1 Initial Files

In your CS 340 directory, merge the initial starting files with the following commands:

git fetch release
git merge release/mp2 --allow-unrelated-histories  -m "Merging initial files"

2 Part 1: Implementing the PNG File Format

The World Wide Web Consortium (W3C) is an international community where [many] work together to develop Web standards¹¹ Quoted from https://www.w3.org/about/. One of the web standards that the W3C published and maintains is the Portable Network Graphics (PNG) Specification (Second Edition). This specification fully and completely defines how PNG image files work.

To complete Part 1 of this MP, you must complete a small PNG library that reads the PNG signature and chunks found in PNG files:

• Read the PNG specification to understand the PNG signature and chunks.
  • Note: integer fields within PNG chunks (specifically the data length and CRC fields) are stored in network byte order (i.e. big endian), but modern systems store integers in memory in various different byte orders (usually little-endian). Your code will need to convert between host byte order and network byte order formats when reading and writing integer fields of PNG chunks.
  • You may find functions like htons, ntohs, htonl, ntohl helpful for converting values between host and network byte order.
• Implement lib/png.c (and the struct in lib/png.h) to complete Part 1.
• We have included lib/crc.{c,h} that defines crc32(const void *data, size_t n_bytes, uint32_t* crc). This function takes in a pointer to data of n_bytes and writes the 32-bit CRC value into the address provided by crc. It also lets you break the data into chunks, passing in the first chunk, then the second, and then checking the result.
  • Your library must calculate and write the CRC when writing PNG chunks.

The design on the PNG library is a file-stream based library where you need only to return the next chunk in the file. Therefore, it is highly recommended you only read the data necessary to complete the function and read the next part of the file the next time a library function is called.

• For example, PNG_open requires only that you verify the PNG signature. It is recommended you do not read any additional data beyond the signature.
• Similarly, PNG_read requires you to return data about the next PNG chunk. It is recommended you only read the data for one chunk when this function is called.
• You can create a data structure to store the full PNG in memory as soon as the file is open but this is not recommended.

You will need to open a file for reading or writing in C. The following C library calls may be helpful:

• fopen to open a new file. Use "r" or "r+" as the second parameter to open the file for reading and "w" as the second parameter to open the file for writing.
• fread to read from an open file.
• fwrite to write to an open file.
• fclose to close an open file. Note that if you write but never close it is up to the OS whether the data makes it to the file or not.

3 Part 2: Understanding the Provided Code

In this MP, we have provided two programs that are already complete:

1. A complete program that analyzes a PNG file, png-analyze.c.
2. A complete program that reads and rewrites the PNG file using the provided PNG library, png-rewrite.c.

The png-analyze.c and png-rewrite.c files use your library from Part 1. If your library is not complete, these programs will not run correctly! Here are several example runs of the programs that you should run to explore what the provided code does for you:

• Run make to compile,
• Run ./png-analyze 340.png to view the chunks in 340.png
• Run ./png-analyze sample/natalia.png to view the chunks in sample/natalia.png
• Run ./png-analyze sample/luther.png to view the chunks in sample/luther.png
• Notice that sample/natalia.png and sample/luther.png both have a uiuc chunk – this is our hidden GIF file! You will need to extract it from the PNG file.

4 Part 3: Extract the Hidden uiuc Chunk

Complete png-extractGIF.c to extract the uiuc chunk from the PNG file specified in the command line (argv[1]) and save the data from the uiuc chunk into the GIF file specified in the command line (argv[2]). You will almost certainly find the provided png-analyze.c as a great starting point for your code.

The hidden GIFs in the files within the sample folder are valid GIF and should be able to be opened by any program that can open images if extracted correctly.

5 Part 4: Hide Your Own GIF

Complete png-hideGIF.c to hide a GIF file within a uiuc chunk in a provided PNG file. The first command line argument (argv[1]) is the normal PNG file that needs to be modified to hide an GIF image and the second argument (argv[2]) is the GIF to be hidden.

Your png-hideGIF.c file will be very similar to the provided png-rewrite.c program except that you need to add an additional PNGChunk somewhere between IHDR and IEND (it cannot be before the IHDR chunk or after the IEND chunk, but it otherwise can be anywhere in between the two chunks).

docker run --rm -it -v "`pwd`":/mp2 cs340 "make clean"
docker run --rm -it -v "`pwd`":/mp2 cs340 "make test"
docker run --rm -it -v "`pwd`":/mp2 cs340 "valgrind --leak-check=full --show-leak-kinds=all ./test"

6 Modifiable Files

In your solution, you must only modify the following files. Modifications of other files may break things:

• png-extractGIF.c
• png-hideGIF.c
• lib/png.c
• lib/png.h

7 Submission and Grading

git add -A
git commit -m "MP submission"
git push