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Assignment overview

In this assignment, you will write a command-line program in C to interact with PNG (Portable Network Graphics) image files. The goal is to familiarize yourself with systems-level C programming through a realistic and challenging project.

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Learning objectives

This homework focuses on developing skills in:

File I/O

Reading and writing binary file formats with precise byte-level control

Pointers & Memory

Dynamic memory allocation, pointer manipulation, and memory management

Bitwise Operations

LSB manipulation for steganography and bit-level data processing

Data Structures

Working with complex binary structures and endianness conversions

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What you’ll build

You will implement a PNG manipulation tool that can:
1

Parse PNG files

Read and validate PNG file structure, including signature verification and chunk parsing
2

Extract metadata

Parse IHDR (header) and PLTE (palette) chunks to extract image metadata
3

Validate integrity

Implement CRC-32 checksum validation for all PNG chunks
4

Hide messages

Encode secret messages into images using LSB steganography
5

Overlay images

Composite smaller images onto larger ones with palette merging

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PNG specification

The official PNG specification is available at: You should familiarize yourself with the specification, particularly:
  • The 8-byte PNG signature
  • Chunk structure (length, type, data, CRC)
  • IHDR chunk format (image header)
  • PLTE chunk format (color palette)
  • IDAT chunks (image data)
  • Big-endian integer encoding

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Project structure

The assignment template includes: 
PNG_HW/
├── include/          # Header files
│   ├── global.h      # Output macros and constants
│   ├── png_chunks.h  # Chunk parsing functions
│   ├── png_crc.h     # CRC-32 implementation
│   ├── png_reader.h  # PNG file reading
│   ├── png_steg.h    # Steganography functions
│   ├── png_overlay.h # Image overlay functions
│   └── util.h        # Utility functions
├── src/              # Implementation files
│   ├── main.c        # Main program (you fill this in)
│   ├── png_chunks.c
│   ├── png_crc.c
│   ├── png_reader.c
│   ├── png_steg.c
│   ├── png_overlay.c
│   └── util.c
└── tests/            # Criterion test files
    ├── test_chunks.c
    ├── test_crc.c
    ├── test_reader.c
    ├── test_steg.c
    └── test_overlay.c
Some files contain “DO NOT MODIFY” notices at the beginning. These files will be replaced with original versions during grading, so do not modify them.

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Allowed libraries

For this assignment: Allowed:
  • glibc (GNU standard C library) including malloc, calloc, fgetc, etc.
  • zlib library for data compression/decompression
  • Sample CRC code from the PNG specification appendix
Disallowed:
  • Binary libraries other than glibc and zlib
  • Existing PNG manipulation libraries
Submitting code that uses disallowed libraries will result in a zero grade and potential academic dishonesty charges.

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Academic integrity

You must write your own code. You are not permitted to:
  • Submit source code you did not write yourself
  • Use code from the internet or other students
  • Use AI-generated code without understanding and documenting it
Exceptions:
  • Base code provided in the template repository
  • Code explicitly permitted in writing by the instructor

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Timeline and expectations

Due date: Sunday February 15th @ 11:59 PM
This is a challenging project. You should:
  • Start immediately - Do not wait until the last week
  • Allocate regular time - Work on it consistently over several weeks
  • Experiment with data - Use hd (hex dump) to examine PNG files
  • Write your own tests - The provided tests are minimal
  • Test incrementally - Don’t wait to test everything at once
It is extremely unlikely anyone will read the handout and immediately write working code. Plan to spend significant time experimenting and debugging.

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Getting help

Resources available to you:
  • README.md - Detailed assignment specification
  • Piazza - Check for updates and clarifications
  • Office hours - Get help from TAs and professor
  • PNG specification - Official documentation
  • Beej’s Guide to C - Excellent C programming reference

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Next steps

Requirements

Review detailed assignment requirements

CLI arguments

Start with Part 1: command-line processing

PNG format

Learn about PNG file structure

API reference

Explore the PNG API functions

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