This is my implementation of the MD5 Message-Digest Algorithm. I did it to learn a little bit about how cryptographic hash functions work. Doing such project can teach you:
- Low-level operations, such as bit-shifts and XORs (I would never dream of dealing with these in my day-to-day, web-development-related job)
- To work with blocks and buffers, and perform paddings when needed
However, it will not necessarily help you understanding why the algorithm works. For this one should explore books such as The Joy of Cryptography.
Of course, this is by no means meant to be used in production. You probably shouldn't be using MD5 anyway.
Apart from the original RFC, the main resource that I used to complete this project was Zunawe's implementation. I took some time to understand some of his decisions but found them to be ingenious in the end. Thanks, @Zunawe! I ended up following a different path, however.
Compiling and running should be easy:
make
./md5
The main function will output some messages and their digests:
message =Hello, World!
expected=65a8e27d8879283831b664bd8b7f0ad4
result =65a8e27d8879283831b664bd8b7f0ad4
You should start by reading the original RFC. It is extremely well-written and not at all hard to follow.
The MD5 algorithm consists of 4 main steps:
- Append Padding Bits
- Append Length
- Initialize MD Buffer
- Process Message in 16-word Blocks
However, it is easier to think about the input as a stream of bytes, and to begin with steps 3 and 4, leaving 1 and 2 to the end.
With this in mind, check md5.c for the actual implementation.