A simple implementation of the AES algorythm, made as part of a self-imposed challange I took after a friend of mine mentioned he'd have to fully implement it by the end of 2018. In the end, this project ended up taking me, roughly, two afternoons (an hour and a half in the first session, and then a whole day in the other) to go from completely clueless as to what AES even was, exactly, to having a functioning implementation.

Generally, encryption to and decryption from AES streams.

Before we can get into any specifics, however, some aspects of the functioning of AES, or, more broadly, the Rijndael Cipher, must be taken note of:

• KBR triplets: Consist in triplets of key length, block size and round count. For Rijndael is a cyclic cipher working on discrete constant-sized blocks of data, this is the single value that will actually determine how the algorythm will run. For AES, specifically, this value will determine which variation of the algortythm, such as AES-128 or AES-256, will be executed (See the "AES KBR table" section for which values correspond to which variations).
• S-Boxes: 16x16 1-byte valued matrices used in determined steps of the cipher algorythm for replacing byte values one to one in a determined fashion. In Rijndael, there are default values for these S-Boxes. For more information on their functionality, one may reffer to this Wikipedia article.

Specifics:

• Encryption in any KBR triplet.
• Decryption in any KBR triplet.
• Custom KBR triplets: This implementation should, in theory, be able to handle any KBR triplet. However, even though it will comply in running any given variation of the algorythm, only very specific triplets, namely, the ones listed under "AES KBR table" have any guarantee of proper functionality. Note that "guarantee" here is used loosely, as I provide no real guarantee aside from the fact they have worked in my personal unit tests, so your mileage may vary.
• Custom S-Boxes: Apart from custom KBR tables, this implementation also supports the use of custom S-Boxes, so long as you provide both the S-Box and its inverse, keep in mind that you are to be sure the inverse is valid, as the implementation does nothing to ensure that.

This is a list of the KBR triplets used in AES:

• {4, 4, 10}: For AES-128.
• {6, 4, 12}: For AES-196.
• {8, 4, 14}: For AES-256.

In order to compile this project, the only requirements are:

• Access to a relatively modern C compiler (although this should be compileable even by c90 compilers).
• Functional implementations for both malloc() and free() (which means this library can run on freestanding environments).
• Optionally, access to a POSIX-compliant version of make, although the library should be simple enough to compile yourself or to integrate into whatever build system one is already using.

If one chooses to run make, however, before doing so it is advisable for them to edit the Makefile provided with the project. Its fields should be obvious enough to need no explaining besides the one already provided as comments in the Makefile itself.

After you're done with the Makefile, simply run make <target>, in which target is one of:

• libaes.a: For a static library.
• libaes.so: For a shared (dynamic) object.
• all: For both libaes.a and libaes.so.