Solving Linux Device Driver Assignments with Caesar Cipher and XOR Encryption

Before writing a single line of C code, it’s crucial to grasp what the assignment actually demands. Most Linux driver projects given in universities follow a similar pattern — the core learning outcome lies in understanding kernel modules and communication between user space and kernel space.

What the Assignment Typically Requires

Your assignment may ask you to:

• Develop a Loadable Kernel Module (LKM) in C.
• Implement the basic driver functions: open, release, read, write, and at least one ioctl function.
• Write a user application to test the driver.
• Add specific functionality — in your case, string encryption and decryption using Caesar Cipher or XOR logic.
• Include a detailed write-up explaining your approach, issues faced, and how they were resolved.

The Caesar Cipher driver focuses on shifting ASCII characters to encrypt and decrypt data. The XOR driver adds another logical layer, combining bitwise operations with key-based encryption — a great way to show your grasp of both low-level bit manipulation and kernel-user data flow.

The Big Picture: Kernel Space vs. User Space

Think of the kernel module as the “engine” and the user program as the “driver’s seat.”

The module sits inside the Linux kernel, interacting with hardware and system resources, while your test program lives in user space and communicates with the module using system calls.

Understanding this separation is key.

Your device driver must safely handle data exchange — that’s where copy_to_user() and copy_from_user() come into play.

The Learning Goals

Assignments like this test multiple skills:

• Proficiency in C programming and memory management.
• Understanding of Linux kernel internals.
• Debugging using dmesg and printk()`.
• Writing Makefiles for automated compilation.
• Building an ioctl interface to control driver behavior (e.g., switching between encrypt and decrypt modes).

So, even though the deliverable is a Caesar Cipher or XOR driver, the real goal is to prove that you can design, implement, and debug a Linux kernel module from scratch.

Step-by-Step Approach to Solving Linux Device Driver Assignments

Successfully completing such a project requires a well-planned approach — not just coding. Below is a structured way to tackle it, step by step.

Setting Up Your Development Environment

Most students lose precious time struggling with setup issues. Since kernel modules are tightly coupled with the operating system, your environment must match your kernel version.

Install the Required Tools

Use the provided commands (from your README or professor) to configure your environment:

sudo apt update sudo apt install gcc-12 sudo apt install pahole sudo ln -sf /sys/kernel/btf/vmlinux /usr/lib/modules/$(uname -r)/build/vmlinux

These ensure that your compiler and kernel debug symbols are compatible. Skipping these steps often causes “missing symbol” or “invalid module format” errors during compilation.

Understand the Directory Structure

You’ll usually have two folders:

• Module/ → contains the kernel driver (e.g., caesar_driver.c)
• Test/ → contains the user program (e.g., test_driver.c)

Each folder will have its own Makefile. Ensure paths are correct and contain no spaces — kernel build scripts are sensitive to this.

Use a Virtual Machine

Always work inside a Linux Virtual Machine (VM). It’s safer — kernel code errors can freeze or crash the host OS. Ubuntu 20.04 or above works perfectly.

Designing the Logic Before Coding

Before you start typing, map out the logic on paper or a whiteboard.

Ask yourself:

• What happens when the user writes to the device?
• What should the driver return when read?
• How will ioctl control encryption and decryption?

A simple plan could be:

1. write() → Get string input from user space.
2. ioctl() → Set mode (Encrypt/Decrypt) and possibly a key.
3. read() → Return processed (encrypted or decrypted) string.

If you’re implementing both Caesar Cipher and XOR, structure them modularly — separate the core logic into helper functions.

Implementing the Device Driver

Basic Structure

Start with the standard Linux module headers:

#include <linux/module.h> #include <linux/kernel.h> #include <linux/fs.h> #include <linux/uaccess.h>

Define your file_operations structure, which connects function pointers like open, release, read, and write to their implementations.

Then, in your init function, register the device with register_chrdev().

Writing the Core Functions

• open() and release(): Initialize or clean up states.
• read(): Copy processed data from kernel buffer to user space using copy_to_user().
• write(): Receive raw user input using copy_from_user().
• ioctl(): Control logic — switch between encryption and decryption modes.

Implementing Caesar Cipher and XOR Logic

Your encryption logic should be independent of I/O handling.

