't explore the gray area of what counts as a "line of code".

Problems

Note that this pset is a little different, as all of the problems require manual inspection and validation for credit to be granted. Gradescope will still tell you whether your solution is functionally correct (look for the PASSED/FAILED output per test case), but will you will not be attributed the relevant points until a TA or the professor verifies that your submission does not use libraries where forbidden (this is covered in docstring problem definitions in ps4.py). Once you've solved all problems in the pset (or as many as you think you will be able to solve), ping one of the TAs and they'll review the relevant problems to attribute credit appropriately.

The only library function you are permitted to use is AES in ECB mode. Still, you must iterate over the input material block-by-block to compute your answer. "One-shot" encryptions/decryptions are not allowed and will receive 0 points.

This assignment consists of 9 problems:

1. Implement PKCS#7 padding
2. Strip PKCS#7 padding
3. Encrypt a plaintext using AES ECB
4. Decrypt a ciphertext using AES ECB
5. Encrypt a plaintext using AES CBC
6. Decrypt a ciphertext using AES CBC
7. Increment counter portion of input iv
8. Encrypt a plaintext using AES CTR
9. Decrypt a ciphertext using AES CTR

Submission

You may submit this assignment in multiple ways described below.

Note that you can submit partial assignment without solving all of the problems first. We actually encourage you to submit early and often as GradeScope will help you testing your solutions.

Gradescope

GradeScope uses pytest for the test runner. It usually shows pretty good error messages. You can see some examples here. If youll need any help with gradescope output, please ask in slack.

Note that we reserve the right to update GradeScope test suite as we come across issues/bugs or find submissions which abuse things.

Python Submission

This is the simplest approach and we recommend that most students take this option. You simply need to submit a file named ps4.py. Note filename is important. Its not problems.py or anything else. Its literally filename == 'ps4.py'. This file should have these 9 functions defined:

• problem1
• problem2
• problem3
• problem4
• problem5
• problem6
• problem7
• problem8
• problem9

You can find overviews of each problem in the ps4.py docstrings. Note that parameter names are important. GradeScope will call these functions with the same parameter names as defined in the stubs. Also some of the functions have doctests defined which provide example output functions should produce.

GradeScope will run this code on Python3.10 so we recommend you use the same Python version locally for development. pyenv might be of use here if you need to manage multiple Python versions.

Binary Submission

See prior assignment READMEs for details on binary and logistics of submissions.