For example:

char encrypt_caesar(char c, int shift) { if (isalpha(c)) { char base = islower(c) ? 'a' : 'A'; return (c - base + shift) % 26 + base; } return c; }

Similarly, XOR logic can be implemented as:

char encrypt_xor(char c, char key) { return c ^ key; }

This separation keeps your code readable and testable.

Testing Your Driver with a User Application

Building the User App

In your Test/ folder, write a simple C program that uses system calls like open(), read(), write(), and ioctl().

Interacting with the Driver

Once compiled, the test program should:

1. Write a string to the device.
2. Use ioctl to set “encrypt” mode.
3. Read back the encrypted result.
4. Switch ioctl to “decrypt” mode and read again.

This shows bidirectional communication between user and kernel space — the heart of the assignment.

Debugging Output

Always monitor the kernel log using:

dmesg | tail

Every printk() in your driver prints here, helping you trace issues like memory corruption or invalid pointers.

Common Issues and How to Solve Them

No Linux device driver project goes perfectly on the first try. You’ll face multiple challenges, but each one is an opportunity to understand the kernel better.

Compilation Errors

If your module fails to compile, check:

• Kernel headers are installed correctly (sudo apt install linux-headers-$(uname -r)).
• The Makefile uses the right path for KDIR.
• You’re compiling with make and not gcc directly.

Module Insertion Errors

When using sudo insmod driver.ko, you might see “Invalid module format.”

This usually means the kernel version mismatch.

Rebuild after running:

make clean make

Permission Denied on Device File

You may need to manually create the device node:

sudo mknod /dev/caesardev c <major> 0 sudo chmod 666 /dev/caesardev

Where <major> is the number printed in your dmesg after loading the module.

Data Not Transferring Correctly

If you read back empty or incorrect data:

• Check copy_to_user() and copy_from_user() arguments.
• Ensure buffer sizes are adequate.
• Use printk() to log intermediate data.

Segmentation Fault in User App

Usually due to file descriptor misuse.

Ensure your test program successfully opens the device (open() should return non-negative).

Documenting Your Work and Write-Up Tips

University professors often allocate up to 20–25% of your grade to the project write-up. The documentation isn’t just a report — it’s proof of your problem-solving process.

Structure Your Write-Up

Use the provided Writeup Template strictly in order:

• Description: What your driver does.
• Approach: The step-by-step plan and logic used.
• Issues and Resolutions: List at least six issues and explain how each was fixed.
• Screenshots: Show compilation and execution.
• Analysis: Discuss performance or observations if required.

Highlight Learning Moments

For example:

“Initially, my Caesar Cipher logic caused segmentation faults because I modified a string literal. I resolved it by using dynamically allocated buffers in kernel space.”

This kind of detail shows understanding, not just results.

Make Screenshots Clear

Capture entire terminal windows, including commands and outputs. Avoid cluttered multi-screen shots. Professors appreciate clean, readable evidence of your work.

Going Beyond the Assignment: Building a Solid Foundation

Working on Linux kernel drivers can feel intimidating, but it’s one of the most rewarding skills you can develop as a computer science student. It prepares you for careers in embedded systems, cybersecurity, and OS development — fields that value deep technical understanding.

Explore More Encryption Methods

Once you’ve mastered Caesar Cipher and XOR, try integrating AES or DES libraries in user space and interface them via ioctl. This demonstrates advanced initiative.

Automate Testing

Write shell scripts to load, test, and unload your module repeatedly — this helps detect memory leaks or stability issues.

Practice Kernel Debugging

Experiment with tools like ftrace and gdb for kernel debugging. These will help you gain confidence in troubleshooting deeper kernel issues.

Conclusion

Solving Linux device driver assignments like Caesar Cipher or XOR encryption isn’t just about meeting submission deadlines — it’s about learning how software interacts with hardware at the system level.

By following a systematic approach — setting up the right environment, designing before coding, modularizing logic, and documenting everything — you can handle even the most complex assignments confidently.

So, the next time your professor says, “Build a driver that encrypts text using Caesar Cipher and XOR,” remember: it’s not about encryption — it’s about mastering how the Linux kernel communicates, processes, and manages data.

And if you ever find yourself stuck midway, guidance from experts who specialize in C programming and Linux device driver assignments can make a big difference — helping you understand, not just complete, your project.